Production of Digital Elevation Models (DEMs) is one of the main tasks within digital photogrammetry. In the process, digital image matching techniques are used to find conjugate points of the DEM grid nodes on both images of stereopairs, and to find their heights. One of the problems in generating DEMs is finding a good approximation for the image matching. This research investigated the use of existing DEM information as the first approximation for the search algorithm. A DEM produced by the United States Geological Survey (USGS) can provide an estimate for the terrain surface which enables optimizing search strategies. The results indicate that the use of existing DEM information has resulted in reduced computational time as compared to methods that do not use approximation.; The matching technique used in this study was grid-based least squares matching. In this method, both the left and right image windows are forced to match along the relief displacement lines to produce a regular grid DEM. The collinearity equations are used as a constraint to relate the images to the object space. Tests have been conducted to study the behavior of this method. In addition, an error detection algorithm has been developed and tested. The results showed improvement of the DEM after applying the error detection algorithm.; The accuracy of the DEMs has been assessed by making comparisons with check points collected manually on a stereoplotter. The results showed that accuracy within about 0.2 {dollar}perthous{dollar} of the flying height was achieved.
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