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process of digitization involves using a computer-aided mouse to draw drainage divides on a scanned map and to trace along lines to determine their length.

Geospatial input to watershed models can be described with vector data.

Points or nodes can be used to represent outlet points, arcs (polylines) can be used to represent streams, and enclosed polygons used to represent the watershed. This is usually done by digitizing the streams and an approximate boundary for a watershed with an image of the site in the background. In this study a digital raster graphic, or DRG, is used as the basis for digitizing as shown in Figure 3.7.

This

is

a

scanned

image

of

a

USGS

topographic

map

produced

at

a

1:24,000

scale, obtained by the USGS (Appendix A). revise other digital data. Once the USGS

DRGs are frequently used to edit and map is scanned, the digital image is

georeferenced to the true ground coordinates and projected into the Transverse Mercator (UTM) for projection consistency (USGS, 1999).

Universal

35

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