數值表面模型(Digital Surface Model, DSM)為包含所有植被、人工構造物等地物高度與地表高程之數值模型。正規化數值表面模型(Normalized Digital Surface Model, nDSM)是由DSM減數值高程模型(Digital Elevation Model, DEM)而得，nDSM代表地物與地表間的高程差。DSM通常是以空載光達系統(Airborne Laser scanner, ALS)掃瞄得到之第一回波點雲內插而得。然而在森林中，部分雷射經由孔隙穿透至樹冠層以下，導致部分第一回波並非在樹木表面，這些第一回波導致DSM網格值降低且與鄰近網格有明顯差異。這些不自然且不規則的網格稱作凹坑(Pit)，凹坑影響DSM的應用與合理性，故需移除。本研究目的為重製全臺DSM、nDSM，解決凹坑問題並維持合理性。所使用的移除凹坑演算法分別為中值濾波(Median filter)、高斯平滑濾波(Gaussian filter)、帶有樹冠型態控制的填補凹坑演算法（Pit filling algorithm with morphological crown control）與免除凹坑演算法（Pit-free algorithm），比較四種演算法在多地物上的表現。研究成果顯示，免除凹坑演算法能有效地減少森林凹坑數量，並且不會造成樹冠表面過渡平滑。當地物下方中空(工廠管道、高架橋)或結構有許多孔隙(電塔)，使部份第一回波低於地物表面，導致地物在DSM、nDSM上起伏不明顯或不合理，免除凹坑演算法是四種方法中唯一能改善此問題的演算法。故最終以免除凹坑演算法重製全臺DSM、nDSM。

Digital Surface Model (DSM) is the digital model that contains the elevation of vegetation and man-made objects. Normalized Digital Surface Model (nDSM) is the difference between DSM and Digital Elevation Model (DEM). The existing method of DSM generation is to interpolate first returns in Airborne Laser scanner (ALS) data, but some laser beams will penetrate into tree crowns. Under this circumstance, some first returns are under tree crown and cause irregular low values called pits in DSM or nDSM. It is necessary to remove pits from DSM and nDSM because that the applications of DSM and nDSM may be influenced by these pits. The purpose of this research is to update existing DSM and nDSM of whole Taiwan, and conducting pits removal method. Pits-removing methods include Median filter, Gaussian filter, Pit filling algorithm with morphological crown control and Pit-free algorithm. The results of these methods on several objects were analyzed in this research. Pits can be removed by Pit-free algorithm effectively in forest and avoid the problem of over-smoothing. Any structure that an off-nadir pulse can peek under results in unobvious elevation of land use in DSM and nDSM, such as overpass, electric tower and eaves of roofs. Except for Pit-free algorithm, the other Pit-removing algorithms cannot solve this kind of problem. Therefore, the DSM and nDSM of whole Taiwan were processed by Pit-free algorithm.

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