Various Methods for Producing Orthophoto out of Central Perspective Imagery

DEM(Digital Elevation Model)-based digital differential rectification is a very popular method for producing Digital Orthophoto Model(DOM) out of central perspective imagery. However, it is by not the unique way. Three other non-DEM methods are proposed and discussed technically in the paper. (1) If central perspective-sensed images are in the form of stereopair, then parallax can be computed by stereo-image matching and directly applied to differential rectification. Thus, the procedure of making DEM is bypassed. (2) If multi-centered perspective images do not meet the requirements of stereopair constitution, but certain spatial distances exist among the centers, then by using the mathematical formula derived in paper, parallax-based differential rectification on the basis of image matching can be done. The figure and formula illustrate that such ill geometrical condition does not seriously affect the results quality. (3) If topographic data, such as DRG (Digital Raster Graphics) or DLG (Digital Line Graphics), of the area distributed with dense objects, are available, then orthorectification can be implemented by registering the feature lines out of DRG or DLG with those extracted from a single frame of image. Besides, a new method, based on mathematical morphology, used for the matching of feature lines, is also presented.