Summary

Description

Bathymetry for Galveston Bay was derived from thirty-three surveyscontaining 209,359 soundings. Twelve overlapping, older, lessaccurate surveys were omitted, and overlap from eight older, lessaccurate surveys was omitted. The average separation between soundingswas 84 meters. The thirty-three surveys used dated from 1931 to 1983with the oldest surveys in the northern and southernmost portionsof the bay. The majority of the surveys were from the early 1960’s. Thesoundings ranged from 1.2 meters to -16.5 meters at mean low water. Meanhigh water values of 0.3 or 0.4 meters were assigned to the shoreline.Twelve points were found that were not consistent with the surrounding points.These were removed prior to tinning. DEM grid values outside theshoreline (on land) were assigned null values (-32676).Galveston Bay has twenty-nine 7.5 minute DEMs and two one degreeDEMs. 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.