CASE II: AUTOMATED METHODS BY DIGITIZING
In Case II watershed boundaries are digitized using ArcView GIS 3.2 from digital raster maps (DRGs) assuming undeveloped conditions. With the DRG placed on the computer screen, new shapefiles are created using the digitizing capabilities of ArcView GIS 3.2 and ArcWorkstation.
A USGS digital raster graphic (DRG) was used as a background in
o30097d6.tif), produced by
DRGs used were the USGS in 1988.
The first step in digitizing is to create a new theme as selected from the ArcView menu. As discussed in Chapter 3, watersheds are delineated in the same fashion as with the paper maps, except with a computer-aided mouse. For each area a polygon was used to represent area, and a polyline to represent the longest flow path. Once the polyline was drawn (or polygon for area), ArcWorkstation was used to convert the new themes to coverages using the shapearc command. After the polylines and polygons were converted to coverages, they were processed using the ArcWorkstation commands build and clean. Once they were built and cleaned, they were opened once again in ArcView and converted back to shapefiles.
During the process of digitization, it became apparent that some areas were difficult to digitize because the contour lines could not easily be seen in areas with highway overpasses. In an effort to incorporate parameter variations that might be encountered due to highway construction, two delineations were