With the ISPRS data, the smallest value of RMSE found was about 0.45 m at W= 45 m and r2 ¼ 0:9. RMSE was calculated by normalizing the ground points in the reference data against the DTM using Eq. (7). where ZDTM ¼ dtm elevation value, ZRef = reference elevation value, and n = number of sample points. In this case ZRef = 0 (ground points).