Thanks a lot for the prompt response. Both were really helpful.
The vertical offset that I was having was in line with what David mentioned in the two links. It was indeed because of the offset between the EGM96 geoid and the WGS84 ellipsoid. I was able to get rid of the vertical offset by using the dem_geoid tool as suggested.
This might be a naive question but I had errors when I try to use the image_align tool to aligns the ASP DEM to srtm DEM and the error says : "RANSAC was unable to find a fit that matched the supplied data. Alignment transform computation failed". This might be because of the difference in resolution between the two dem's (1m vs 10m)
I know the suggested tool was pc_align but I do not have a point cloud for the srtm dem which is why I am unable to use that tool. Am I using the image_align tool correctly or is there some parameter that I should specify? Also, what is the correct method to use the pc_align tool for my case?
Thanks again for the help.