Validation of Remote Sensing Data and Direct Field Observation in Mapping of Gully Erosion for Accuracy with Respect To Terrain Variation

Abstract

Many Digital Elevation Model (DEM)-based methods have been proposed to enable automated gully mapping. However, the accuracy of a gully map derived from a DEM is inevitably affected by the DEM resolution. Related literature evaluated the accuracy of DEMs generated from stereo imagery of different satellite sensors; however, limited work has been done on quantitative analysis concerning the capability and accuracy of satellite DEMs in Gully erosion mapping. Therefore, this study was centered to map gully erosion and to investigate the accuracy of measured heights of gully erosion using field acquired data (RTK GNSS) and Global Satellite DEMs obtained from ALOS World 3D 30m and 30m ASTER GDEM. The Etim Umana Gully Erosion site in Uyo Local Government Area of Akwa Ibom State, Nigeria was used as the test site for this study. These datasets were referenced to orthometric height, therefore were converted to ellipsoidal height using the Earth Gravitational Model (EGM) 2008. Digital Elevation Model of the field data were generated using various kriging method (simple, ordinary, universal) of which the ordinary method assumed the best with the least Root-Mean-Square Error(RMSE) of 1.326199. Ground Control Points (GCP) (32) values were used to validate the accuracy of the field acquired data and obtained a RMSE of 1.125463 and a Mean Absolute Error (MAE) of 0.8. The spot height values obtained from field were used to validate the satellite DEMs by calculating their RMSE from the difference between the measured values and the predicted values. The ALOS had a RMSE of 10.89154 when compared to the ASTER which had the least RMSE of 10.71999 which indicates the propagation of error. The Mean-Absolute-Error was calculated recoding MAE of 8.539147 for ALOS and ASTER 9.346781 and a Standard deviation of 9.176663 for the ALOS and ASTER 10.53712. ALOS had a mean error of -8.5174 and ASTER 7.706339. The result from this research indicates that they are similar levels of error propagation between the ALOS and ASTER and this can further be ascertained by carrying out validation on different terrain surfaces. These results indicate that it can be map to scale with contours based on the specifications of the National Mapping Accuracy Standards (NMAS)