Hi everybody, Currently I am working on the Geographic Coordinate System whit IDL6.3. The Map_Proj_Inti() Fuction does have the Datum keyword inside but this keyword just defined the Ellipsoid's Semimajor and Semiminor, there' seems NO WAY to apply x, y, z offsets form the WGS84 datum to get a local datum. But a local Datum is realy important to the Geographic Coodinate System. I have checked the !Map system variable, hope that I can change the !Map variable Directly to fit the problem. But I don't know what the !Map strtucture's fields realy represented. Any help will be appreciated. Thanks. Huxz