The Impact of Neighbourhood Size on Terrain Derivatives and Digital Soil Mapping

Abstract

Slope gradient, slope aspect, profile curvature, contour curvature, and other terrain derivatives are computed from digital elevation models (DEMs) over a neighbourhood (spatial extent). This chapter examines the combined effect of DEM resolution and neighbourhood size on computed terrain derivatives and its impact on digital soil mapping. We employed a widely used regression polynomial approach for computing terrain derivatives over a user-specified neighbourhood size. The method first fits a least squares regression polynomial to produce a filtered (generalized) terrain surface over a user defined neighbourhood (window). Slope gradient, slope aspect, profile, and contour curvatures are then computed analytically from the polynomial. To examine the effects of resolution and neighbourhood, we computed terrain derivatives using various combinations of DEM resolution and neighbourhood size and compared those values with corresponding field observations in two Wisconsin watersheds. In addition, we assessed the effects of resolution and neighbourhood in the context of knowledge-based digital soil mapping by comparing soil class (series) predictions with observed soils. Our results show that a neighbourhood size of 100 feet produces the closest agreement between observed and computed gradient values, and that DEM resolution has little impact on the agreement. Both profile curvature and contour curvature are more sensitive to neighbourhood size than slope gradient, and sensitivity is much higher at small neighbourhood sizes than at large neighbourhood sizes. Because of the importance of terrain derivatives in the knowledge base, predictive accuracy using a digital soil mapping approach varies strongly with neighbourhood size. In particular, prediction accuracy increases as the neighbourhood size increases, reaching a maximum at a neighbourhood of 100 feet and then decreases with further increases in neighbourhood size. DEM resolution again does not seem to impact the accuracy of the soil map very much. This study concludes that, at least for knowledge-based soil mapping, DEM resolution is not as important as neighbourhood size in computing the needed terrain derivatives. In other words, assuming the DEM resolution is sufficient to capture information at the optimum neighbourhood size, there is no predictive advantage gained by further increasing DEM resolution.