Standards for Hydrographic Surveys

7 Quality Control

7.1 Introduction

To ensure that the required accuracy is achieved it is necessary to check and monitor performances. Establishing quality control procedures shall be a high priority. All related pertinent documentation should be preserved for further consultation.

7.2 Positioning

Quality control for positioning involves monitoring the proprietary hardware/software quality indicators for accuracy, precision, signal strength, signal to noise ratio, cycle tracking, solution type, etc. A position check by the survey vessel occupying or offset to a known ground position must be done at the start of a survey, periodically during, and at the end of the survey. Redundant lines of position or redundant satellites must always be observed. The use of a position monitor station to monitor position accuracy and system performance is desirable but often not practical. The use of two independent positioning systems along with ground position checks is also a very desirable situation.

7.3 Depths

A standard quality control procedure shall be to check the validity of soundings by conducting additional depth measurements. Differences shall be statistically tested to ensure compliance of the bathymetric data with the standards given in Table 1. Anomalous differences shall be further examined with a systematic analysis of contributing error sources. All discrepancies shall either be resolved by analysisor re-survey during progression of the survey task.

7.4 Check lines

Check lines crossing the regular lines shall always be done to confirm the accuracy of the positioning, the depth measurement and other depth corrections. They shall be run as close to perpendicular to the principal lines as possible. The differences between principal lines and check lines shall fall within the limits of the survey order. If possible, check lines shall be collected using an independent system, different survey vessel and/or time and on a rough bottom.

Check lines crossing the principal sounding lines shall always be run to confirm the accuracy of positioning, sounding, and depth corrections. Check lines shall be spaced so that an efficient and comprehensive control of the principal sounding lines can be done. As a guide, it may be assumed that the interval between check lines shall normally be no more than 15 times that of the principal sounding lines.

7.5 Sounding Density requirements

7.5.1 Single-beam Echo Sounders (SBES)

Depending on the characteristics of the seafloor the line spacing from Table 1 may have to be reduced or, if circumstances permit, expanded. Check lines shall be run at discrete intervals (see § 7.4 Check lines).

7.5.2 Side scan Sonar (SSS)

Where SSS is being used in conjunction with SBES or MBES, the line spacing from Table 1 may be increased, whilst ensuring adequate coverage of the area directly beneath the towfish.

7.5.3 Multibeam Echo sounders (MBES)

MBES have great potential for accurate seafloor coverage if used with proper survey and calibration procedures. An appropriate assessment of the accuracy of measurement with each beam is compulsory when full bottom coverage is required for use in areas surveyed to Exclusive Order, Special Order and Order 1 standards. If any of the outer beams have unacceptable errors, the related data are to be excluded or weighted accordingly. If not hampered by geographical constraints, all lines shall be crossed, at least once, by a check line to confirm the accuracy of positioning, depth measurement and depth corrections – squat, draft, tide, and sound speed. Accuracy’s can also be confirmed by redundant measurement on a small seafloor target.

7.5.4 Multitransducer systems (MTES)

Multitransducer (sweep) systems provide one technology for ensuring the accuracy while a full bottom coverage is required for Exclusive Order, Special Order and Order 1 standards. It is essential that the distance between individual transducers shall be matched to ensure a 100% bottom coverage. If not hampered by geographical constraints, all lines shall be crossed, at least once, by a check line to confirm the accuracy of positioning, depth measurement and depth corrections – squat, draft, tide, and sound speed. Accuracy’s can also be confirmed by redundant measurement on a small seafloor target.

7.5.5 Airborne systems

Airborne laser systems are capable of measuring depths to 50 m or more provided the water is clear. Hazards to navigation detected by airborne laser shall be examined using an independent method (see § 4.4.3 Shoal Examination). A check line to confirm the accuracy of positioning, depth measurement and depth corrections shall cross all lines, at least once.

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