Summary

Description

Bathymetry for Chesapeake Bay was derived from two hundred ninety-sevensurveys containing 3,178,509 soundings. Thirty-five older, lessaccurate, overlapping surveys were entirely omitted before tinning. Partialoverlap from other older, less accurate surveys was also omitted prior totinning. The surveys used dated from 1859 to 1993. Thirty-six surveys datedfrom 1859 to 1918, thirty-seven from the 1930s, ninety-one from the 1940s, sixty-six from the 1950s, twenty-five from the 1960s, twenty-four from the 1970s,fourteen from the 1980s, and four from the 1990s. The total range of soundingdata was 3.7 meters to -50.4 meters at mean low water. Mean high water valuesbetween 0.2 and 1.2 meters were assigned to the shoreline. Fifteen pointswere found that were not consistent with the surrounding data and were removedprior to tinning. DEM grid values outside the shoreline (on land) were assigned null values (-32676).Chesapeake Bay has two hundred eighteen 7.5 minute DEMs and ten onedegree DEMs. The 1 degree DEMs were generated from the higherresolution 7.5 minute DEMs which covered the estuary. A DigitalElevation Model (DEM) contains a series of elevations ordered fromsouth to north with the order of the columns from west to east. TheDEM is formatted as one ASCII header record (A- record), followed bya series of profile records (B- records) each of which include ashort B-record header followed by a series of ASCII integerelevations (typically in units of 1 centimeter) per each profile.The last physical 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.