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I'm a new ERDAS Imagine user, so it's probably a beginner problem I'm facing: I get negative elevations in my DTM after automatic tie points setting..

I am geo-referencing a stereo-pair of ASTER images to a reference image (with x,y,z, coordinates) in order to generate a DTM from them. When I set the GCPs in the point measurement tool of LPS without the use of tie points, then triangulate, I can generate a quite useful DEM. However, if I use the option 'automatic tie generation' and check the coordinates after triangulation, I get, what seems, random elevations for the tie points (although x and y seem ok, both in the ASTER-images and the reference image): basically any value between -30'000m and +50'000m. Consequentially, the resulting DEM is useless (but this also confuses me somewhat, as I don't expect the 'DTM extraction' process to use the z-coordinates of the tie points, but only the ones of the GCPs). Anybody has experience with this? Any clue what I am missing? I'm glad for every hint!

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