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San Francisco Bay Bathymetry

The process of collecting and correcting bathymetric soundings in tidal waters is complex and beyond the scope of this web site. For more details on hydrographic surveys visit the NOAA web site at: http://chartmaker.ncd.noaa.gov/HSD/H-Manual.html.

In the example sequence given below, the first step in the preparation of regular grids displayed on this web site begins with irregular hydrographic survey data (soundings) that have been corrected to a common datum (1). NOAA is the primary resource for obtaining these original soundings, although other agencies--including COE, CA-DWR, USBR, and USGS--have contributed local studies. Once the soundings are in hand they are contoured, and shoreline and marsh perimeters are added and combined into a GIS (2). All data layers must be adjusted to a common horizontal and vertical datum and all depths must have the same orientation and units. At this point a grid can be generated.

Quality control is an iterative process, performed on the resulting grid by comparing it with the original soundings (3, 4). Errors are computed, plotted and repaired when appropriate. Errors are usually a result of incorrect unit tags on the source data or digitizing mistakes, but some are due to gradients in bathymetry that cannot be resolved by a single grid cell. The final grid (5) can be adjusted to a different tidal datum using an adjustment grid. This grid is produced by assigning tide levels observed at shore stations to co-tidal lines from the TRIM-2D model (6, 7).


Obtain the most recent published depth soundings and shoreline data (see our Related Links list)

Hydrographic survey data


Plot soundings and contour soundings. The South Bay figure here contains over 100,000 points and depth-colored contours.

Depth-colored contours


On closer examination we can see the individual points and identify inconsistencies. This figure shows depth contours with colored, labeled soundings.

Depth contours with colored, labeled soundings


Check for errors. Here is an example where incorrect depth codes were assigned to shallow water soundings. A depth code for meters was assigned, the soundings were actually decimeters.

Comparing depth contours with the original soundings


Final grid produced using the ArcInfo command Topogrid.

Final grid produced using the ArcInfo


Datum adjustments for MSL, MHHW, NGVD. Since the adjustment for various tidal and non-tidal datums is not constant or is a simple function of distance, we use cotidal contours developed by the TRIM model. In this figure the cotidal contours are shown.

Cotidal contours


Based on the tide records at stations around the perimeter of the Bay we can assign adjustment values to these cotidal contours and produce an adjustment grid. The example given here is the adjustment grid for MLLW to MHHW.

MHHW Correction

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