垂直平行投影獲得之真實正射影像，可以展示物體的正確位置與形狀，為具有圖面量測性質的資料。除了圖面量度外，豐富的地表影像資訊還可應用於GIS視覺分析、河川流域監控等不同的層面。航照影像之正射糾正必須具有地物資訊，才能完全糾正地物之高差移位。相對於DEM，DSM是較合適應用在正射糾正的幾何面。空載光達點雲具有地物及地表的三維坐標資料，有利於DSM的製作，故本研究以空載光達點雲為基礎，探討如何製作符合需求之真實正射影像。

為了要完整糾正航照影像之高差移位，正射糾正所使用的DSM解析度必須要與原始影像相當。基於空載光達可獲得密佈於地表的點位與多重反射的特性，理論上，第一回波的點雲即是DSM觀測點，然而受到空載光達點雲的不規則分佈與足跡造成之建物邊界內縮或外擴的影響，不僅使製作之DSM產生邊界不平整的問題，更會影響正射糾正成果。因此，本文以點雲過濾所獲得之DEM，與數值地形圖之建物邊界線構成的簡易建物模型，結合DEM建物模型進行DSM的製作。經由實際測試，本文所提之方法在建物邊界正確的情形下有效、可行，且能會得良好的糾正成果。

然而，在建物模型的建構中，可能面臨的問題有：(1)建物邊界錯誤(2)建物邊界遺漏(3) 建物變遷。針對第一個問題，本文藉由初步的正射糾正成果，進行邊界線的編修。針對遺漏及變遷問題，本研究提出以航照影像、建物點雲與地形圖獲取建物邊界線的方法。對於萃取之建物邊線可能會帶有錯誤，故仍須藉由初步正射糾正成果的輔助，進行人工檢核與編修，以確保DSM的正確性。此外，在正射糾正過程中，本研究也透過建物模型與成像原理，對航照影像進行遮蔽區之偵測並對遮蔽區加以標記。透過對標記為遮蔽之區域進行影像填補的作業，以製作出具有完整影像資訊之真實正射影像。

The generation of true orthoimages is a procedure to rectify photographs of perspective projection to the images of orthogonal projection. True orthoimages provide correct shape and position of ground objects and correct measurement of angles and distances as a traditional line-based map. Therefore, they can serve as a background of a thematic map and as a means to developing and updating thematic maps. True orthoimages can be applied in many fields, such as city planning and river basin monitoring, etc. Complete correction of relief displacement for true orthoimages generation should be done bases on digital surface model (DSM), rather than digital elevation model (DEM). Airborne Lidar is a highly efficient and accurate method to obtain data for determination the spatial data of the topographic surface. This paper investigates the benefits of using Lidar data for true orthoimages generation.

In order to acquire qualified true orthoimages, the resolution of DSM should be equivalent to the resolution of aerial photographs. To prevent from jagged edges of DSM generated by first return of airborne Lidar data, combining DEM and DBM to form DSM is more preferable. In this paper, DEM is generated from Lidar data by filtering out the non-ground points and DBM is obtained by using building boundaries of topographic map and Lidar point on the roof. Experiments on some test data reveal several problems: (1) incorrect building boundaries (2) missing of building boundaries (3) new buildings not shown in topographic map. Manually modifying the incorrect DBM is usually needed to cope with the first problem. For the missing part or new buildings, one could digitize the new building boundaries on the aerial images to update the topographic map.

In urban area there are inevitable hidden areas which contain no information of the occluded areas. The presence of hidden areas results in double mapping effects on true orthoimages. Refinements are needed for the hidden areas to make true orthoimages look reasonable. The detection of occluded areas is based on visibility analysis by tracing perspective rays from the top surface back to the perspective center. Those occlusions should be refilled by other available aerial images.

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