Abstract
This study aims at the detection of gully-affected areas by applying object-based image analysis in the region of Taroudannt, Morocco, which is highly affected by gully erosion while simultaneously representing a major region of agro-industry with a high demand of arable land. As high-resolution optical satellite data are readily available from various sensors and with a much better temporal resolution than 3D terrain data, an area-wide mapping approach to extract gully-affected areas using only optical satellite imagery was developed. The methodology additionally incorporates expert knowledge and freely-available vector data in a cyclic object-based image analysis approach. This connects the two fields of geomorphology and remote sensing. The classification results show the successful implementation of the developed approach and allow conclusions on the current distribution of gullies. The results of the classification were checked against manually delineated reference data incorporating expert knowledge based on several field campaigns in the area, resulting in an overall classification accuracy of 62%. The error of omission accounts for 38% and the error of commission for 16%, respectively. Additionally, a manual assessment was carried out to assess the quality of the applied classification algorithm. The limited error of omission contributes with 23% to the overall error of omission and the limited error of commission contributes with 98% to the overall error of commission. This assessment improves the results and confirms the high quality of the developed approach for area-wide mapping of gully-affected areas in larger regions. In the field of landform mapping, the overall quality of the classification results is often assessed with more than one method to incorporate all aspects adequately.
Highlights
Situated between the High Atlas and Anti-Atlas, the region around the city of Taroudannt in theSouss Valley is characterized by high population dynamics and changing land use
Erosion is a form of soil erosion that is mainly induced by heavy rainfall events, but that depends on soil properties, geomorphological factors and land-use management [2]
Reference data were derived from the subset satellite image incorporating expert knowledge: Gullies were manually delineated from the QuickBird-2 image at a scale of 1:1500 using the ArcGIS software
Summary
Situated between the High Atlas and Anti-Atlas, the region around the city of Taroudannt in theSouss Valley is characterized by high population dynamics and changing land use. The region has been strongly affected by gully erosion, leading to continuous loss of arable land [1]. The area-wide mapping of gully-affected regions would enable investigations on the environment in which gullies occur and lead to a better understanding of the (spatial) context of gully erosion [4]. It would allow quantitative estimations on gully density in a given region and continuous monitoring of gully erosion. The derived polygon data set was refined in a second iteration to increase the validity of the data set This gully map was applied only as reference data for the accuracy assessment of the classification results. These detail views show: (1) part of a gully neighboring plantation; (2) gullies within a heterogeneous environment; and (3) gully-affected areas that incise riverbanks
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