空載光達點雲穿透率探討

Abstract

空載光達可在短時間內獲取大量且高密度的點雲資料，其中包含了地面點與地物點。在評估數值高程模型資料品質優劣時，地面點點雲密度為重要指標之ㄧ。地面點密度高且分布均勻時，較能正確地展示地形面。地面點密度除受原始點雲密度之影響外，穿透率為另一重要因子。而雷射點穿越覆蓋面的比率(穿透率)，主要受地表覆蓋物之性質、雷射入射角與航高等因子影響。 本研究之目的在探討穿透率之表示方法，與其各種影響因子的關係。探討之項目，包括：(1)比較格網方式及TIN三角形面積排序的密度指標方法，了解不同地表覆蓋物與穿透率指標之間的關係。(2)分析實測航帶，探討雷射入射角對光達穿透率的影響，(3)討論覆蓋物之遮蔽效應，分析航高、入射角、覆蓋物高等影響因子，並試驗在不同航帶重疊率時，遮蔽陰影區域的補償效果。 經由實驗發現以格網方式評估密度時，當以不同面積為統計單位時，密度指標數值不同，相對上TIN三角形面積排序方法較不會有高密度區與低密度區互相補償的情形，可獲得較詳細的點雲密度資訊。當雷射入射角越大時，穿透率越低，反之亦然；航帶重疊可以改善遮蔽問題，但若在兩建物太靠近的情況下，即使有航帶重疊，也可能無法改善遮蔽問題。

The point cloud obtained from airborne LiDAR is composed of ground points and non-ground points. The density of ground points is an important index for characterizing the quality of DEM. When ground point density is high and distributed evenly, topography can be presented with high fidelity. There are a number of factors which may influence the ground point density. In addition to the density of the original point cloud, penetration rate is another essential factor. Penetration rate is influenced by land-cover types, incidence angle, flying height, etc. This study investigates the indices for measuring penetration rate and the relationship among various factors. Three study subjects are included. (a) The penetration rate indices based on grid and TIN with different land-cover types are compared. (b) The influence of incidence angle on penetration rate is evaluated with real data set. (c) The relation between the building height, overlap ratio, and the shadow is derived, and validated with real data set. The experiments revealed that the area size of statistic unit has high influence on the density computed. The triangle ranking indices based on TIN are less influenced by the compensation between high and low density regions in the unit area. It is also proved from real data analysis that the penetration rate is higher with smaller incidence angles. Overlap between flight lines can reduce the building shadow effectively, but not when the buildings are closely spaced.