Swath Bathymetry Surveys of the Monterey Bay Area from Point Año Nuevo to Moss Landing, San Mateo, Santa Cruz, and Monterey Counties, California

Bathymetric Uncertainty Estimation

Methods

Standard deviation values were exported from CARIS as a 2 m BASE surface and were gridded in ArcGIS using the same procedures as for the bathymetry grid. Standard-deviation values were multiplied by two and normalized by water depth. Figure 6 shows 2-sigma depth-normalized standard deviation for the project area. The mean standard deviation of bathymetric soundings in the 2-m cells of the Monterey Bay data set was 0.23 m (1 sigma). Areas of high standard deviation are primarily associated with nearshore rock outcrops (figure 7), steep slopes, and areas near the range limit of the sonar, particularly in Soquel and Monterey Canyons (figure 8 and figure 9), as well as one area west of Soquel Canyon where survey-line spacing approached the sonar range limit.

Along-track artifacts are noticeable throughout most of the survey area, particularly on the smooth shelf bathymetry. The artifacts appear as "ridges" in the center of tracklines and are on the order of 0.05% of water depth higher than neighboring bathymetry. The along-track artifacts appear to be a flaw in the sonar system itself.

Five tie-lines were run perpendicular to trackline data for the purposes of uncertainty analysis. The 2-sigma uncertainty across the alongtrack artifacts is about 1% of water depth (figure 10). Mean depth uncertainty for all tie-line to track data is 0.4% of water depth, with 2-sigma uncertainty of 1.4% of water depth. Uncertainty increases substantially over rough terrain, such as nearshore rocks (figure 11).

Comparison with IHO standards

Depth-normalized 2 sigma standard deviation was compared with International Hydrographic Organization (IHO) minimum standards for hydrographic surveys to evaluate the performance of survey equipment and methodology. The IHO defines several survey orders based on a combination of coverage, depth, and accuracy (International Hydrographic Organization, 2008). For the two highest order surveys (Special Order and Order 1a), full bottom search is required, which means that nadir gaps must be filled by adjacent survey lines. This was not an objective of the Monterey Bay survey; however comparisons with Special Order survey accuracy requirements are included for completeness.

The IHO defines maximum allowable total vertical uncertainty (TVU) by using a depth-dependent formula with two additional variables as defined below (from IHO Standards for Hydrographic Surveys 5th Edition):

IHO standards require a 95 percent confidence level, defined as 1.96 x standard deviation, which essentially is identical to the 2-sigma standard deviation (standard deviation x 2) calculated for this survey. IHO survey orders use the following values as minimums for their survey orders (note that descriptions are generalized and Order 1 and 1a are combined because they have the same values for minimum TVU):

A comparison of the Monterey Bay survey with IHO standards can be seen in figure 12. As was true with standard deviation values, the main areas where TVU fell outside of IHO standards were areas where the surface elevations varied significantly within a single grid cell. Figure 13 and figure 14 show comparison of standard deviation values with IHO standards in shoals near Point Año Nuevo and near the head of Monterey Canyon, respectively.

For more information, contact David Finlayson.