Thanks for the comments everyone! Some technical info on the Mars data; the data format is a gridded 2-byte integer binary file. The data is not a standard USGS DEM which are an ASCII format. Off the top of my head I can't think of the actual ground resolution in Kilometres but it's very good for planetary data (I'll have a check). If anyone is interested in this data let me know and I'll post the link. The Wilbur program is freeware and a very good utility to play round with and check DEM data out. The link is: http://www.ridgenet.net/~jslayton/software.html