比較不同移除凹坑演算法於空載光達nDSM

A Comparison of Pit Removal Algorithms for Airborne Laser Scanner nDSM

10.6574/JPRS.201803_23(1).0004

林志樺(Zhi-Hua Lin)；王驥魁(Chi-Kuei Wang)

正規化數值表面模型 ； 空載光達 ； 移除凹坑演算法 ； normalized digital surface model ； airborne laser scanner ； pit removal algorithm

航測及遙測學刊

23卷1期（2018 / 03 / 01）

43 - 59

繁體中文

正規化數值表面模型（Normalized Digital Surface Model，nDSM）是由數值表面模型（Digital Surface Model，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，以該DSM所產製的nDSM也會有相同情況。這些不自然且不規則的網格稱作凹坑（Pit），凹坑影響DSM、nDSM的應用與合理性，故需移除。移除凹坑演算法的目的是移除植被上的凹坑，但測區內可能會有植被以外的地物存在，所以本研究的目的為探討四種移除凹坑演算法在植被nDSM與非植被nDSM上的表現。比較不同移除凹坑演算法移除植被凹坑的能力，以及該演算法是否能保留地物表面原始特徵且更合理地展示表面起伏。四種演算法分別為中值濾波（Median filter）、高斯平滑濾波（Gaussian smoothing filter）、帶有樹冠型態控制的填補凹坑演算法（Pit filling algorithm with morphological crown control）、免除凹坑演算法（Pit-free algorithm）。研究成果顯示，帶有樹冠型態控制的填補凹坑演算法、免除凹坑演算法皆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（孤立木、建物）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。

Normalized Digital Surface Model (nDSM) represents the elevation of the object with respect to the ground surface, which can be obtained by subtracting Digital Elevation Model (DEM) from Digital Surface Model (DSM). The DEM can be obtained by interpolating the elevations of the first returns from the airborne laser scanner (ALS) data. However, in forest area, the laser can penetrate through the openings of the canopy, where the first returns of ALS point clouds are not necessary on top of the canopy. As a result, the DSM interpolated based on the first returns may show lowered elevation values. In turn, the nDSM obtained from such DSM may show similar phenomenon. In DSM and nDSM data, the raster with lowered elevation value is called pit, which need to be removed for the purpose of data integrity. This study compares the performance of four pit removal algorithms, i.e., median filter, Gaussian smoothing filter, pit filling algorithm with morphological crown control, and pit-free algorithm, in vegetated and non-vegetated areas with the focus on pit removal in vegetated area while keeping the surface integrity. The results show that pit filling algorithm with morphological crown control and pit-free algorithm can effectively reduce the number of pits in forest area while avoid creating overly smoothed results. For single tree, building, and electrical tower, only pit-free algorithm can reasonably mitigate the pits in DSM and nDSM.

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