Readers of this forum who don't keep up with the active and extremely useful IDL newsgroup (comp.lang.idl-pvwave) may be unaware of two recent developments with map projections that can cause inadvertent errors in your programs. First, there is a bug in all versions of IDL up through 8.1 that causes a UTM map projection using the WGS84 ellipsoid as the datum (the most common ellipsoid to use with this projection!) to produce incorrect results when coordinates are being translated from lat/lon to projected XY and visa versa. The work-around is to avoid the WGS84 ellipsoid and use the virtually identical Walbeck ellipsoid instead. You can learn more about this problem in this article: http://www.idlcoyote.com/map_tips/utmwrong.php The second problem concerns itself with the silent substitution of a different map projection and ellipsoid when trying to set up a map projection with Map_Proj_Init. It is essential that if you are selecting a GCTP map projection that you also set the GCTP keyword in the Map_Proj_Init call. Otherwise, you may well be using a different map projection and a different ellipsoid than the ones you specified in the call. It is unlikely that you will discover this until it is too late to do anything about it. You can see an example of the problem and learn more about how to make sure this "feature" doesn't bite you in this article: http://www.idlcoyote.com/map_tips/choosemap.php