A Contour-Directional Detection for Deriving Terrace Ridge From Open Source Images and Digital Elevation Models

Agricultural terraces, as typically artificial landforms, have maintained an important role throughout human history. Terrace ridges, which form the main constructive part of terraces, face the greatest risk from gully or gravity erosion. Mapping terrace ridges can benefit agricultural production management and soil-and-water conservation. This study presents a contour-directional detection method that automatically maps terrace ridges by considering the image features and topographic features of terraces. First, the terrace areas were extracted via object-based image analysis. Second, the Canny edge detector was employed to extract all edges from the image, and a digital elevation model (DEM) was used to generate the contour direction. The contour direction and edge image were then overlaid to implement the contour-directional detection method, which detects ridges along the contour direction. Finally, those candidate terrace ridges which lengths exceed an appropriate threshold were identified as terrace ridges. Five study areas located in the Loess Plateau of China were used to validate the proposed method. The results showed that the correctness, completeness, and quality ranged from 82.57% to 94.92%, from 82.51% to 88.39%, and from 70.28% to 84.40%, respectively. However, the proposed method is widely available and can be easily applied by using open global images and DEMs, such as Google Earth images and ASTER GDEM.