1 GENERATING TRUE ORTHOIMAGES FROM URBAN AREAS WITHOUT HEIGHT INFORMATION

Orthoimages are very important geospatial datasets, forming the database for a great variety of applications. So far, orthoimages are generated from aerial photographs by differential rectification in order to correct the effects of relief displacements. Digital Terrain Models (DTM) describe the topographic surface but they ignore trees, man-made objects, etc. Therefore, effects of leaning buildings or bent bridges appear, especially in urban areas. Detailed and expensive Digital Surface Models (DSM) describe the entire surface including the mentioned objects. This enables the gen-eration of orthoimages of a high quality which are called »True Orthoimages«. Now, recent developments of optoelectronic line-scanning cameras like HRSC or ADS 40 allow for a totally new approach for orthoimage generation. The nadir-looking channels of these cameras provide image data in parallel projection along the flight line, and in central perspectives across. If a scene is imaged twice in flight lines perpendicular to each other, the first image strip provides correct ground coordinates of any object in one direction, the second image strip in the other direc-tion. The new approach takes advantage of these particular geometric properties by combining the correct ground coordinates of each point. Thus, the generation of a true orthoimage becomes pos-sible without any height information. Furthermore, due to the mixed projections of pushbroom sen-sor imagery, the relief displacements occur only in one direction, and therefore occluded areas are significantly smaller than in conventional aerial photographs. And the new approach can also take advantage of the particular pattern of certain urban areas, which makes it easier to fill up remaining gaps with image information