Precision Agriculture, 4, 179±192, 2003 # 2003 Kluwer Academic Publishers. Manufactured in The Netherlands. Comparison of DEM Data Capture and Topographic Wetness Indices FRANK SCHMIDT fsch.midt@beak.de Foerstereistrasse 38 01099 Dresden, Germany ANDREAS PERSSON andreas.persson@jti.slu.se Swedish Institute of Agricultural and Environmental Engineering (JTI), P.O. Box 7033, 750 07, Uppsala, Sweden Abstract. Digital elevation models (DEMs) can be captured and analysed by various methods. Elevation capturing with RTK-GPS and airborne laser scanning is presented and evaluated in terms of height accuracy of raw data and interpolated DEMs for study sites in Sweden and Germany. Applications for precision agriculture are based on the connection of land surface and the movement of water in the landscape. Three methods of deriving potential flow accumulation from DEMs are discussed. Results indicate that the Topographic Wetness Index can be used to assess the pattern of potential soil moisture on a field and changes in soil texture caused by erosion processes. The quality of the soil moisture assessment varies with both flow algorithm and terrain characteristics. Correlations up to r 2  0.64 were found. Best results were obtained on undulating farmland using the DEMON algorithm and a formbased approach. However, in areas with low relief, the concept did not lead to valuable soil moisture maps. Keywords: GPS, laser scanning DTM, topography, wetness index Introduction Terrain relief controls the movement of water in a landscape. It influences the spatial pattern of soil attributes and is one of the most important natural factors causing heterogeneity on arable land and yield (Afyuni et al., 1993, Stone et al., 1985). The functions of the terrain can be represented using digital elevation models (DEM). The DEM is a stable factor compared to other data sources needed for precision agriculture. DEMs have been under investigation for agricultural applications for a number of years (e.g. Bishop and McBratney, 2002; Nolan et al., 2000; Nugteren and Robert, 1999; Russel et al., 2000; Yao and Clark, 2000) and will be more common with increasing availability and quality in the near future. Digital terrain analysis can support the creation of application maps for soil tillage, site-specific seeding, irrigation, fertilizing and pesticide spreading. For this purpose, topographic attributes such as slope, aspect, drainage area (flow accumulation) or the Topographic Wetness Index (TWI) have to be derived from the DEM. The Wetness Index ln(A s /tan b) is a compound terrain attribute calculated from specific catchment area of a point (A s ) and the local slope gradient tan b. The concept was first presented by Beven and Kirkby (1979) and further developed in the 1990s (Wilson and Gallant, 2000). In this article, the importance of sample geometry for the generation of digital terrain models and algorithms for terrain analysis will be presented for two study sites and two