A continuous surface representation (surface grid) of each bathymetric survey was created using the Arc/Info Topogrid routine. Input data was a combination of point soundings and hand-drawn depth contours (see table below). Each historical bathymetric surface is defined by more than 300,000 grid cells (each cell is a 50-m square). Bathymetric change grids were generated by differencing bathymetric grids and applying vertical and horizontal corrections to bring surveys to a common datum. Creation of accurate grids involved several steps.

Example from 1887 survey

Starting with mylar copies of the original 1:20,000 scale NOS Hydrographic Sheets, NOS contours were checked for accuracy and additional contours added in areas where point soundings were sparse. These annotated H-Sheets were scanned at 300 dpi and registered using Latitude/Longitude graticules and hard shoreline features in the National Wetlands Inventory. Point soundings and depth contours were digitized from the registered image. The existing NOAA/NOS database of soundings, supplemented by digitized depth contours, were used for the 1951 and 1983 surveys. Input data were gridded and the grids compared to the input data to check for problem areas. The final step was regridding after adding point soundings and contours to force grids to accurately represent historical bathymetry.

The focus of this analysis was bathymetric change, not shoreline change. In most cases the H-Sheets have incomplete shoreline and marsh front information and were supplemented with shorelines from scanned, larger scale (1:40,000) navigation charts. Portions of the 1887 and 1898 shorelines are a composite of 1856 and 1922 data. In the future we plan to improve these estimates using shore surveys from USCGS Topographic.