It is estimated that the accuracy of the Bathymetric DEMs is 2% of depth or 1 meter for depths grater than 20 meters and 2 % of depth or 0.20 meters for depths shallower than 20 meters.
THESE DEMs SHOULD NOT BE USED FOR NAVIGATION.
There are three types of DEM vertical errors: blunder, systematic, and random. These errors are reduced in magnitude by editing but cannot be completely eliminated. Blunders are errors of major proportions and are easily identified and removed during interactive editing. Systematic errors follow some fixed pattern and are introduced by data collection procedures and systems. Systematic error artifacts include vertical unsampled elevation shifts, relative spacing of the source soundings, misinterpretation of terrain surface caused by softness or poor reflectivity and by the resolution of the collected soundings (feet, feet & fractions, fathoms, fathoms & fractions, meters, tenths of meters etc.). Random errors result from unknown or accidental causes. The 1 degree (DSQ) DEMs are generated from 30 m grids on UTM projection. The rms difference between these surfaces is an estimate of the vertical accuracy of the DSQ DEMs.
Null values (encoded -32767 within the DEMs) are used to represent all locations either above (outside) the local high water datum (the shoreline as defined by the National Ocean Service) or outside the extents of the estuary as delineated by the CAF.
Bathymetric elevations within these data sets are referenced to the local tidal datum which typically is Mean Lowest Low Water (MLLW) averaged over a 19 year tidal epoch. Elevations above this datum (between the datum and the shoreline) have positive values (meters to centimeter resolution) while those below are negative. Note that this datum is different from that used by USGS for land elevation data that it distributes in DEM form which are referenced to Mean Sea Level (MSL).