VERTICAL ACCURACY EVALUATION OF THE FOREST AND BUILDINGS REMOVED COPERNICUS DEM (FABDEM) OVER THE PHILIPPINES

Abstract. This study evaluated the accuracy of the Forest And Buildings removed Copernicus DEM (FABDEM) over the Philippines. The evaluation was conducted at the country and island levels using 17,013 Geodetic Control Points (GCPs) partitioned into 3 independent sets. Results suggest that the FABDEM generally underestimates terrain elevation in the Philippines, as exemplified by a mean error of −1.44 m which is the average among the three sets of the GCPs used. Negative errors are primarily dominant in Luzon Island in the northern part of the Philippines, and positive errors are prevalent in the Visayas and Mindanao islands. At the country level, the DEM has an average Root Mean Square Error (RMSE) of 4.74 m and average linear errors of 7.80 m and 9.80 m at 90% and 95% confidence levels, respectively. At the island level, the DEM's accuracy varies, with some islands having RMSEs as low as 3.03 meters and as high as 5.80 m. This DEM is also most accurate at elevations less than 100 m and slopes less than 2 degrees. Care must be taken when applying the FABDEM in an archipelagic country like the Philippines due to several factors, such as the general tendency of this DEM to underestimate elevations, the dominance of negative errors in the northern part of the country, the differences in accuracies of elevations from one island to another, and the pronounced influence of elevation and slope to its accuracy. Nevertheless, the FABDEM is one of the most accurate among several freely available global DEMs covering the Philippines. Its accuracy in Mindanao Island is comparable to AW3D30 and superior to SRTM DEM.