Digital Shorelines for Boston Harbor
A selection of existing low and medium-resolution shoreline files is displayed in figure 2 (scales 1:2,000,000 to 1:80,000). Medium and high-resolution shoreline files are shown in figure 3 (scales 1:80,000 to 1:10,000). NOAA«s Medium Resolution Digital Vector Shoreline (NMRDVS) is common to both. Whereas the NOAA shoreline has an average mapping scale of approximately 1:70,000, the chart used for Boston Harbor (Massachusetts Bay, #13267) is 1:80,000.
Figures 2 and 3 display shoreline files superimposed upon a mosaic created from a set of orthophotographs based on recent aerial photographic coverage of the greater Boston area (MIT and MassGIS, 1996). The differences between NMRDVS, the MCOAST (1:24,000) , and the new BHS file (1:25,000,1:10,000) emerge at larger scale, depicted in the inset maps (Figs. 4-9 and legend). MCOAST is an earlier high-resolution shoreline file as described in section V.. There is often a systematic deviation between the MCOAST and the other lines. Some differences are clearly due to changes in the harbor since the data in the original topographic maps were compiled. Others may originate in differing tidal levels, or processing. In the ensuing discussion we will limit ourselves mainly to the more recent shorelines, i.e. NMRDVS, and BHS. In general, all three files tend to show a systematic westward offset with a mean of about 30 m from the orthophotos. We have not yet identified the source or sources of this offset.
To Download a copy of the new BHS file, click here. (available in MapInfo Interchange and ARC/INFO Ungenerate formats).
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