Hydrographic Survey Management Guidelines

7 Quality control

For all aspects of a hydrographic survey, checking procedures (checklists, etc.) must be in place to monitor that the data is being recorded or calculated correctly and that the manual inputs (system configuration values, navigational system data changes, etc.) are entered correctly. The guidelines outlined in the CHS Standards for Hydrographic Surveys, § 7 - Quality Control must be followed. Checks and verifications should be done following the regional Hydrographic Survey QMS procedures in place.

All adjustments, changes or alterations made to the various recording instruments must be documented and approved.

Efforts should be made so that all survey parties use the same version of the software. During the course of a hydrographic survey, changes or modifications to the various software packages used to process field data must not be made without approval from the regional office.

7.1 Bathymetry

The validity of soundings should be verified by an independent qualified hydrographer other than the data processor. The HIC must approve the list of shoal to be examined and where additional hydrography is required. He/She will have the final decision regarding agreement between soundings obtained from regular sounding lines, check lines, shoal exams and other verifications. The depth accuracies for the various survey orders are found in Table 1 of the CHS Standards for Hydrographic Surveys.

The final digital files and, if required, the field sheets must be thoroughly checked before being submitted.

7.1.1 Overlap of surveys

It is the responsibility of the HIC to compare the bathymetry obtained during the present survey against the bathymetry obtained from previous surveys. It is also the responsibility of the HIC to ensure that all contiguous datasets overlap in order to facilitate the verification of survey accuracies and ensure continuity of survey coverage.

A surveyed area can be divided into separate datasets. In most cases, the limits of the datasets to be completed are such that the limits of the new datasets are contiguous to the limits of previously completed field sheets or datasets. In such a case, the datasets produced during the same field season, using the same methods and equipment, and by the same vessels and HIC, do not require any overlap. But datasets where one or more of the limits are the same as the limits of previously completed datasets must have sufficient overlap to verify the accuracy of the new data and ensure that it compares well with the previous survey.

In the digital world, the bathymetric data can be shown as a series of data sets, each having its own set of limits within one large data set, showing where the present survey data ends. The new limits may or may not coincide with the limits of a previously surveyed area. Either way, as is the case with field sheets, the new data coverage must be such that there is a certain overlap in order to verify the accuracy of the new data and ensure that it compares well with the previous survey.

As a general rule, the overlap with previous field sheets or data sets shall be sufficient to ensure that the hydrography shown in the overlap area is in agreement with the previously obtained data. The amount of overlap will depend on the purposes (scale and density) for which the current data and previous data were collected and may vary from as little as 50 metres to as much as 2 Kms. The overlap section should also show the shoreline and foreshore details as well as the contour lines.

The depths should be within the accuracies stated in the Table 1 of the CHS Standards for Hydrographic Surveys. If depths in the overlap area do not appear to agree with the depths of the previous surveys, the results obtained from the survey underway must be thoroughly checked. If the disagreement still cannot be resolved, then check lines must be run across the previous survey area to determine the extent of disagreement. Any discrepancies in the overlap areas must be fully resolved before the datasets are certified to be correct and approved for use in chart production.

7.2 Other computations

All computations should be independently verified by a qualified hydrographer. Once completed, there should be a clear indication that the computations have in fact been verified as correct and that the results are acceptable for use.