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Parameter Sensitivity in Hydrologic Modeling - page 136 / 163

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below the mean using traditional methods of parameter extraction. For Area 1 and Area 2 the differences between paper map methods and digital automated methods were not as apparent.

The reason for the large deviation from the mean using traditional methods in area, longest flow path, and perimeter measurements is most likely due to the error associated with 1) taping two topographic maps together to make a map large enough to cover the whole drainage area, 2) difficulty in determining flow path and drainage divides from the map contours and 3) the accuracy of manual measurement techniques using a map wheel and a planimeter.

6.1.2

On-Screen Digitizing of Raster Graphic Maps

The process of on-screen digitizing of raster graphic maps closely resembles paper map methods. The methodology is analogous, except that the process of on-screen digitizing of raster graphic maps involves using a computer- aided mouse and a scanned topographic map to draw in watershed boundaries and flow paths. Once the watershed boundaries and flow paths have been determined, a GIS (or similar system) computes drainage area, longest flow path length, and perimeter. Slope is calculated by taking the difference in elevation between map contours, as done with paper maps.

On-screen digitizing eliminates the error of taping maps together, and interpreting lengths and areas from hand-held instruments. As with paper maps,

determining flow paths and drainage divides

subjective

process.

On-screen

digitizing

also

is a has

very time-consuming and the advantage over paper

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