Hydrographic Survey Management Guidelines

Table of Contents

6 Data acquisition and processing

Data acquisition involves the proper collecting and recording of information. Care must be taken so that all written information (descriptions, sketches, calculations, etc.) be entirely understandable to anyone who may be required to process or verify any of the data at a later date.

One of the main requirements of data processing is to be able to ensure or qualify that you are in fact collecting what you say you are collecting and to verify that what is being acquired meets the standards specified for the survey. If a product is to be derived, data processing may be necessary to condition acquired data to meet the product specifications.

Ideally, all acquired data should be processed, coded and validated as the survey progresses. All data shall be processed according to the practices described in the regional Hydrographic Survey QMS procedures in use.

All physical records are to be properly filed and kept in a safe place.

Backups must be made of the data contained in the various computers used for processing. The medium used to store this information (CD, DVD, tapes, separate hard disk, etc.) should be kept in a different location than the processing room.

Measures should be in place to ensure that all original survey data be taken to a safe place should the processing room aboard a ship or on shore be threatened by fire, flood or other damage.

6.1 Bathymetry

Acquisition

Bathymetric data shall be obtained using equipment capable of attaining the accuracy and coverage defined for each survey (See Table 1 of the CHS Standards for Hydrographic Surveys and CUBE Bathymetric data Processing and Analysis (CHS February 2012)). For each typical area, the coverage and line spacing shall be in accordance with those specified in the table. In some cases, it may not be appropriate or desirable to follow these recommendations. It is then possible to attain other accuracies or coverage type. The resultant accuracies must then be computed and documented.

The CHS uses a variety of echo sounders to obtain depths. All echo sounders must be properly calibrated in the field before being used to acquire data. Evidence that field calibration procedures were followed must be filed with sounding notes linked to the survey project. During data acquisition, care must be taken to ensure that the true bottom is being portrayed.

In order to systematically sound a specific area, it will be necessary to run a series of parallel lines and log depths obtained from the echo sounder. This work can be done using single beam echo sounders or, if striving for full bottom search, done using multibeam or multi-transducer systems, or with a single beam echosounder augmented with a side scan sonar system.

To attain the accuracies for each survey, all error sources such as: accuracy of positioning, sounder accuracy, sound speed, attitude sensor, water level measurement, distance from tide gauges, etc. shall be taken into account while computing the horizontal and vertical accuracies, as well as the feature detection capability.

To attain the accuracy and detect the targets as specified in Table 1, it is necessary to adjust the data acquisition methodology such as sounder parameters, line overlapping and vessel speed. When determining the speed at which to conduct sounding, the hydrographer must consider the depth of water being sounded, the pulse repetition rate of the sounder and the sounder characteristics. High speed combined with a slow pulse repetition rate may lead to small but significant features being missed, particularly in shallow water. The hydrographer is to adjust the speed so that for the nadir beam of the echo sounder, such that a target as indicated in Table 1 receives a minimum of five pulses of sound from the echo sounder.

During all sounding operations, the following information should be recorded:

  • general survey area, project, etc.,
  • date,
  • vessel name,
  • operator (s),
  • type of sounding line (regular, check line, shoal exam),
  • sounding equipment (positioning, sounder, attitude sensor, etc.),
  • initial sounder settings and setting changes (include dates),
  • sound velocity profile being used.
  • weather conditions, problems, Beaufort Scale value, other vessel traffic (which might impact the data i.e. vessel wake, water level variation), other particularities, etc.

Data processing

All depths and associated features obtained during the survey should be processed and validated as the survey progresses. While cleaning the data, care must be taken to ensure that the true bottom has been validated.

A comparison of cleaned sounding data obtained from regular sounding lines, check lines and shoal exams must be done to resolve any discrepancies that may arise during the survey.

This comparison is also necessary to ensure that, before leaving a survey area, sufficient sounding detail, coverage, and bottom search have been obtained and shown, verified to substantiate the existence or non-existence of shoals and other hazards, and to enable accurate and detailed portrayal of the contours around shoals, critical depths, channels, anchorages, banks, etc. and the detailed portrayal of the bottom. All dangers, shoals and hazards charted on the existing chart(s) of the area being surveyed must be verified prior to leaving the survey area.

Bathymetric data shall be processed to meet or exceed the accuracies defined for the survey (see Table 1 of the CHS Standards for Hydrographic Surveys and CUBE Bathymetric data Processing and Analysis (CHS February 2012)).

Evidence that field calibration procedures were followed must be found in the files and documents linked to the survey. This includes the calibration of the echo sounder, positioning system, tide gauges, etc.

6.1.1 Corrections for sound speed

Acquisition

A majority of bathymetric data are obtained using acoustic echo sounders where depths are derived from time measurements of an acoustic pulse traveling in a column of water. The velocity of the sound wave in the water column will vary depending on a series of factors, temperature and salinity being the major ones. The depths obtained must be corrected for this variation in sound speed throughout the sounding area in order to compute true depths. A MultiBeam Echo Sounder (MBES) system must have a Sound Velocity Profile (SVP) applied while a Single Beam Echo Sounder (SBES) system or a Multi-transducer (MTES) system may have either a sound speed profile, a single sound speed, or a bar check correction table applied.

Data Processing

Correcting depths for the variation in sound speed throughout the water column may be performed during post processing. A proper record or log should be kept to identify the SVP corrections applied.

Systems employing a MTES or SBES that have a single sound speed set during acquisition will require either bar check or SVP corrections to be applied in post processing, if not already applied during acquisition.

6.1.1.1 Bar check (for MTES and SBES systems)

Acquisition

A “Bar Check” is the field procedure for calibrating the MTES and SBES and involves a metal cone or plate device lowered to a maximum depth of 60 meters and recording the true depth versus the measured depth and compiling a depth correction table that will be used later to correct the measured depths.

A bar check can be conducted in one of two methods. In one method, a shallow bar check is performed to verify vessel draft supplemented by a deeper bar check, obtained up to the maximum depth acquired during the daily operation, to ascertain a harmonic speed of sound for the water column. If the bar check cannot cover the depths recorded, then appropriate calibration devices such as velocimeters, etc. should be used.

The other method also includes a shallow bar check to verify vessel draft (deeper bar check may be conducted as a rough check to verify that the system is working at depth) and a default value is set for the harmonic speed of sound. In the latter method, an accurate sound velocity profile (see § 6.1.1.2) is acquired and applied, during post processing, to the sounding record to adjust the default value of the speed of sound at a given depth.

A bar check should be performed daily or more often as local conditions dictate or when moving location.

Data Processing

If the SVP is not entered into the digital echo sounder, the corrections should be applied at the processing stage of the bathymetric data. A sound velocity profile can be derived and applied from the values obtained during a bar check. The corrections are applied during post processing.

Confirm that the bar check provides information to the maximum depth obtained during the daily operation. If a bar check cannot cover the depths recorded, then corrections obtained from a velocimeter or CTD should be applied.

6.1.1.2 Sound speed profiling (for MTES and SBES systems; mandatory for MBES)

Acquisition

When determining the speed of sound in the working area using a sound velocity profiler (a velocimeter or CTD), the probe is lowered to the maximum depth of the site or at a depth at which the acceptably uniform speed of sound is obtained. The resultant cast from a velocimeter or CTD will provide a table listing the speed of sound at recorded depth intervals. The results obtained from the sound speed cast should be entered directly into a multibeam echo sounder if the particular system has that capability. Data acquired with a MBES, MTES and SBES may have the sound speed corrections applied during post processing.

To adequately ensure to minimize the temporal and spatial uncertainty with respect to the water column, sound velocity profiles should be collected as often as operationally possible, and spatially throughout the survey area while also considering seabed relief and other factors that would affect the local oceanography.

The results of these casts should be checked against a second probe or another system in order to verify that the sound speed is correctly being measured. These checks should be performed at least once a week or as the conditions permit.

A proper record or log should be kept to identify the SVP casts acquired during the project including their time, date and their geographical locations. Sound speed profilers should be capable of measuring sound speed to a precision of at least 0.1m / s.

Data Processing

Profile location and time acquired should be recorded in the header of the SVP files in order to apply SVP corrections in post processing.

Profiles should be edited for anomalies or spikes in the data. When applying SVP corrections in post processing, ensure that the data within the profile contains information for depths up to or greater than the soundings being corrected. If required, an SVP editor may extend the profile to depths greater that the soundings being corrected.

6.1.2 Static and dynamic draft

Acquisition

The draft of a vessel may vary on a daily basis from a few centimeters to some decimeters depending on water and fuel consumption, the number of people onboard, equipment, etc. The echo sounders should be adjusted so that the static transducer draft will be applied to measured depths. Changes in transducer draft will have to be monitored during sounding operations and during echo sounder calibration. This value can be set by adjusting the transmission line of the echo sounder (MBES, MTES or SBES) while performing a bar or cone check with the bar set under the transducer at 2 or 3 meters below the surface of the water. Record and apply any horizontal offsets from bar to transducer due to point of displacement and for a vertical offset caused by current.

Draft can also be obtained by measuring the distance between the water surface and the face of the transducer or by using RTK GPS heights to determine the dynamic draft. All vessels are subject to squat and settlement. At certain speeds, the stern of the launch may sink or squat into the water, thus changing the angle and draft of the transducer. This may be less appreciable if the transducer is located near amidships. The use of a speed sensor to measure the Speed Through the Water (STW) is recommended in order to apply adequate dynamic draft to the data. Settlement is the general lowering in level of a moving vessel, relative to what its level would be were it motionless. Settlement is due to the regional depression of the surface of the water in which the vessel moves.

Tests should be performed at various speeds over flat bottom, preferably before the sounding platforms leave for the field. RTK GPS or orthometric leveling heights measured at the transducer location, at rest and at speed increments of one knot (through water), can be used to derive a squat/settlement table. Hydrographers should also be aware of long period changes in transducer draft attributed to vessel fuel consumption or ballasting. Any discrepancies should be noted and corrections should be applied durint the acquisition or at the processing stage.

Data Processing

Static and dynamic draft values are to be applied in post processing when they cannot be applied during acquisition. Dynamic draft tables usually take the form of a listing of draft changes versus speed of vessel. A record of applied values is to be kept with the raw data set.

6.1.3 Heave, pitch, roll and yaw

Acquisition

Sounding platforms float on or below the water surface and the position and attitude of onboard sensors will therefore be affected by changes in heading, speed, wave and current action. Position and attitude relative to the platform are described by a platform coordinate system, defined in terms of the direction of the platform and the ground. The x-axis is relative to the centerline of the platform and is positive to the bow. The y-axis is perpendicular to the x and is positive to starboard. The z-axis points to the nadir direction, and is positive downward. Roll is the side to side rotation about the x-axis, pitch is the forward and backward rotation about the y-axis, yaw is the horizontal rotation about the z-axis and heave is the up and down motion along the z-axis.

These parameters are measured using an attitude sensor and are logged and used to correct the bathymetry in real time.

For sounding launches not equipped with attitude sensors, the only practical method of minimizing vessel motion is to set a limit on the maximum allowable sea state under which sounding operations can be conducted.

For MBES or MTES surveys, a properly calibrated MRU (Motion Reference Unit) is mandatory.

Data Processing

Heave, pitch, roll and yaw are logged during the acquisition stage and are to be checked and processed to meet the specifications of the systems used and for the order of the survey specified. Spikes and anomalies within the attitude and heading sensor data should be corrected or removed.

Where consistent anomalies are found a system re-calibration may be required before acquisition continues.

Filtered, latent heave data (e.g. True Heave™), if acquired, is to be applied during post processing. These files are to be referenced with the corresponding raw data files.

6.1.4 Positioning

Acquisition

The positioning system used for sounding operations should be checked on a regular basis. Confidence checks should be done prior to daily sounding operations, after equipment replacement or repair or whenever there is any doubt in the positioning system’s performance. The use of mission planning software is recommended. When possible, compare the position obtained against a known position (wharf corner, fixed object, etc.) or by comparing the position against an independent positioning system (survey platform’s positioning system, a second independent positioning system, etc.).

Data Processing

Positioning is to be reviewed, and spikes or anomalies not falling within survey specifications must be corrected or removed. When possible, compare the position obtained against a known position (wharf corner, fixed object, etc.) or by comparing the position against the data collected with an independent positioning system (survey platform’s positioning system, a second independent positioning system, etc.).

6.1.5 Single beam work

Acquisition

This method consists of acquiring depths using an echo sounder interfaced with a single beam transducer normally mounted in the hull of the sounding platform. Depths obtained from the echo sounder and their corresponding positions obtained from a navigation source are time-tagged and logged onto a computer using a hydrographic navigation software package which, among other things, also allows the launch to be conned along predetermined tracks.

At the beginning of each sounding day, a bar check should to be performed to verify or adjust transducer draft and to acquire values in order to correct the soundings for the varying speed of sound in water; a bar check should also be done at the end of the day to ensure that the sounding parameters have not changed. Bar checks should also be performed if any echo sounder components have been changed or if changes in sound speed and transducer draft are suspected.

Speed of sound corrections may also be obtained by sound speed probe. The results can be logged in the acquisition software and duly noted so that the results may be applied in post processing.

The sounding line pattern should be set so that the lines are run as perpendicular as possible to the natural bathymetric contours of the sea floor. The line spacing should be chosen to conform to the values contained in Table 1 of the CHS Standards for Hydrographic Surveys. In the vicinity of wharves, lines are to be run as perpendicular as possible to the wharf face. Additional lines should be run along the wharf face.

In most cases, based on water depth, the echo sounder used should be operated at the highest pulse rate possible. The speed of the vessel should be such that a bottom target receives enough pulses to detect a specific target size as specified for the different Survey Order in Table 1 of the CHS Standards for Hydrographic Surveys (generally, 5 pulses (or pings) of sound on a target should ensure detection of most of the significant features).

Data Processing

Applicable regional Hydrographic Survey QMS procedures must be employed. An outline of the steps required in post processing single beam work entails applying: bar check or, SVP data, attitude sensor data and navigation corrections, dynamic draft tables and then removing noise or other anomalies from the sounding data. In most cases where a dual frequency system is used, the highest frequency recorded with the echo sounder shall be selected as the primary depth record when possible and used when post processing depths. The soundings are to be attributed according to the CHS Product Specification and Coding Guide (PSCG).

Check lines are to be compared against the regular sounding pattern and used to verify that the acquisition project is meeting the specified survey order. Sounding line patterns and spacing should also be reviewed for conformance with the values prescribed in Table 1 of the CHS Standards for Hydrographic Surveys.

According to Survey Order in Table 1 of the CHS Standards for Hydrographic Surveys, review the raw data to ensure that the bottom target receives enough pulses as specified by the specifications.

6.1.6 Multi-transducer work

Acquisition

This method consists of acquiring depths from a series of transducers mounted in the hull and along retractable booms, which when lowered to the water surface are perpendicular to the ship’s centerline. Multi-transducer sounding systems are run using sophisticated computer software systems that record set-up and corrections parameters as well as attitude sensor data so that corrected depths, positions and other required information may be logged.

The system needs to be calibrated before sounding operations can begin. Each transducer array may obtain a different value on the same target. This means that every transducer must be measured, calibrated and compensated for. A series of very close lines on a flat bottom should be run to ensure a consistent bottom measurement from all the transducers. This calibration should be performed at least twice a year or when any changes in the configuration of the system are suspected or occur.

On a daily basis, only one transducer is to be used to verify sound speed using a bar check and all other transducers are to be set to this reference transducer.

The speed of sound through the water column can be measured using sound velocity profilers, the results of which are entered into the sounding system’s processing unit.

The line spacing will depend on the coverage required. The necessary coverage should be specified in the Field Survey Instructions.

A multi-transducer system requires an attitude sensor so the depths obtained from all the transducers can be calculated at their true position. Vessel attitude relative to the X, Y and Z platform coordinate system must be measured. Yaw, will affect the position of the soundings when the sounding platform deviates from its centre line. Any defects in the platform attitude measurements shall be detected during the acquisition stage. As it is difficult to correct this kind or error, all the affected lines must be resurveyed.

Another parameter, which might have to be factored in, is the fact that the vessel draft may decrease as fuel is consumed during sounding operations. In this case, changes in draft will have to be monitored over time. A good knowledge of the platform’s draft variation is essential. A draft variation table must be established for each platforms and applied either during the acquisition or the processing.

Data Processing

Applicable regional Hydrographic Survey QMS procedures must be observed. An outline of the steps required in post processing multi-transducer work entails:

  • review and correct attitude sensor data and navigation,
  • review and correct attitude sensor data and navigation,
  • apply latent heave data (e.g. TrueHeave™),
  • apply SVP corrections,
  • attribute TPE values
  • apply water level reductions, and
  • remove noise or other anomalies from the sounding data.

Sufficient coverage and sounding density should be ensured to meet the specified survey order. The soundings are to be attributed according to the PSCG.

Check lines are to be compared against the regular sounding pattern and used to verify that the acquisition project is meeting the specified survey order. Check line patterns and spacing should also be reviewed for conformance with the values prescribed in the Standards. (See the CHS Standards for Hydrographic Surveys § 7.4).

6.1.7 Multibeam work

Acquisition

Multibeam work consists of acquiring depths from an array of narrow beams formed from a single or dual transducer system. Normally this system is hull-mounted, however pole-mounted systems are also common.

Depth data must be corrected for vessel attitude relative to the X, Y and Z platform coordinate system.

Sounding lines should be run parallel, as much as possible, to the natural bathymetric contours of the sea floor. The line spacing and survey methodology will depend on the required coverage. See CUBE Bathymetric data Processing and Analysis (CHS February 2012).

Survey instructions may require that the multibeam backscatter be logged and rendered at the end of the survey.

Data Processing

Applicable regional Hydrographic Survey QMS procedures must be observed. Detailed procedure can be found in CUBE Bathymetric data Processing and Analysis (CHS February 2012).

Check lines are to be compared against the regular sounding pattern and used to verify that the acquisition project is meeting the specified survey order. Check line patterns and spacing should also be reviewed for conformance with the values prescribed in the Standards. (See the CHS Standards for Hydrographic Surveys § 7.4).

6.1.8 Side scan sonar work

Acquisition

Side scan systems are generally used to satisfy the full bottom search requirement when conducting single beam surveys. However, they may be employed in multibeam or multi-transducer surveys when a higher resolution of potential hazards to navigation is required. Spacing of the survey lines must be sufficient to ensure the full insonification of the bottom.

Side scan sonar is best suited for searching for obstructions such as wrecks, where small features may be present within the water column, like mast structures, suspended buoys and other pinnacle-like hazards to navigation. The high-resolution functionality of side scan sonar is most suitable for sensing small targets, which may not otherwise be detectable by multibeam or multi-transducer systems.

Hull or pole mounted systems are recommended as heave, pitch, roll and yaw can be collected with an attitude sensor and values applied to the sounding data.

Although the heights of targets may be approximated, side scan sonar should not be used for depth determination but as a reconnaissance tool to seek out targets and seabed anomalies requiring more detailed and accurate investigation by survey echo sounders.

Data Processing

Applicable regional Hydrographic Survey QMS procedures must be observed. Care must be taken in the interpretation of side scan records in order to identify true targets from anomalies such as surface returns (sea clutter, waves and vessel wakes), multiple echoes, cross talk or marine animals. Vessel attitude corrections need to be applied to data acquired from hull-mounted systems in order to create a clean digital mosaic of the side scan records.

Where side scan is employed within a single beam survey to detect features between sounding lines, any features found should be recorded and communicated to the acquisition team for further investigation.

6.1.9 Interferometric work

Acquisition

Interferometric sonar (IFMS) is an emerging technology which provides significant advantages when sounding in shallow water. Unlike multibeam, IFMS systems are not beam-forming but measure depths through the use of exactly spaced phase differencing transducer elements which measure the phase offsets of acoustic returns. This is converted into the angles from which the return was received and is used in combination with range based on two way time travel to calculate the positions on the seafloor relative to the transducer. IFMS provides both high density bathymetry and side-scan imagery which enables advanced display and quality control capability with seafloor datasets.

The major advantage of IFMS is that in shallow water, it covers a much larger swath than a comparable multibeam system; for example, a typical IFMS swath could be 12 times the water depth as compared to 4 times for a typical MBES. This could significantly reduce the time survey crews spend in dangerous inshore waters which would considerably increase production. The disadvantages are limited sounding depth due to transducers being required to be mounted at an angle deflected from the vertical; typically by about 30°. This creates another problem in that the seabed directly beneath the vessel is not covered during the current swath and must be covered from adjacent lines. IFMS systems have had issues in the past resulting in noisy data mostly attributed to ambient and internal noise, as well as difficulty resolving multiple angles of arrival. Recent improvements in electronics and algorithm advancements along with the use of increased numbers of elements have improved the precision of this technology.

Sounding with IFMS follows the same principles as multibeam in that depth data must be corrected for vessel attitude relative to the X, Y and Z platform coordinate systemSounding lines shall be run parallel, as much as possible, to the natural bathymetric contours of the sea floor. The line spacing will depend on the required coverage (100%, 200% or other).

Data Processing

Applicable regional Hydrographic Survey QMS procedures must be observed. An outline of the steps required in post processing IFMS work entails:

  • review and correct attitude sensor data and navigation,
  • apply draft corrections, dynamic draft,
  • filter latent heave data (e.g. TrueHeave™),
  • apply SVP corrections,
  • attribute TPE values,
  • apply water level reductions, and
  • remove noise or other anomalies from the sounding data.

Sufficient coverage and sounding density should be ensured to meet the specified survey order. The soundings are to be attributed according to digital coding guides.

Check lines are to be compared against the regular sounding pattern and used to verify that the acquisition project is meeting the specified survey order. Check line patterns and spacing should also be reviewed for conformance with the values prescribed in the Standards. (See the Standards for Hydrographic Surveys § 7.4).

6.1.10 Airborne (LIDAR) work

Acquisition

Airborne system such as LiDAR (Light Detection And Ranging) is an optical remote sensing technology which measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use laser pulses. Like the similar radar technology, which uses radio waves instead of light, the range to an object is determined by measuring the time delay between transmission of a pulse and detection of the reflected signal. Depth range is dependent upon water clarity, and can vary from 0m to 40m. In the ocean, turbidity, breaking waves and kelp can render LiDAR measurements difficult to impossible. Low level (ground) fog and the presence of ice will also prevent LiDAR measurements. Poor depth ranges occur where there is abundant sediment or other suspended material (turbidity) in the water column. Typically 2-3X secci dish readings are expected.

Low ceiling heights will restrict bathymetric coverage and often prevent flight operations. Attention must be given to planning Airborne LiDAR surveys around known seasons of inclement weather, the presence of ice and high sediment load (turbidity). Satellite remote sensing imagery such as MODIS provides an excellent means for pre-survey and in-situ analysis of turbidity conditions.

An area of overlap between acoustic surveys and the LiDAR survey is preferable.

To calibrate the depth measurements, ideally full bottom coverage groundtruthed patches should be surveyed in the LiDAR survey area. Such groundtruth must cover the depth range of the LiDAR survey at different locations. The positional accuracy of the system can also be checked in-situ against precisely positioned terrestrial features such as buildings.

Data Processing

The Canadian Hydrographic Service doesn’t own such a system, and this type of survey is normally done under contract to the private sector. A CHS representative must be present during the acquisition to supervise the contract and optimize the operations in conjunction with the contracted company. The processing and verification process is normally done by the contracted company and CHS involvement is minimal. A final report should be submitted with the data and must include all pertinent information and metadata pertaining to the data.

Upon reception of the data, thorough quality controls of the data are necessary and consist of validating the survey methodology (positioning, calibration, etc.), comparing the resulting depths against the groundtruth and other data available in the survey area, and validating the water level values applied. Any anomalies should be reported to the contractor for revision.

6.1.11 Spot soundings

Acquisition

Spot soundings may be conducted with a graduated rod, a lead line, or with a single beam transducer sounding through ice.

When the depth of water is measured with a lead line or other type of graduated rod, the depth as well as the position, date and time at which the depth has been taken must be recorded. The results must be manually entered either in the logging system or in the main processing system.

Through-ice sounding entails clearing the ice of snow, lubricating the ice surface and transducer face, placing the face of the transducer on the ice surface and recording depths for the time required to obtain an adequate redundancy of depths.

Data Processing

The depth values obtained from a graduated rod or lead line should be corrected for tide.

Depths acquired with the through-ice technique are to be corrected for tide and sound speed. Noise or other anomalies from the sounding data should be removed and one representative sounding must be selected for each spot.

All soundings are to be attributed according to the PSCG.

6.1.12 Shoal examination

Acquisition

A shoal is a distinct rise of the seabed, which could be a hazard to navigation. A 10% rise in the seabed depending on the depth of the surrounding area may indicate the existence of a shoal or some other serious hazard to surface navigation and should therefore be investigated. Consideration must be made for the type of vessel traffic expected. When taking into account the draft of some modern tankers, any isolated indication of shoaling of less than 50 meters may be of sufficient importance to warrant an examination for a possible shoal. In some instances, it may be necessary to further develop shoaling areas deeper than 50 meters.

Potential shoals are, for the most part, identified by analyzing the depths obtained from regular soundings or historic data. It is therefore imperative that the processing of the bathymetric data be kept up to date and be available for shoal identification. In other cases, information pertaining to shoals and other hazards may come from various reports and local knowledge.

Whenever there is any indication of a shoal, or if a shoal is suspected, additional investigation must be done to determine the position and the least depth.

If single beam systems are used, an adequate shoal examination shall be conducted to identify the least depth, then a series of short lines (a minimum of 3) shall be run in either a star or grid pattern over that least depth. Lead line depth verification may be required if weeds or kelp are present, making the sounder’s data non-conclusive. A bottom sample should be recorded and can be acquired with a bottom grab or an armed lead line.

When a multi-transducer or multibeam system is used to investigate shoals, the ping density and the vessel speed should be set such that the chances of missing the shoal is reduced and that there is sufficient overlapping soundings.

Data Processing

When post processing sounding data from a shoal exam: ensure that adequate coverage is obtained to meet survey specifications, the least depth is identified and the recorded bottom classification is assigned to that sounding. Any potential new shoals should be identified and forwarded to the acquisition team for further investigation. All soundings are to be attributed according to the PSCG.

6.1.13 Check lines

Acquisition

As a means to validate a hydrographic survey, check lines should be run perpendicular to the regular sounding lines. The check lines should be spaced according to the specifications stated in the CHS Standards for Hydrographic surveys.

The information obtained from these check lines will confirm the precision of the positioning system, the depth measurement and corrections applied to depths such as sound velocity, draft and tide adjustments. These lines should be run on a different day (when possible) than the regular sounding lines and if possible using a different survey vessel.

Data Processing

Verify that the check lines are spaced according to the specifications stated in the CHS Standards for Hydrographic Surveys (§ 7.4). The difference in the depths between the regular sounding lines and the check lines must fall within the depth accuracy limits (95% of the time) of the Survey Order as specified in the Field Survey Instructions (the various survey orders are found in Table 1 of the CHS Standards for Hydrographic Surveys).

The comparison of the check line soundings over a DTM generated from the regular lines on a relatively flat bottom is a good way to obtain difference values.

When discrepancies exceed the permissible amount, the soundings must be carefully examined to determine the possible sources of error: tide or water level, sea state, beam angle, position errors, calibration errors, corrections used etc. Once the sources of error have been determined, corrective action must be taken. This work should be done as soon as possible as the survey progresses. In some cases, it may be necessary to resurvey the portion of the area that is subject to disagreement with the check lines.

6.1.14 Track and reconnaissance survey

Acquisition

In some cases, it may be required to run sounding lines in uncharted waters in order to obtain valuable depth data in advance of regular surveys or provide additional soundings in sparsely sounded areas.

Depths obtained for these purposes should be obtained and processed in the same fashion as a standard survey.

Data Processing

Depths obtained for these purposes should be obtained and processed in the same fashion as a standard survey. (See § 6.1.5 – 6.1.7 inclusive.)

6.1.15 Reducing soundings to datum from water level survey

Acquisition

All soundings must be reduced to a sounding datum as specified in the survey project instructions (also see §5.2 Vertical Datum). The water levels used to reduce the soundings may be acquired by readings from a permanent water level gauge, a temporary water level gauge, a water level staff or by leveling to the waterline from known benchmarks. In some cases, it might be necessary to use tidal predictions to reduce the soundings to a known datum. In this situation, it is advisable to verify the local predictions by leveling to the waterline from benchmarks at the station used to produce the predictions.

All water level readings, whether they are obtained manually or telemetrically, should be measured with instruments capable of attaining the required accuracy as specified in the CHS Standards for Hydrographic Surveys.

The water level gauges should be checked daily against readings obtained from a stable water level staff or by leveling from known benchmarks.

Data Processing

Corrections to the water levels should be applied before using them to reduce soundings. Procedures as outlined in the Canadian Tidal Manual should be followed.

The final datasets must contain a note describing the method used to reduce the soundings and include a reference elevation to at least one benchmark, a reference port in the case of tidal predictions or the reference port and number of the co-tidal chart used.

A record of the water levels used to reduce the soundings should be included with the sounding notes.

6.1.15.1 Reducing soundings to datum from Real-Time Kinematics’ (RTK) Differential GPS Surveys

Acquisition

Kinematic GPS positioning is capable of yielding sub-decimeter accuracy on a moving survey platform – both horizontally and vertically. This technology can provide real-time or post-processed elevations of survey vessels, which can be related to the local chart datum surface. If adequate attitude compensation equipment is used, and an accurate water level datum model, such as separation table from ellipsoid to chart datum, have been developed, accurate water level reductions can be directly obtained without observing water levels.

High quality geodetic GPS receivers capable of processing the necessary information are required in order to use this method. The height from the GPS ship antenna to the water line, and consequently to the transducer draft set on the echo sounder, must be accurately measured. The height of the reference station antenna to the horizontal control point must also be accurately measured. The elevation of the horizontal control point above local sounding datum must also be accurately determined.

This method of obtaining reduced depths is usually applicable to a specific area where tidal information is accurate and has been recorded for long periods allowing the generation of accurate tidal models.

Specific Field Survey Instructions will be required before leaving for the field.

Data Processing

When the real time water levels are not applied directly to soundings it will be necessary, in post-processing, to run software equipped with algorithms capable of extracting, filtering and applying water level corrections to the soundings. See CUBE Bathymetric data Processing and Analysis (CHS February 2012).

6.2 Navigational aids

Acquisition

If navigational aids (fixed or floating) have to be observed, the position and a detailed description must be taken. Note that this task is not required for aids entered in SIPA, unless requested by the Canadian Coast Guard (CCG).

Data Processing

Unless otherwise stated, it will be necessary to verify the position of all fixed and floating aids established and maintained by Federal Government (if requested by the CCG) or private agencies found in the area of work and report the positions obtained to the regional CCG offices for inclusion in the CCG database (SIPA). It is a good practice to submit coordinates to Provincial Geodetic Agencies for inclusion in their database.

Survey positions and heights (where applicable) of fixed or floating aids according to CHS Standards for Hydrographic Surveys and describe their characteristics. Information is to be tabulated according to a format sufficient for charting and CCG applications.

The position and characteristics of these fixed and floating aids must be compared with values published in SIPA and the List of Lights (if required). If they do not correspond with SIPA and the List of Lights and the differences are such that they may be considered hazardous for navigation, they must be reported to CCG - see § 9.1 Reporting Dangers. If the differences are not considered to be dangerous to navigation they must still be reported to the CHS Regional Director and to the Coast Guard offices.

6.2.1 Fixed aids

Acquisition

A fixed aid can be defined as any stationary man-made structure erected for navigational purposes (day beacon, lighthouse, light structure including range lights, sector light, etc.) and must be positioned to the accuracy mentioned in the CHS Standards for Hydrographic Surveys.

Private fixed aids of navigational significance should also be positioned whenever possible.

Fixed aids must be described using the CHS Fixed Aid Tabulation (FAT) form which will eventually be submitted to CCG for their records (see § 8 Data rendering and submission). The description shall include the geodetic position, the elevation, the height of the light structure to the ground, the height of the structure from the ground to the vane (focal height), etc. Color photographs (analogue or digital) of the fixed aid should be taken and filed.

The elevations of fixed aids must be obtained (see § 6.7 Elevations).

Data Processing

The processing entails to calculate the position of the fixed aid as per regional Hydrographic Survey QMS procedures. The CHS FAT form should be completed.

The light history should be investigated to determine if the light has been previously positioned, has moved or been altered since the previous survey, and whether the structure is the same type as shown in the List of Lights. Check if the light is in the same position, relative to the shoreline, as shown on the largest scale chart.

6.2.2 Floating aids

Acquisition

Private floating aids not included in SIPA should be positioned and described. The agencies or organizations responsible for the maintenance of the private floating aid should be noted.

Floating aids must be described (shape, colour, light characters, letters and numbers, etc.) and any significant feature noted (whistle, bell, horn, radar reflector, etc.). Colour photographs (analogue or digital) should be taken.

Data Processing

The position of CCG and private floating aids (from the SIPA database) should be verified to the accuracy given for the specified survey order as found in Table 2 of the CHS Standards for Hydrographic Surveys.

The Color photographs (analogue or digital) taken should be filed.

6.2.3 Ranges

Acquisition

In addition to being positioned to the accuracy mentioned in the CHS Standards for Hydrographic Surveys, the true bearing (geodetic azimuth from seaward) of the range line must also be determined. This is done by drifting across the range line approximately at right angles and at several locations along the usable length of the range. Good fixes (positions) must be obtained when the beacons are exactly in line.

Survey the usable portion of the range line, i.e. visibility and depths. Detailed soundings must be taken along and adjacent to range lines in order to validate the depths available. This sounding coverage should be designed so as to ensure that all hazards within the navigable limits of the range line are detected, examined and verified. The anticipated draft, size and maneuvering ability of the largest vessel likely to navigate the range should be considered in determining the extent and detail of the survey.

Where possible, the soundings should be acquired using a multi-transducer or multibeam system according to standards prescribed in Exclusive or Special Order surveys (Table 1 of the CHS Standards for Hydrographic Surveys).

Less critical range lines can be surveyed using single beam echo sounders. A minimum of 3 sounding lines should be run, one along the centre line of the range line and one on each side of the centre line. Additional parallel lines and checklines must be sounded if the depths along the range are suspect or do not agree with charted depths. This may necessitate a full bottom search of the range.

Data Processing

In addition to surveying the position of the fixed light structures (see § 6.2.1 Fixed Aids) that make up a range line, the true bearing (geodetic azimuth from seaward) of the range line must be computed and checked against the published value.

Inverses computed from the various obtained positions and the Front Range light will yield a geodetic azimuth – statistical means can be used to determine the average azimuth, which should then be compared to the published value. The maximum allowable difference between the theoretical and drift azimuths is given in Table 2 of the CHS Standards for Hydrographic Surveys.

Verify the least depth available by processing the soundings taken along and adjacent to range lines. The anticipated draft, size and sea room required for the largest vessel likely to use the range should be considered when determining the extent along ranges to which the verification and examination of hazards and shallower depths are to be carried out.

To process multi-transducer or multibeam data, see § 6.1.6 and 6.1.7.

When single beam echo sounders are used, verify that a minimum of 3 sounding lines have been run, one along the centre line of the range line and one on each side of the centre line. Report to CCG and the Regional Director if the depths along the range are suspect or do not agree with charted depths. Additional parallel sounding lines may be required.

A comprehensive comparison with the charted depths must be made and all discrepancies (in azimuth or depths along the range line) shall be verified and reported. If any discrepancies are found, the procedure described in § 9.1 Reporting Dangers should be followed.

6.2.4 Sector lights

Acquisition

Sector lights separate zones where it is safe to navigate (usually a white light) from zones which are unsafe when the vessel has gone too far to starboard or port (red and green lights respectively). On a chart, the various navigation zones are separated by lines drawn according to the true bearing of each sector line limit.

The true bearing of the various sector lines must be determined and checked by drifting across the limit between two zones and obtaining a series of positions which can then be used to compute the bearing of the various limits (see § 6.2.3 Ranges).

Data Processing

In addition to surveying the position, the true bearing of the various sector lines must be determined and checked from the series of positions taken (see § 6.2.3 Ranges).

6.3 Wrecks and obstructions

It might be necessary to search for wrecks and obstructions using information, which may have been obtained through Mariner reports, local authorities or found on documents obtained through the Navigable Waters Protection Act or other sources.

In some circumstances, care should be taken not to disclose some wreck information to the public, as these sites may be protected by federal or provincial regulations in respect of marine heritage.

6.3.1 Position of wrecks and obstructions

Acquisition

All wrecks and other obstructions such as submerged cribs, piles, artificial islands, etc. are to be accurately positioned using applicable regional Hydrographic Survey QMS procedures. The obstruction should be sufficiently surveyed in order to determine its orientation and extents. Positions should also be acquired for the portions of the hull, superstructure or masts that may be visible above water.

If wrecks or other obstructions are located in areas where it is dangerous or impractical to approach by launch or ship, the position must be obtained by intersection or by helicopter equipped with GPS. Side scan sonar can be used as a reconnaissance tool to locate wrecks and obstructions.

Data Processing

Determine the orientation and limits of the obstruction as it exists above and below chart datum and/or high water. Verify positions obtained and the definition of the portions of the hull, superstructure or masts that may extend out of the water.

New wrecks or wrecks which have shifted position and are hazardous to navigation must immediately be reported to the Regional Director. Additional action may also be required (see § 9.1 Reporting dangers).

The photographs and pertaining documentation of the wrecks and obstructions are to be filed.

6.3.2 Least depth over wreck

Acquisition

The least depth over the wrecks and obstructions must be determined. In the simplest of cases when the obstruction is in shallow water, the depth can be verified by using a lead line or graduated pole. In certain situations, SCUBA divers could be used to ensure that the least depth is measured.

Least depths over wrecks and obstructions found in deeper water must be obtained using echo sounders. The use of multibeam or multi-transducer systems with precise positioning and increased coverage is recommended. Modern MBES’s have the ability to record volume backscatter data throughout the water column which can be very useful in identifying least depth of wrecks, particularly their masts.

If located using side scans sonar, the least depth and its position should be confirmed by using another method.

Data Processing

The least depth over the wrecks and obstructions must be determined.

Depths acquired by lead line, or by echo sounders are to be processed as per § 6.1.5 – 6.1.7 inclusive. All depths are to be reduced to a sounding datum as specified in the Field Survey Instructions (also see § 5.2 Vertical Datum).

For wrecks that have been found during a hydrographic survey for which no previous information exists, a report containing least depth and position must be forwarded to a Transport Canada “Receiver of Wreck” official or any other designated organizations.

6.4 Submerged cables and pipelines

Acquisition

The positions of submerged cables and pipelines should be acquired from existing as-built plans or by survey. The type of cable (e.g. telephone, power, ferry etc.) and pipeline (e.g. oil, gas, water intake and outfall) should also be noted. Structures such as manifolds, cribs or diffusers should also be surveyed. Side scan sonar can be used as a reconnaissance tool to locate cables, pipelines and structures related to these installations.

Warning signs, such as those prohibiting anchorage or warning buoys at the end of pipelines must also be positioned.

Data Processing

The positions of submerged cables and pipelines, and the position of the cables used to pull ferries from one shore to another should be extracted from the sounding lines. Soundings may be overlaid with any existing as-built plan, gridded multibeam imagery and/or a side scan sonar mosaic to verify the survey.

Validate cables and pipelines against existing nautical publications.

6.5 Clearances

Acquisition

Clearances under bridges, cables and other overhead structures are vital to safe navigation. The elevations must therefore be measured with care and precision to the accuracy specified for the various survey orders given in Table 2 of the CHS Standards for hydrographic Surveys. The elevation of the lowest point over the navigable channel should be obtained.

The elevations may be determined using an optical range finder, laser measurement system or by measuring zenith angles with a theodolite. Furthermore, the observer should be at right angles (as close as possible) to the bridge, cable, etc. and be in a position to measure to the lowest point of least clearance over the water surface.

In some cases, the elevation of a bridge may be determined by using a measuring tape or by levelling to the waterline or to a benchmark. If a tape is used, the measurement should be taken at least twice for verification. When feasible, this method should be used to verify the results obtained by measuring zenith angles.

In situations where the accuracy requirements are lower, other devices such as range finders, hand levels, etc. may be used. At least three measurements must be taken and they must be within the accuracy specifications listed in Table 2 of the CHS Standards for hydrographic Surveys.

The elevation of cables and bridges will be affected by load whether vehicular or electrical, as well as by ice, snow, wind, etc. In addition to measuring elevations, the following information should also be recorded, as it will help determine the vertical elevation that will be charted:

  • date, time temperature and barometric pressure
  • elevation of observer above the water level at the time of the observations
  • nature of the overhead cable
  • sketch, or photograph (analogue or digital) and description of object measured

Cable owners, i.e. provincial hydro electric authorities, telephone companies, etc. must be contacted to determine the maximum catenary for a cable considering temperature, ice, load, etc.

It is important to acquire the position of the support structures of bridges and overhead cables, especially if they are in the water area and/or areas where they are conspicuous and can be used for taking visual positioning, bearings.

Data Processing

Apply any tidal corrections and catenary load corrections relative to collection time of clearances obtained. Validate clearances against existing nautical publications and the authorities responsible for these installations.

6.6 Conspicuous objects

Acquisition

A conspicuous object can be described as any natural or artificial object which is distinctly and notably visible from seaward and may be used for navigational purposes (distinct topography such as bluffs, cliffs or rock pinnacles, church spire, chimney, cross, silo, water tank, communications tower, aeronautical beacon, etc.).

Conspicuous objects must be positioned and their elevation determined to the accuracy given for the different survey orders found in Table 2 of the CHS Standards for Hydrographic Surveys. The methods described in § 5.1 Horizontal Control should be followed.

Each conspicuous object must be given a unique identifying number and described as specified in QMS documentation (see § 5.1.2 Station Description). Colour photographs (analogue or digital) should be taken and filed.

Data Processing

Ensure that positions and elevations obtained meet the required survey order specified for the project.

Validate conspicuous objects against existing nautical publications.

Refer to the appropriate regional Hydrographic Survey QMS procedures and forms for processing elevations.

File colour photographs (analogue or digital) as applicable.

6.7 Elevations

Acquisition

The elevation above a vertical datum (HHWLT) of all horizontal control points, fixed aids, major wharves and conspicuous objects positioned must be determined as accurately as possible and be within the limits set in Table 2 of the CHS Standards for Hydrographic Surveys.

The elevations can be obtained using GPS, laser/reflectorless measurement system, by physically measuring the height of the structure to the ground and levelling to a benchmark, by physically measuring the height to the water level and applying a water level reduction, or by using a trigonometric height measurement method from known horizontal points with accurate elevation values.

Data Processing

Ensure that elevations obtained meet the required survey order specified for the project.

Refer to the appropriate regional Hydrographic Survey QMS procedures and forms for processing elevations.

The elevations of all major wharves in the survey area should be reduced and shown above a vertical datum of HHWLT (tidal areas) or a reference datum (non-tidal areas).

6.8 Seafloor classification

Acquisition

The nature of the seabed must be acquired to indicate its suitability for vessels anchoring, trawling or taking ground. The specific requirements for bottom sampling are given in the chapter 5.3 of CHS Standards for Hydrographic Surveys. Sampling density could be increased in potential anchorage areas, in fishing grounds, and in areas which could be used to ground a vessel in emergencies. The sampling equipment generally used is a tallow-armed sounding lead. This method is not very efficient and will only retrieve relatively fine-grained samples. However, an experienced user will be able to tell the difference between rock, sand and mud. The use of a small bottom grab, scoop or dredge type sampler will yield more sample but will prove difficult in a bottom consisting of boulders. This method is also useful if the bottom samples are to be retained for later analysis by others. A small submersible camera can also be used.

Bottom characteristics can also be determined from the acoustic information recorded from single beam, multibeam and multi-transducer systems. This method requires ground-truthing using the above mentioned methods. Correlation tables may also be used.

Bottom characteristics determined from samples shall be classified in accordance with CHS Chart Number 1.

The size criteria table listed below can serve as a General Guide:

  • Mud
    • Clay less than 0.002 mm
    • Silt 0.002 to 0.0625 mm
  • Sand
    • Fine 0.0625 to 0.25 mm
    • Medium 0.25 to 0.50 mm
    • Coarse 0.50 to 2.0 mm
  • Gravel 2.0 to 4.0 mm
  • Pebbles 4.0 to 64.0 mm
  • Cobbles 64.0 to 256.0
  • Boulders greater than 256.0 mm
  • Rock bedrock or compacted bottom

All seafloor classification information must be geo-referenced. This information (position and bottom characteristic) shall be captured by the acquisition system aboard the vessel or entered later in the main processing system.

Data Processing

Verify the position and bottom characteristics and verify that the sampling interval is maintained.

Bottom characteristics may be determined from the acoustic information recorded from single beam, multibeam and multi-transducer systems by analysing the backscatter strength. As a general rule hard surfaces such as rock and gravel provide high backscatter and sand or mud provide low backscatter. Areas attributed with similar backscatter strengths can be delineated. However, ground-truthing is required to assign seafloor classification. Correlation tables may also be used.

All seafloor classification information must be geo-referenced. This information (position and bottom characteristic) should be entered later in the main processing system, if not entered in the acquisition system in the vessel.

6.9 Shorelining

Acquisition

The shoreline shown on charts is referred to the Higher High Water Large Tides (HHWLT) or the Highest Astronomical Tide (HAT), which must be verified in the field. It is therefore necessary to delineate the HHWLT or the HAT using remote sensing and land survey techniques. In some cases it will only be necessary to do spot checks, but in low-lying areas where the coastal gradient is gentle, where flooding occurs or where tidal ranges are great, more data points will be required to properly position the HHWLT or HAT line.

The nature of the foreshore should also be recorded, as this information can be very useful for navigation.

Topographical maps, air photos, satellite images or other tools used to generate the shoreline on charts should be checked in the field and discrepancies resolved using land survey techniques as described above. New construction, landfill, accretion or erosion not found on existing plans and photos must be surveyed using land survey techniques. Navigable Waters Protection Act records should be referenced for proposed works. In the case of facilities, installations, etc. under construction, the names of the companies carrying out the work should be obtained as well as copies of the proposed work.

The drying line (defined by LLWLT, LAT, etc.) must also be well delineated. It is of the utmost importance that the drying line of the mainland, of islands and of all drying features, especially offshore features, is accurately surveyed. The limits of Canada’s internal waters, territorial sea, fishing zones and exclusive economic zones established from points based on this. According to the rules defined by UNCLOS, the limits listed above may be extended seaward if a drying rock or rock awash exists within the twelve nautical mile territorial sea.

Both the high shoreline line and the drying line must be positioned to the accuracy specified for the various survey orders found in Table 2 of the CHS Standards for Hydrographic Surveys using the methods and procedures outlined in the regional Hydrographic Survey QMS procedures.

Foreshore should be classified and the various limits geo-referenced.

Areas with structures in ruin should be delineated. If structures are no longer visible, it may be necessary to acquire additional bathymetry in order to validate the existence or the absence of ruined structures. Colour photographs (analogue or digital) or historical data should be consulted to obtain more information on those ruins.

Data Processing

Refer to the appropriate regional Hydrographic Survey QMS procedures and forms.

When processing the positions for both the HHWLT line and the drying line, verify that the accuracy specified for the survey has been met.

When multiple positions have been logged for point data, calculate an average position for the feature.

It may be necessary to apply transformations to digitized topographical maps, aerial photography, and satellite imagery to geo-reference the information they contain.

Foreshore should be classified and the various limits geo-referenced.

6.10 Revisory survey

Before updating of nautical publications surveys are required to acquire data describing additions, deletions and other changes that have taken place in an area. A revisory survey must incorporate all changes that may affect the safety of navigation.

6.10.1 Revisory work during a standard hydrographic survey

Acquisition

During the course of a standard hydrographic survey, it is imperative to verify the source information for the area being surveyed. Care must be exercised to verify the positions of existing charted information such as wharves, piers, shoals, rocks, dolphins, cribs, etc. The charted depths, especially shoals, wrecks, hazards and reported dangers including “Doubtful dangers” labeled ED-“existence doubtful”, PA-“position approximate”, PD-“position doubtful”, SD-“sounding doubtful”, Rep “reported danger”, etc., must be verified and their existence or non-existence confirmed.

If the new soundings do not agree with the charted depths within the tolerances indicated in the CHS Standards for Hydrographic Surveys, a thorough investigation will be required to confirm the discrepancies.

In critical areas such as harbours, berths, canals, dredged channels, etc., it may be necessary to make use of a multibeam or multi-transducer system (if not already used), assisted with a side scan sonar system in order to prove the existence or non-existence of the feature being investigated. Enough information must be obtained to confirm that the feature in question does not exist in its charted or reported position. If the feature is non-existent, a report should be made to the regional office, which may then provide additional information as to the source of the charted feature.

Conversely, all new dangers must be thoroughly checked using the steps outlined in these guidelines and reported to the regional office for immediate charting action. The navigation community must also be informed through Notice to Shipping or Notice to Mariners (see § 9.1 Reporting Dangers to Navigation).

The shoreline must be verified and all changes (landfill, construction, etc.) must be surveyed using standard land survey techniques if the information is not available from the latest remote sensing information or topographical maps. Photographs should be taken whenever possible.

Data Processing

Data processing is conducted as per a standard survey using the appropriate regional Hydrographic Survey QMS procedures and forms.

In some cases, it may be necessary to provide a written report to explain the changes found while conducting the hydrographic survey, especially if features no longer exist. These may be included in a hard copy document or in a digital form. Photographs should be filed with the Revisory reports to support any changes found.

6.10.2 Revisory work for Nautical Publication

A survey team may be required to perform a revisory survey of a specific area as requested by the chart production unit. The work is performed as a standard hydrographic survey except that sounding operations may be limited to areas critical to navigation such as harbours, wharves, berths, entrance channels, ranges, reported shoals, etc. Revisory surveys will also verify existing hazardous shoals and other dangers to navigation. The line spacing and methodology shall be chosen to satisfy the required survey order (Table 1 of the CHS Standards for Hydrographic Surveys). Photographs should be taken whenever possible.

Prior to leaving for the field, it will be necessary to obtain information specifying where changes are known or suspected to have taken place in order to establish priorities to the work to be done. Related source information will have to be studied and taken into the field if necessary. Lists detailing additional hydrographic data to be obtained, inquiries, etc. will also have to be obtained in order to assess the work to be done in the field.

This type of survey is prepared and executed as a standard survey, using the CHS Standards for Hydrographic Surveys, these guidelines and the regional Hydrographic Survey QMS procedures. Consult § 3.0 Survey Planning.

Data Processing

Data processing is conducted as per a standard survey using the appropriate regional Hydrographic Survey QMS procedures and forms.

In some cases it may be necessary to provide written report to explain the changes found while conducting the hydrographic survey, especially if features no longer exist. These may be included in a hard copy document or in a digital form. Photographs should be filed with the Revisory reports to support any changes found.

6.11 Sailing Directions

Acquisition

During the course of a hydrographic survey, it is imperative to verify the information contained in the Sailing Directions as applicable to the area being surveyed. Landmarks may have been removed or conspicuous new construction, such as communications towers, churches and tall buildings, may have been erected; shoals may have been removed; beacons may have become obscured by trees or destroyed; new harbour regulations may have come into effect; limits may have been altered, etc.

Surveys may find that certain areas within the Sailing Directions may be poorly described and require additional detailed information. In such a case, the information required covers a wide variety of subjects, among which are: descriptions of the coasts, harbours, dangers, landmarks, etc. as well as directions for entering harbours and for coastal navigation.

Special instructions may be given by the person responsible for the regional Sailing Directions. These may include a requirement to acquire oblique aerial photographs of specific areas and approaches.

Data Processing

Refer to regional QMS instructions for the procedures for submitting data to the regional Sailing Directions representative.

Ensure that any special instructions given by the person responsible for the regional Sailing Directions have been followed.

6.12 Chart nomenclature

Acquisition

In remote areas, it may be necessary to obtain the local names of the features that are significant for charts and Sailing Directions. In such instances, the source reference (local authority, etc.) should be recorded in case additional information is required.

Data Processing

All geographical names to be included on CHS documents and publications must be checked against the Geographical Names Board of Canada database or other provincial organisations such as the database maintained by the “Commission de toponymie” of the Quebec government.

It is possible to suggest names for unnamed entities. The methods and procedures required to undertake such an action are outlined in the document entitled Principles and Procedures for Geographical Naming 1999 published by the Canadian Permanent Committee on Geographical Names, Natural Resources Canada.

All suggestions are subject to approval by the Regional Director’s Office.

6.13 Use of photographs

Descriptive photographs are very useful for reporting and can be used for many tasks such as training, technical reference, public relations, etc. The use of video may also provide valuable information.

The photographs and videos taken must properly be identified, coded and stored in the hydrographic data centre (HDC) according to the regional Hydrographic Survey QMS procedures in use.

6.13.1 General use

Acquisition

Photographs should be taken of all horizontal and vertical control points occupied or installed during the survey. At least two photographs should be taken: one with a view of the general area and one showing the control point or benchmark with its immediate surroundings. The photographs should be taken from a horizontal viewpoint or a low level oblique.

Photographs or videos may be necessary to better describe features such as:

  • Navigation aids (single lights, range lights and markers, beacons, buoys, etc.),
  • signs signalling directions or giving navigational information,
  • conspicuous objects and special features such as wrecks, pilings, critical shoals, etc.,
  • revisory work (new construction, new features, changes, modifications, etc.). These can be submitted with the revisory report,
  • Installations of the positioning system, the tide gauge set-up etc.,
  • equipment in use during the survey (launches, instruments, equipment, office and processing facilities, etc.).

Data Processing

The photographs and videos taken must properly be identified, coded and stored in the hydrographic data centre according to the regional Hydrographic Survey QMS procedures in use.

6.13.2 Coastal photographs for Sailing Directions

Acquisition

Coastal photographs included in the Sailing Directions provide the mariner with a visual means to identify land features referred to in the text. Photographs most suitable for reproduction in the Sailing Directions need to be taken with a camera capable of producing sharp black and white prints having good contrast. They are usually taken from a ship or aircraft. It may be necessary to employ the services of an experienced photographer and may require the need of a helicopter.

Photographs of the following features are particularly useful:

  • salient points of the land, especially those which would be sighted first on approaching the coast, or which would be rounded on passage,
  • islands that lie near the usual routes followed and that are valuable aids for fixing position,
  • entrances to narrow straits and channels,
  • harbour entrances,
  • landing places, leading beacons, etc.,
  • conspicuous natural and cultural features.

In taking coastal photographs, the following general rules should be observed:

  • Photos should be taken from about the height of the bridge of ship, if possible, and looking forward or on the bow, in order to show the appearance of the land from the normal direction of approach. Photos taken on or abaft the beam (when a vessel is passing or has already passed the land) are seldom of much value. Care should be taken to avoid including parts of the ship’s structure such as the foremast.
  • If the land is low, say 60 meters or less above the water level, photos should be taken at a range of about 2 to 3 kilometres. At any greater distance, low land will not show up well.
  • If the land is of medium height, say 70 to 250 meters above the water level, photos can be taken at ranges of about 5 to 7 kilometres. If the land is high, say 300 meters and upwards, the range may be 9 kilometres or more.
  • In the case of entrances to straits, channels and harbours, it is always desirable to include both entrance points (both sides of the entrance) in the picture. If the angular distance between them is too great for inclusion in one photo, two or more photos, overlapping horizontally, should be taken from the same position so that the prints can subsequently be cut and joined to make a continuous panorama. Wide angle camera lens may be used.
  • Helicopter photos can be valuable provided they are taken form a moderate height, e.g. 15 to 50 meters, taking into consideration the height of land in the area and that the position from which they are taken can be determined with fair accuracy.
  • In all circumstances, the photos should be taken when weather is clear and that all features are optimally identifiable.

It is essential that the position from which each photo was taken and the direction in which the camera was pointed be known. This may be determined by taking a bearing and a distance to a distinctive object that must appear in the photo. The bearing to roughly one degree and the distance to about 0.5 kilometres (0.25 nm) should be adequate. However, depending upon the circumstances and the distance away, these rough tolerances may be exceeded. All the information should be recorded immediately after a photo is taken.

Data Processing

A brief report should accompany the photographs taken specifically for Sailing Directions. The report should include:

  • the reference numbers of the photos (corresponding to those on the prints),
  • dates and times when photos were taken,
  • positions of photo stations relative to land features shown,
  • indication of the area shown in each photo depicted on a chart or topographic map,
  • name of the ship,
  • other details, e.g. taken from helicopter, part of panorama, etc.

The photographs and videos taken must properly be identified, coded and stored in the hydrographic data centre according to the regional Hydrographic Survey QMS procedures in use.

6.14 Current measurements

Acquisition

Current information is required mainly as an aid to navigation especially in entrances to harbours, in narrow or shallow channels, in anchorage areas and in the vicinity of wharves, piers or other areas where the margin for navigational error is small and where current velocities appear to exceed 0.5 knots.

Specific and detailed Fields Survey Instructions should be obtained prior to leaving for the survey area. The instructions should give the location where measurements are required, the instruments to be used and the length of time that data should be recorded. The time required to perform this work must be taken into account when establishing timetables for the total tasks to be done during survey operations.

Whenever possible, the following guidelines should be followed for the collection of tidal stream and current information:

  1. Current observations should be made using a recording device.
  2. Observations should be measured at depths between 0 and 10 m below the surface.
  3. Simultaneous observations of water level height and meteorological conditions should be made. In the case of non-tidal waters, simultaneous observations of river discharge should be obtained.
  4. The period of observation in tidal waters should normally extend over a period of at least 29 days, at intervals not greater than 1 hour. In the case of non-tidal waters, the observations should extend over the range of factors causing the currents (i.e. river discharge, etc.).
  5. The speed and direction of the tidal stream should be measured to 0.1 knot and the nearest 10° respectively, at 95% confidence level.
  6. Where there is reason to believe that seasonal river discharge influences the tidal streams, measurements should be made to cover the entire period of variability.

In other cases, it may be enough to get instantaneous readings from a portable current meter or other methods to get an idea of the current, which might affect low powered vessels and other small craft.

Instructions for the measurement of tidal streams and currents are given in Chapter 8 - Current Measurement of the Canadian Tidal Manual.

It should be noted that it will not always be possible to follow all of these guidelines completely and survey parties are encouraged to collect any information possible on the direction and speed of tidal streams and currents.

Data Processing

The current data must be processed according to the regional Hydrographic Survey QMS procedures in use.

A brief report should accompany the current data. This report should detail the instruments and the methodology used to measure the currents, as well as the calibrations and precisions records. The position of every current measurement must be recorded precisely, as well as the date / time, depths, speed and direction of each measurement. The depths must be reduced to chart datum. Any pertinent information that may help in the interpretation of the data must be noted in that report.

6.15 Field checking (before leaving the field)

If processing cannot be conducted during the survey, certain steps should be taken to ensure that post processing may take place off site and that the survey coverage and the data quality respect the survey specifications as defined in the Field Survey Instructions.

6.15.1 Bathymetry

  • Ensure that all sensors aboard the sounding platform have been adequately calibrated to meet the survey’s specifications.
  • Ensure that the sound speed measurements casts and bar checks are performed within the areas surveyed and correlate with the times of data acquisition. Positions of the sound speed measurements and bar check sites should be recorded and filed. Ensure that the recorded values are valuable and not corrupted.
  • Review sounding pattern for adequate coverage.
  • Verify that check lines have been run.
  • Verify that the shoals, obstacles or hazardous areas have been properly investigated.
  • Randomly select subsets of data while collecting to verify consistency and quality of data.
  • Ensure that water level and current information has been gathered and is available.
  • Create a DTM of sounding data if possible and monitor anomalies.

6.15.2 Other Data

  • Ensure position files may be read and contain the required datagrams.
  • Ensure good quality of all the data collected.
  • Ensure all data required for forms, such as FAT forms are available or have been collected.
  • Ensure adequate positioning has been done to geo-reference digitized sources.
  • Ensure that photographs taken for Sailing Directions meet the required needs as per § 6.13 Use of Photographs

6.16 Other computations

All computations must be done with care and attention by a qualified hydrographer. In some cases (newly established horizontal control points, new benchmarks, temporary water level gauge installation, etc.) the computations should be checked and approved before sounding operations begin.

All comments, annotations, explanations, etc. recorded while performing revisory survey work for charts or sailing directions should be reviewed as soon as the fieldwork is completed and while the information is still fresh in the mind of the hydrographers that did the work. All annotations, comments, etc. pertaining to changes concerning the chart being revised should be included in a separate report.

Descriptions of horizontal and vertical control points or any other sketches drawn during field operations should also be reviewed as soon as possible after the data have been collected. All photographs taken during such operations should be developed or downloaded, identified and labelled as soon as possible.

The results of the various calculations and the various original observation forms should be included in a computation file or notebook and be readily accessible for consultation. All pertinent information must be clearly documented and classified.

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