This dataset integrates several sources of data describing the historical features of South Bay tidal marshes. The primary source is the maps of the United States Coast Survey (USCS; later US Coast and Geodetic Survey), a federal agency renowned for the accuracy and detail of its 19th-century maps of America's shoreline. In most parts of the country, these maps provide the best early pictures of coastal and estuarine habitats prior to substantial Euro-American modification.
Nine Coast Survey maps (T-sheets) were individually georectified and vectorized. The georectification included transforming historical latitude/longitude markings on the original map to the modern coordinate grid and, subsequently, a locally-derived adjustment to maximize correspondence between persistent physiographic features such as "ghost" channels in diked baylands, road intersections, and hilltops. Marsh features, in particular tidal channels and ponds, were vectorized using pattern recognition software and careful on-screen digitizing by GIS technicians. The T-sheets were then appended into a single polyline coverage.
This coverage also incorporates several additional sources to those described above -- including one additional Coast Survey map (T 2312) and 1939 aerial photography -- to fill gaps in either the spatial coverage or the level of detail in the T-sheets. These sources and the estimated associated certainty levels are recorded in the coverage attributes. Three distinct kinds of certainty are recorded: the certainty of our interpretation of the feature, its size/shape, and its location. The coverage attributes also record the original source, original surveyor, year of survey or flight, the GIS layer author, and the methods used.
As with any map or GIS layer, confidence or certainty varies geographically due to differences in source data or methods. This coverage provides direct information about the certainty level of different areas and features to allow the user to intelligently assess the applicability of the data for the chosen technical question.
The dataset provides a voluminous set of information about early geomorphic and ecological characteristics of South Bay tidal marshlands. Based upon our research on early surveyors and map accuracy, most of this dataset represents a consistent level of detail and accuracy. However, there are areas of less detail that should be recognized, particularly towards the upland edge of the extreme South Bay, and in several other specific areas. Also because several later sources had to be used to fill in these gaps, the extent of anthropogenic influence increases in these places, resulting in some modified channels and associated features such as landings and ditches. Technical users are strongly encouraged to read the process documentation to gain a stronger understanding of the strengths and limitations of the dataset.
