The normalized topographic method: an automated procedure for gully mapping using GIS

Abstract

Many studies in agricultural areas focus on the place where the head of rills and gullies start, but little information is available on defining their limits in the transversal direction. Here, we compare the ability of image interpretation and terrain analysis to delineate gully limits accurately by using topographical criteria, and we propose an automated procedure for mapping gullies. A 600m-long gully network was surveyed through an airborne campaign in order to obtain a high resolution elevation model (0.06m pixel size) in combination with differential GPS measurements for defining the reference perimeter. The gully was located near the city of Cordoba (southern Spain) with annual crops in a typical Campiña landscape. In all cases, the techniques based on terrain analysis over-performed those requiring visual interpretation of aerial images. Orthophotography interpretation was hindered by the existence of a crop-gully interface covered by weeds, that led mainly to overestimation of the gully area, although different trends were found among operators. 24 The slope criterion used to determine gully areas produced good results for medium-high resolutions, whereas the profile curvature was preferable when only high pixel sizes were available. An optimal resolution of around 0.5m was found for defining the gully perimeter when using topographic factors, showing that intermediate relative resolutions may be the most effective way to describe the geometry of the gully (5-10% of the mean gully width). The automated procedure featured the combination of the slope and the local lowness index and the use of rendering display methods. It proved to be accurate when compared with the terrain analysis methods based on manual digitising and was found to be applicable on the landscape scale. The sequential implementation of the procedure for increasing resolutions improved the accuracy of the gully mapping by adapting the pixel size to the variability of gully dimensions.JRC.H.4-Monitoring Agricultural Resource