If you are not working in a known coordinate system like UTM or State Plane, then the current coordinate system should be set to Undefined.
Coordinate Systems and Map Projections
Undefined
Use this setting when coordinate systems aren't of interest to your project.
Latitude / Longitude
Latitude and Longitude values can be specified in either Nad27 or Nad83 datums. For more information, see Latitude - Longitude Formats.
Map Projections
The following map projections are available:
Universal Transverse Mercator (UTM) (NAD83 or NAD27)
State Plane Coordinates (SPC83 or SPC27)
Albers Equal Area
Lambert Conformal Conic
Mercator
Polar Stereographic
Polyconic
Note: The State Plane 27 projections can be computed using either the 'standard' table-based solution, or using the 'exact' calculated solution. The table-based solution matches the projection tables that were created in the 1930s when hand-calculating state plane coordinates. The exact solution computes the projection constants on the fly with greater precision than the original projection tables.
Datums
NAD83
North American Datum of 1983. Uses the GRS80 ellipsoid.
NAD27
North American Datum of 1927. Uses the Clarke 1866 ellipsoid.
Grid Shift Files
Datum conversions are possible using grid shift files. For Canadian transformations, use the NTv2 grid shift file. For transformations on the continental US, use the NADCON grid shift files.
