This GIS layer provides information about the timing of the conversion of South Bay tidal marshes to salt ponds. Individual present-day salt ponds were researched to determine the period when the pond was converted. Salt ponds were assigned one of four periods of conversion, based on available source materials:
1857-1896
1897-1931
1932-1951
1952-1960
By comparing different map/photo sources, the general era of conversion to a salt pond could be established. For example, USCS maps from 1897 could be compared to USCS maps from a later date to quickly establish an initial range of dates. If an area was mapped as tidal marsh in 1897 by the USCS, but was diked by 1931, then the conversion occurred between those two dates. Use of these sources helped establish a date that all or most of the pond was converted. Since some levee patterns have changed over time, some lumping of smaller ponds was necessary. Establishment of later salt ponds could often be verified by aerial photography.
Sources used in research include the maps of the United States Coast Survey (USCS; later US Coast and Geodetic Survey); historical USGS quadrangles; and early aerial and oblique aerial photography. The USCS maps and the aerial photography were found to be the most useful; USGS quadrangles often depicted data derived from earlier USCS maps and thus were more difficult to use to assign a specific date of origin.