Historical bathymetry soundings between 1916 and 1920 around the Mississippi and Alabama barrier islands

In order to obtain XYZ sounding data from historical maps, T-sheets and H-sheets were georectified using ArcMap 9.3 georeferencing tool. All five T-sheets were rectified using transformations North American Datum of 1927 (NAD27) supplied by the National Oceanic and Atmospheric Administration (NOAA) (Buster and Morton, 2011). Six H-sheets, which display soundings from the 1916-1920 time period, were georectified to the georeferenced T-sheets (Buster and Morton, 2011).

After georectifying the H-sheets, the soundings previously digitized by the NOAA contractors did not exactly lie on center of handwritten soundings on H-sheets (H04000, H04021, and H04171). These three sets of soundings were rubbersheeted within ArcMap (with greater than 25 control points) to their respective locations on the rectified H-sheets. Soundings on H-sheets H03960, H04020, and H04023 had not been digitized by NOAA and therefore all the soundings on these three H-sheets were digitized for this project. Both NOAA digitized soundings and soundings digitized for this project were initially entered in feet and then converted to meters.

The soundings depths were originally in the mean low water (MLW) datum. To adjust each value to reflect mean sea level trends, the NOAA tide gage at Pensacola was used as it is at a geologically stable open-ocean location and has a long historical record of tides and sea level trend values. The relative rise in sea level at Pensacola, FL (mean sea level trend) as checked in July 2017 was 2.31 millimeters (mm)/year (yr). To account for the historical rise in sea level, linear regression was applied to the tide-gage data from Pensacola to obtain water-level corrections for the bathymetric-survey period, using 1992 as the midpoint of the most recent 19-year tidal epoch. For each survey period, there is a range of years during which the bathymetric surveys were conducted (1916-1920). The water-level correction, which was averaged for the years included for the survey period was calculated at 0.171 mm/yr and was applied to each sounding value (column header includes the abbreviation for sea level (SL).

After the water-level adjustment was applied, the soundings were run through NOAA's datum transformation software Vdatum version 3.6. The data were transformed from a horizontal datum of NAD27 and vertical datum of mean low water (MLW) and derived products are relative to the UTM NAD83 horizontal datum and NAVD88 GEOID12B vertical datum.

Added keywords section with USGS persistent identifier as theme keyword.