Standards for Hydrographic Surveys

1 Classification of Bathymetry

To accommodate different accuracy requirements for areas to be surveyed and to classify old surveys, six orders of survey are defined. These are described below and in Table 1 and summarize the overall accuracy requirements.

Unlike IHO S44, the 100% bottom search is not compulsory. In the CHS, it is strongly recommended to obtain 100% bottom search in critical areas, but in certain circumstances (client need, costs, time, etc.), it may not be achievable.

One other major difference with the IHO S44 is the way CHS classify surveys. The classification is divided into four components: the horizontal accuracy, the vertical accuracy, the target detection capability and the type of coverage. For instance, a survey can attain an horizontal accuracy of Special Order, a vertical accuracy and a feature detection of Order 1a and the type of coverage could be 1a (complete coverage).

Table 1 Standards for Hydrographic Surveys

ORDER Exclusive Special 1a 1b 2 3
(Imprecise)
Examples of Typical areas Shallow water in Harbours, berthing areas, and associated critical channels with minimum under-keel clearances or engineering surveys Harbours, berthing areas, and associated critical channels with minimum under-keel clearances Areas shallower than 100 metres where under-keel clearance is less critical but features of concern to surface shipping may exist. Areas shallower than 100 metres where under-keel clearance is not considered to be an issue for the type of surface shipping expected to transit the area. Areas generally deeper than 100 metres where a general description of the sea floor is considered adequate. All areas where the accuracies do not meet the requirements of the previous orders
H Horizontal Accuracy (95% Confidence Level) 1m 2m 5m + 5% of depth 5m + 5% of depth 20m + 10% of depth > 20m + 10% of depth
V Depth Accuracy for Reduced Depths (95% Confidence Level) Footnote 1 a = 0.15m
b = 0.0075
a = 0.25m
b = 0.0075
a = 0.5m
b = 0.013
a = 0.5m
b = 0.013
a = 1.0m
b = 0.023
Same as order Footnote 2
D System Detection Capability Features > 0.5m cubed Features > 1m cubed Features > 2m cubed in depths up to 40 m; 10% of depth beyond 40m Footnote 3 N/A N/A N/A
C Type of coverage (M270)
1. complete coverage (multibeam, multi-transducer, acoustically swept);
2. systematic survey (single-beam echo sounder lines run parallel at pre-planned line spacing, LiDAR);
3. sparse coverage (lead-line surveys, reconnaissance, track soundings, spot soundings);
4. unsurveyed

Guidelines for single beam and punctual surveys

SBES Maximum Line Spacing Footnote 4 The lesser of: 3x average depth or 25m in depths to 10m; or 50m in depth of 10-40m; or 100m in depths deeper than 40m. Closer line spacing may be required in doubtful areas. The lesser of: 3x average depth or 200m.For Bathymetric LiDAR a spot spacing of 5x5 metres or less. The lesser of: 3x average depth or 1000m. N/A

The rows of Table 1 are explained as follows:

  • Row 1 "Examples of Typical Areas" gives examples of areas to which an order of survey might typically be applied.
  • Row 2 "Horizontal Accuracy" lists minimum positioning accuracy for each depth sounding to be achieved to meet each order of survey.
  • Row 3 "Depth Accuracy" specifies parameters to be used to calculate minimum accuracy of reduced depths to be achieved to meet each order of survey.
  • Row 4 "System Detection Capability" specifies the detection capabilities of systems used when 100% bottom search is required.
  • Row 5 “Coverage type” specifies the seabed coverage bases on the system and the methodology used to achieve a survey.
  • Row 6 "Maximum Line Spacing" is to be interpreted as:
    • Spacing of sounding lines for single beam sounders and spot soundings surveys.

This table gives the different accuracy requirements for different areas to be surveyed according to a specific order of precision. The highest accuracy requirements are found in Exclusive order while the least order of precision is given in Order 4.

Feature detection implies that the bottom will be completely insonified for the width of the multibeam or the multitransducer array and that there will be no gaps (areas of no insonification) between sounding lines. When the feature to be detected is smaller, 200% coverage is recommended. It implies that the surface insonified by the multibeam or multitransducer will be covered at least twice from a minimum of two separate passes or swaths.

This, however, does not necessarily mean that all targets will have been detected. The detection of targets as described in the various survey orders will depend on the following factors:

  • the speed of the sounding platform
  • the depth of water (multitransducer systems will not give full bottom coverage in shallow waters)
  • the stability in maintaining strait line navigation
  • beam angle
  • beam width
  • ping rate

Even though an echo sounding system may be capable of detecting target features as defined in Table 1 System Detection Capability, efforts will have to be taken to ensure that all the cubic features are found when conducting a full bottom search.

1.1 Exclusive Order

Exclusive Order hydrographic surveys are based on the IHO Special Order with higher accuracy and their use is intended to be restricted to shallow water areas (harbours, berthing areas and critical channels) where there is an optimal use of the water column and where specific critical areas with minimum under-keel clearance and bottom characteristics are potentially hazardous to vessels. This order also applies to high precision engineering surveys. All error sources must be minimized. Exclusive Order requires very precise positioning systems, closely spaced lines (when target detection is required) and a rigorous control on all aspects of the surveys.

The use of side scan sonar or multi transducer arrays or high-resolution multibeam echo sounders is required to detect the feature size to be detected. In required areas, appropriate sounding equipment and methodologies must be employed in order to ensure that all features greater than 0.5m cubed are detected. The use of side scan sonar in conjunction with multibeam or multi-transducer echo sounders may be necessary in areas where pinnacles and dangerous obstacles may be encountered.

1.2 Special Order

Special Order hydrographic surveys are intended to be restricted to specific critical areas with minimum under-keel clearance and where bottom characteristics are potentially hazardous to vessels. These areas have to be explicitly designated by the agency responsible for survey quality (harbours, berthing areas, and associated critical channels. All error sources must be minimized. Special Order requires the use of closely spaced lines (when target detection is required).

The use of side scan sonar, or multi transducer arrays, or high-resolution multibeam echo sounders is required to detect the feature size to be detected. In required areas, appropriate sounding equipment and methodologies must be employed in order to ensure that all features greater than 1m cubed are detected. The use of side scan sonar in conjunction with multibeam or multi-transducer echo sounders may be necessary in areas where pinnacles and dangerous obstacles may be encountered.

1.3 Order 1

Order 1 hydrographic surveys are intended for harbours, harbour approach channels, recommended tracks, inland navigation channels, and coastal areas of high commercial traffic density where under-keel clearance is less critical and the properties of the seafloor are less hazardous to vessels (e.g. soft silt or sand bottom). Order 1 surveys shall be limited to areas with less than 100 m water depth. Although the requirement for seafloor search is less stringent than for Exclusive Order and Special Order, full bottom search may be required in selected areas where the bottom characteristics and the risk of obstructions are potentially hazardous to vessels. In required areas, appropriate sounding equipment and methodologies must be employed in order to ensure that all features greater than 2m cubed in water depths up to 40m, or features representing 10% or more of the depth in areas deeper than 40m are detected.

1.4 Order 2

Order 2 hydrographic surveys are intended for areas with depths less than 200 m not covered by Exclusive Order, Special Order and Order 1. These are areas where a general description of the bathymetry is sufficient to ensure there are no obstructions on the seafloor that will endanger the type of vessel expected to transit or work the area. Full bottom search may be required in selected areas where the bottom characteristics and the risk of obstructions may be potentially hazardous to vessels.

1.5 Order 3

Order 3 hydrographic surveys are intended for all areas not covered by Exclusive Order, Special Order, and Orders 1 and 2 in water depths in excess of 200 m.

1.6 Imprecise order

This order is intended for the classification of old imprecise surveys. It must not be used to determine the precision of a new survey.

1.7 Order classification

The choice of technology is normally made to meet a specific order. The classification of the bathymetry is applied to a set of data and may include as many systems and/or survey vessels. The order is determined by the worst horizontal and vertical accuracy found in the data of a dataset. It is recommended to separate the data with the same type of coverage to help the identification of the survey type. It is strongly recommended to classify surveys by statistical method after completion of these surveys. If the use of statistical method is not feasible or practical, the survey techniques shall be rigorously controlled to ensure the best results.

Notes:

  • For Exclusive Order, Special Order and Order 1 surveys the Project Manager may define a depth limit beyond which a detailed investigation of the seafloor is not required for safety of navigation purposes in the surveyed areas.
  • Side scan sonar shall not be used for depth determination but to define areas requiring more detailed and accurate investigation.
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