The tech tip you reference is for adding a datum to ENVI that is *not* supported by the projection engine. The datum you mention, ETRS 1989, is supported in the projection engine but it does not show up in the list of datums in ENVI. In this case, you need to add the datum to the datum.txt located in the \envixx\map_proj directory in the typical manner. The offsets you use won't really matter because the projection engine will use its own. You also need to add the datum to the datum overrides file as described in the projection engine reference here: http://www.ittvis.com/ser...echtip.asp?ttid=4455 So, you first need to identify the projection name you need, which from the the EnviPEDatums_v10.txt link looks like the name should be: PE_D_ETRS_1989 6258 /* European Terrestrial Ref. System 1989 In the datum overrides text file located in C:\Program Files\ITT\IDL71\resource\pedata\user_defined, you would add in the datum name: ETRS1989 , D_ETRS_1989 where 'ETRS1989' is the datum name you used in the datum.txt.