Each Projection has both projected coordinates and geodetic coordinates corresponding to the same point on the earth. These geodetic coordinates are the latitude/longitude values corresponding to locations on the sphere or ellipsoid upon which projections are based.
The earth can be measured in degrees, minutes and seconds. MicroStation PowerDraft uses the standard system which describes the earth using:
While the distances between east-west lines (lines of latitude) is constant from equator to pole, distances between the lines running north-south (lines of longitude) decreases as the lines of longitude converge at the north poles and the south pole.
During the transform process, mapping coordinates are transformed using the following sequence:
- Projected coordinates in the source projection are obtained (usually meters, miles, etc.)
- These coordinates are transformed into their latitude/longitude equivalent for the source datum
- The source latitude/longitude values are related to the WGS84 datum (shift to WGS84 lat/long is performed)
- The WGS84 latitude/longitude coordinates are related to the target datum (shift from WGS84 lat/long is performed)
- The target latitude/longitude coordinates are projected into the target coordinate system
Latitude/Longitude values of one datum (Example: WGS84) are different from other datums (Example: WGS72) for the same point on the earth. In other words, it is not enough to have a "latitude/longitude" value without also knowing the datum upon which it was defined.
Changing latitude/longitude values between the various datums is referred to as "datum shift."