Bathymetry for Massachusetts Bay was derived from thirty-eight surveys containing 297,628 soundings. Two older, less accurate, overlapping surveys were omitted before tinning. The average separation between soundings was 57 meters. The thirty-eight surveys used dated from 1940 to 1970. The total range of soundings was 3.0 meters to -71.0 meters at mean low water. Mean high water values between 2.7 and 2.9 meters were assigned to the shoreline. Fifty-six points, predominantly from two surveys, were found that were not consistent with the surrounding data. These were removed prior to tinning. DEM grid values outside the shoreline (on land) were assigned null values (-32676). Massachusetts Bay has fifteen 7.5 minute DEMs and two one degree DEMs. The 1 degree DEMs were generated from the higher resolution 7.5 minute DEMs which covered the estuary. A Digital Elevation Model (DEM) contains a series of elevations ordered from south to north with the order of the columns from west to east. The DEM is formatted as one ASCII header record (A- record), followed by a series of profile records (B- records) each of which include a short B-record header followed by a series of ASCII integer elevations (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 the Universal Transverse Mercator (UTM) projection. It provides coverage in 7.5- by 7.5-minute blocks. Each product provides the same coverage as a standard USGS 7.5-minute quadrangle but the DEM contains over edge data. Coverage is available for many estuaries of the contiguous United States but is not complete.