About Changing the Specified Coordinate System of a DEM File
Use the Prospector tree to specify or change a DEM file coordinate system. If the DEM file coordinate system is different from the current coordinate system of the drawing, you can specify a coordinate system for the DEM file. The coordinate system you specify for the DEM file should match the data defined in the DEM file itself.
About DEM Files and Memory Usage
Large TIN surfaces, such as those built from DEM files, can require a substantial amount of memory (RAM and virtual memory) and disk space to build and save the surface. TIN surfaces require about 100 bytes per point, whereas grid surfaces require about 16 bytes per point.