本研究之目的是利用地形分析及影像處理技術，以計算建築物之視角遮蔽區，並摘取該區域之未受遮蔽之影像。其作業程序為(l)利用地理資訊系統所提供之功能，將數值高程圖層與建物圖層套疊，(2)再利用空間查詢功能，找出各建物坐落範圍內之高程點，(3)將建物之高度值加至相應的數值高程點，產生含建物之地表高程模型，(4)選定透視點，以該點之位置，計算建物之地面遮蔽區，(5)利用算得之地面遮蔽區，從相關影像中摘取其不受遮蔽之影像，以獲取其地表真實影像。本研究以地理資訊系統軟體，進行圖層套疊、空間查詢、高程整合、遮蔽區計算，再以影像處理軟體摘取不受遮蔽之影像，其作業過程皆以現有之套裝軟體進行。其所摘取之地表影像，可作為正射像片製作時，處理都會區被高樓之遮蔽區域之影像。

The purpose of the study is to set up an operation model for abstracting images obstructed by the buildings. The operational procedures are listed as following: (1) Registrate the layers of digital elevation model (DEM) and the building layer using a Geographic Information System (GIS), (2) Locate the elevation points of DEM inside each building sites, (3) Add the heights of the buildings to the elevation of the DEM, (4) Select the perspective position and compute the obstructed ground areas by the buildings, and (5) Extract the ground images from the related images of the buildings obstructed areas. A GIS software is used to perform data registration, spatial query, building height and ground elevation integration, the obstructed area computation. A digital processing software is used for extracting the images unobstructed area. The output of the procedures can be used for producing orthophoto of urban area to improve the image visibility.