Digital Shorelines for Boston Harbor
Computerized digital mapping of coastal waterways and their environmental conditions requires data files that define the coast and provide a frame of reference for data being mapped. We have reviewed existing digital shoreline files for Boston Harbor and created a new file derived from the nautical charts of Boston Harbor (NOS, 1991,1993 ). This work is in support of studies of chemical contaminants in bottom sediments, and contaminant transport in Boston Harbor and adjoining parts of Massachusetts Bay. For summaries of these investigations see Butman et al, 1992, Bothner et al, 1994).
The most common way to provide shoreline files is in vector format, i.e. sets of data points that define connected line segments. This might seem a relatively simple and straightforward matter, but increasing need for digital shoreline data has pointed up variables and problems in their acquisition and use (see Section II). Examples include the definition of the shoreline as the boundary between land and sea. Which sea level measurement does one choose, and with what tidal corrections? Should one use high water marks, mean sea level, or low-water based data, which will encompass features like sand bars and mudflats? Geographical Information System (GIS) and gridding and mapping systems generally require completed shoreline "polygons", i.e. the shoreline must form a closed loop. In the case of streams and canals, how far upstream should artificial closures be drawn? How much of man-made constructions like bridges, docks and jetties be shown?
Acknowledgemnts: Millington Lockwood, National Oceanic and Atmospheric Administration / National Ocean Survey (NOAA/NOS) gave permission to use a briefing report, and provided valuable supplementary information. Christian Jacqz, Data Center for the Executive Office of Environmental Affairs (MassGIS) transferred to us the Massachusetts digital shoreline derived from USGS topographic maps, and provided information on the recent cooperative orthophoto coverage of Boston Harbor. Norman Vine, of Woods Hole MA, cooperated in the earlier data transfers.
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