Sitespecific Decisionmaking Based on RTK GPS Survey and Six Alternative Elevation Data Sources: Watershed Topography and Delineation

Soil erosion modeling and assessment requires substantial and accurate topographic data to obtain meaningfulresults for decisionmaking regarding soil and water conservation practices. Todays precision farming equipment includesGlobal Positioning System (GPS) technology to determine the location of spatially distributed data. Besides the main purposeof tagging sitespecific information to a unique location (x and y), the elevation data (z) recorded has the potential to be usedfor topographic analysis, including delineation of flowpaths, channels, and watershed boundaries. In addition to GPSbaseddata collection at various accuracy levels, surveying companies and the U.S. Geological Survey also provide alternativesources of topographic information. Spatial statistical tests were performed to determine if some of these data sources . inparticular the ones free of charge or gathered with inexpensive equipment . are sufficiently accurate to represent field orwatershed topography and meaningfully apply detailed, processbased soil erosion assessment tools. The most expensivealternatives were most useful for determining elevation and slopes in the flow direction, while there was not much differencebetween alternatives in obtaining upslope drainage areas and delineation of the channel network and watershed boundary.This is the first of two articles analyzing the impact of the accuracy of six alternative topographic data sources on watershedtopography and delineation in comparison to GPS measurements using a surveygrade cmaccuracy GPS.