*2.4. Spaceborne Topographic DSM*

The building of the satellite-based topographic DSM requires both panchromatic imageries to be radiometrically converted from digital number to top-of-atmosphere radiance, then to bottom-of-atmosphere reflectance values by considering the calibration factors (.IMD file), the atmosphere composition and sun irradiance (see [5] for details). The reflectance imageries were used to retrieve a 3D point cloud using a dense point matching algorithm [28]. The matching algorithm seeks for the pairwise pixels of two imageries by shortening the epipolar 2D to 1D, based on the rational polynomial coefficients (RPCs), then by reducing the length of the epipolar line with the global multi-resolution terrain elevation data 2010 dataset. The point cloud was then gridded at 0.3 m by converting the XY coordinates into the WGS84 datum, UTM zone 6S, and referencing in Z to the mean sea level (Figure 4). The topographic validation accuracy was estimated by the mean absolute error (MAE) and the root mean square error (RMSE) between the modeled and observed values (*N* = 12). The MAE and RMSE attained 0.84 and 1.11 m, which corroborates the results from previous WorldView-3 works [24].
