Submitted to: Transactions of the ASAE
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/1/2002
Publication Date: 12/31/2002
Citation: Renschler, C.S., Flanagan, D.C. Engel, B.A., Kramer, L.A., Sudduth, K.A. Site-specific decision-making based on RTK GPS survey and six alternative elevation data sources: Watershed topography and delineation. Transactions of the American Society of Agricultural Engineers. 2002. v. 45(6). p. 1883-1895. Interpretive Summary: It is becoming more and more important to be able to estimate soil erosion from landscape areas. In order to do this in an efficient way, digital geographic elevation information can be used from a variety of sources. One source is elevation data that is derived from existing paper maps, while newer methods use multiple satellite signals to determine the elevation at a position on the Earth's surface (Global Positioning System - GPS). Many farmers already have simple GPS systems for monitoring grain yields across a field. This paper provides information on determining watershed characteristics (boundaries, area, slopes, etc.) using a variety of digital information, from less accurate to more accurate sources. Ultimately we wanted to see how detailed (and expensive) a GPS system is needed to make satisfactory soil erosion predictions for small watersheds. We found that all of the systems could reasonably outline the watershed boundary and locate the channels. However, the more accurate and expensive systems did a much better job of determining elevation and slope steepness. The work impacts recommendations for the type of GPS needed. Low-cost GPS systems can be useful in locating watersheds and channels - which can get a user started in setting up soil erosion model simulations. However, for more detailed information on slopes, more accurate equipment or techniques may be needed.
Technical Abstract: Soil erosion modeling and assessment requires substantial and accurate topographical data to obtain meaningful results for decision-making regarding soil and water conservation practices. Today's precision farming equipment includes Global Positioning System (GPS) technology to determine the location of spatially distributed data. Besides the main purpose of tagging site-specific information to a unique location (x and y), the elevation data (z) recorded has the potential to be used for topographic analysis including delineation of flowpaths, channels and watershed boundaries. In addition to GPS-based data collection at various accuracy levels, surveying companies and the U.S. Geological Survey also provide alternative sources of topographic information. Spatial statistical tests were performed to determine if some of these data sources - in particular the ones free of charge or gathered with inexpensive equipment - are sufficiently accurate to be able to represent a field or watershed topography and meaningfully apply detailed process-based soil erosion assessment tools. The most expensive alternatives were most useful for determining elevation and slopes in the flow direction, while there was not much difference between alternatives in obtaining upslope drainage areas and delineation of the channel network and watershed boundary. This is the first paper of a two-paper-series analyzing the impact of the accuracy of six alternative topographical data sources on watershed topography and delineation in comparison to GPS measurements using a survey-grade GPS with cm accuracy. Information presented here impacts farmers, land managers, soil conservationists, engineers and others involved in assessing runoff and sediment loss from watersheds.