Summary

Description

Bathymetry for St. Helena Sound was derived from nineteen surveys containing83,246 soundings. No surveys were omitted. The average separationbetween soundings was 49 meters. Sixteen of the nineteen surveys useddated from 1934. The remaining surveys, located in the southwest, dated from1956 and 1973. The total range of sounding data was 3.7 meters to -23.5meters at mean low water. Mean high water values between 1.5 and 2.1meters were assigned to the shoreline. Six points were found that were notconsistent with the surrounding data. These were removed prior to tinning.DEM grid values outside the shoreline (on land) were assigned nullvalues (-32676).St. Helena Sound has eleven 7.5 minute DEMs and a single one degreeDEM. The 1 degree DEMs were generated from the higher resolution 7.5minute DEMs which covered the estuary. A Digital Elevation Model(DEM) contains a series of elevations ordered from south to northwith the order of the columns from west to east. The DEM isformatted as one ASCII header record (A- record), followed by aseries of profile records (B- records) each of which include a shortB-record header followed by a series of ASCII integer elevations(typically in units of 1 centimeter) per each profile. The lastphysical record of the DEM is an accuracy record (C-record).The 7.5-minute DEM (30- by 30-m data spacing) is cast on theUniversal Transverse Mercator (UTM) projection. It provides coveragein 7.5- by 7.5-minute blocks. Each product provides the samecoverage as a standard USGS 7.5-minute quadrangle but the DEMcontains over edge data. Coverage is available for many estuaries ofthe contiguous United States but is not complete.