You can use ground control to enter known elevations. You might have a look at this tech tip: http://www.ittvis.com/ser...echtip.asp?ttid=3995

Thanks indeed for your respond, i have already used GCPs but i still had negative values. i think the most important problem is that, at the first stage of generating DEM by using DEM Extraction wizard, the minimum elevation value is almost -108 which is keyed in automatically.Also if i change minimum elevation value to any positive value, i still have the negative values in output DEM.

The default value is an estimate from the RPCs that are generated from the dataset. If you have good ground control with accurate elevations, then the RPC model should be improved. Are the datasets near sea level? You may need to contact ITT Technical Support directly for assistance so they can work with your data.

Thanks indeed, surely i will contact the technical supporters.In regard to your question about may dataset, i generate DEM for Malaysia which is almost the same level as sea and unfortunately my image is full of clouds.

Thanks indeed, surely i will contact the technical supporters.In regard to your question about may dataset, i generate DEM for Malaysia which is almost the same level as sea and unfortunately my image is full of clouds.

If your images are near sea level with a lot of cloud contamination, then the DEM generation may be difficult. It is not surprising that some values are negative. I would also think that the area is fairly flat with not much relief. The model for extracting a DEM and the resolution of ASTER may not be sensitive enough to get an accurate DEM for your area. Is it possible to get a scene with fewer clouds and try?