Hi Dan,
Thanks for your reply. I resapmled my DEMs to the same resolution(25x25m) before processing them. I have defined the spatial extent myself, by clipping the DEMs to a vector outline through the Image Analysis menu.
My other problem is that when I turn these three DEMs into contour maps two of them doesn't draw. I have imported the third DEM's Coordinate system, also the 'project' tool and tried several other systems, nothing works, whereas they all show up correctly before turning them into contour maps.
Ideally, all layers should be in the same coordinate system, but somehow it is just not possible in my case. For instance, my vector files are in 1993 Lambert system and my Landsat images are in WGS 1984. They perfectly overlap this way, but if I change my vector to WGS 1984 it won't draw. I have been using ArcMap actively for a year by now, performing different tasks but i still can't figure out why this is happening.
Any idea?
Many thanks,
Laszlo