Do you have data in the GGRS 1987 geographic coordinate system? Are you working in the Greek system and wish you were working in Universal Transverse Mercator (UTM) instead? Have no fear, because ESRI’s ArcMap v.10 has a perfected algorithm for transforming between WGS 1984 and GGRS 1987. It took me months to discover this fact, since no one else had written about it online, so I’m happy to pass on the information to my colleagues working in Greece.
For the transformation to work properly, the Greek data must be set to the “Greek_Grid” projection (Projected Coordinate Systems \ National Grids \ Europe). Do not be deceived by the file called “GGRS 1987” (Geographic Coordinate Systems \ Europe), which is also based upon the GCS_GGRS_1987 coordinate system – using this to define your projection will result in a 350 meter error after reprojecting.
Tool: Define Projection
Next, the Greek data can be projected to UTM in one simple step. I always project my data to “WGS 1984 UTM Zone 34N” (Projected Coordinate Systems \ UTM \ WGS 1984 \ Northern Hemisphere) because this is a transverse mercator projection, meaning that your GIS sessions will be in meters rather than decimal degrees. This is important when running spatial analyses, since the ArcMap tools give outputs in the session units. Unless you want to measure distance in degrees, you should always use projected coordinate systems. Happy projecting!