Quantifying soil variability in GIS applications I. Estimates of position

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Abstract
Geographical information systems (GIS) commonly make use of soil information which consists of soil maps, descriptive text, and tables of attribute values. However for optimal use, published soil maps first need to be digitized and registered and subsequently the variability associated with tabular soil attribute information needs to be quantified. Validity of individual field observations depends on combined accuracy of an attribute estimate, sampling position location, and interpolation or extrapolation error among measured values. In this paper (part I), we evaluate the accuracy of sampling position location by comparing Global Positioning System (GPS) field measured values with visual estimates obtained from aerial photographs. Results indicate that such visual estimates of position, registered using standard procedures, can be in considerable error. We concluded, that best estimates of position on a GIS map are obtained when coordinates of position are computed from kriged estimates, or a projection type registration procedure.

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