A datum is established by tying a reference ellipsoid to a particular point on the earth. The following Datums table lists the details for each datum:
- The number used to identify the datum in the MAPINFOW.PRJ file.
- The datum's name
- The maps for which the datum is typically used
- The datum's reference ellipsoid
For a comprehensive list of supported projection datums, see Projection Datums.
To create a custom datum, see Defining Custom Datums.
For a list of datum changes by version, see Coordinate System Enhancements by Version.
Converting Coordinates from One Datum to Another
When converting coordinates from one datum to another, MapInfo Pro has used the Molodensky (3-parameter) and Bursa-Wolfe (7-parameter) methods. These are general-purpose methods that can convert coordinates from any datum to any other datum.
After the NAD 83 datum was introduced, NOAA developed a program called NADCON, which stands for North American Datum CONversion. This is a very specialized program that converts coordinates only from NAD 27 to NAD 83 and vice versa. For this specialized task, it is much more accurate than the Molodensky general-purpose method; NADCON is accurate to about 0.1 meter, and Molodensky is accurate to only 10-30 meters. Most U.S. government agencies, including the Census Bureau, have standardized on NADCON for converting between NAD 27 and NAD 83.
The NADCON algorithm is used to convert coordinates between NAD 27 and NAD 83 if those coordinates lie within the areas covered by NADCON (United States, Puerto Rico, and the Virgin Islands). If the coordinates lie outside those areas, or if they use datums other than NAD 27 or NAD 83, MapInfo Pro uses the Molodensky or Bursa-Wolfe conversion methods.
Due to the file access required, the NADCON conversion method can be slightly slower than the Molodensky method. If you want to turn off the NADCON conversion, add a "NADCON" entry to the registry. The registry entry should have this path:
HKEY_LOCAL_MACHINE\Software\MapInfo\MapInfo\Common\NADCON
If this entry is set to zero, then the Molodensky conversion method will be used instead of NADCON.
NADCON=0