Google and Microsoft Virtual Earth both use a Spherical Mercator projection for their geospatial data. This is represented by the EPSG code of 900913.
This particular coordinate system converts the earth into a perfect square. This fits perfectly with the square image tiles that are delivered by the Google and Microsoft Systems.
For “real GIS work”, this coordinate system isn’t going to be the Coordinate system of choice. But – "real accuracy" doesn’t matter when you are just presenting a picture to the public.
Image Web Server is a proper Geospatial Image Delivery solution. It is designed to be highly accurate. However, if you use the “inaccurate” Google Projection, you may need to tell IWS to “loosen” its accuracy requirements for that particular projection system.
This is very easy to do. Simply navigate to the “datumshift.xml” file. This is found at:
C:\Program Files\ERDAS\Image Web Server\ermlib\BMG_DATA
Open the datumshift.xml file and tell it to relax the maths involved in processing from your original datum / projection to the Google EPSG code. In the example, my original projection is 2227.
For the Datum:
| <DatumShiftLookUpTable> |
| <!—relax math processing for 2227 to Google Proejction --> |
| <DatumShift HDatum1="EPSG:2227" HDatum2="EPSG:900913" DatumShift="NULL" Notes="GOOGLE stuff"/> |
| </DatumShiftLookUpTable> |
| |
For the projection:
| <ProjectionDatumShiftLookUpTable> |
| <!-- relax math processing for EPSG 2227 to Google --> |
| <DatumShift CoordSys1="EPSG:2227" CoordSys2="EPSG:900913" DatumShift="NULL" Notes="Google stuff..."/> |
| </ProjectionDatumShiftLookUpTable> |
| |
Easy!