Product: MapInfo Professional
Version: 8.0, 7.8, 7.5 b21, 7.0 b22, 6.5 b19, 6.0, 5.5, 5.0, 4.0
Platform: All Windows Platforms
Custom Datum documentation.
How to create additional custom datums in MapInfo.
Defining Custom Datums/Datum Conversion
A datum is a mathematical description of the earth's shape and orientation. Because the earth's shape is not uniform, there are many different local datums used in different parts of the world. These local datums provide a close approximation to the earth's surface in a particular area.
Each Earth coordinate system uses a specific datum to approximate the earth's surface. If two coordinate systems use different datums, then MapInfo must perform a datum transformation when it converts coordinates from one coordinate system to the other. MapInfo uses the Bursa-Wolfe datum transformation method, which is generally accurate to within 10 meters. (When MapInfo converts between two coordinate systems that use the same datum, no datum transformation is performed, and the results are generally accurate to within 0.1 meter.)
Most coordinate systems use one of MapInfo's predefined datums, listed in the Appendix. If a datum is required that isn't in the list, and the datum's mathematical parameters are known, then the coordinate system can be defined using a custom datum. MapInfo uses the following information to define a datum:
**In case the Greek letters are not displaying correctly, see the MapInfo UserGuide.pdf available on the MapInfo Professional CD.
- An ellipsoid, also called a spheroid. This is an ellipse rotated around its minor axis to form a three-dimensional surface. The ellipsoid is described by two mathematical parameters: the length, in meters, of its semi-major axis (denoted by the letter a) and its degree of flattening (denoted by the letter f). MapInfo supports over 40 predefined ellipsoids, which are listed in the table below.
*It is required to use a predefined ellipsoid contained in the following table. If it does not appear here, please contact MapInfo Technical Support with the information including ellipsoid parameters so that it may be included in the next release.
- Three shift parameters specifying the distance, in meters, to shift the ellipsoid along each of its axes. These parameters are usually denoted by dX, dY, and dZ. They may also be denoted by X, Y, and Z, or by u, v, and w.
- Three rotation parameters specifying the angle, in arc-seconds, to rotate the ellipsoid around each of its axes. These parameters are usually denoted by EX, EY, and EZ. They may also be denoted by X, Y, and Z, or by , , and .
- A scale correction factor specifying the amount, in parts per million, to adjust the size of the ellipsoid. This parameter is denoted by the letter m, or sometimes .
- The longitude of the prime meridian, in degrees east of Greenwich. The prime meridian specifies which location on earth is assigned longitude 0°. Most datums use Greenwich as the prime meridian, so this parameter is usually zero. However, some datums use a different location as the prime meridian. For example, the NTF datum uses Paris as its prime meridian, which is 2.33722917 degrees east of Greenwich. If the NTF datum is used in a coordinate system, all longitudes in that coordinate system are relative to Paris instead of Greenwich.
A custom datum can be defined in any coordinate system definition. The Appendix describes how to define a coordinate system. To define a custom datum in a coordinate system, use datum number 9999 followed by the datum parameters, in this order:
9999, EllipsoidNumber, dX, dY, dZ, EX, EY, EZ, m, PrimeMeridian
Some datums specify only an ellipsoid and shift parameters (dX, dY, dZ), with no rotation parameters, scale correction, or prime meridian. In those cases, use datum number 999 instead of 9999, to simplify the definition: (See Example LD-3 below):
999, EllipsoidNumber, dX, dY, dZ
The ellipsoid number must be chosen from the following list. If an ellipsoid needing to be used does not appear on this list, please notify MapInfo Technical Support so that it can be added as an ellipsoid to a future MapInfo release.
|Airy 1930 (modified for Ireland 1965)||6377340.189||299.3249646|
|Bessel 1841 (modified for Schwarzeck)||6377483.865||299.1528128|
|Bessel 1841 (modified for NGO 1948)||6377492.0176||299.15281|
|Clarke 1866 (modified for Michigan)||6378450.047484481||294.9786982|
|Clarke 1880 (modified for Arc 1950)||6378249.145326||293.4663076|
|Clarke 1880 (modified for IGN)||6378249.2||293.4660213|
|Clarke 1880 (modified for Jamaica)||6378249.136||293.46631|
|Clarke 1880 (modified for Merchich)||6378249.2||293.46598|
|Clarke 1880 (modified for Palestine)||378300.79||293.46623|
|Everest 1830 (modified for Kalianpur)||6377301.243||300.80174|
|Everest 1830 (modified for Kertau)||6377304.063||300.8017|
|Everest 1830 (modified for West Malaysia)||6377295.664||300.8017|
|Everest 1830 (modified for Timbalai)||6377298.556||300.8017|
|Fischer 1960 (modified for South Asia)||6378155.0||298.3|
|IAG 75||6378140.0||298.257222 |
|New International 1967||6378157.5||298.25|
|ATS 77 (Average Terrestrial System 1977)||6378135.0||298.257|
The shift and rotation parameters describe the ellipsoid's orientation in space, as compared to the WGS 84 datum. It's important to make sure that these parameters have the correct signs (positive or negative). Usually, a document describing a local datum will list the parameters required to convert coordinates from the local datum to WGS 84. (This is the same as saying that the parameters were derived by subtracting the local datum from WGS 84.) In that case, use the parameters exactly as they appear in the document. However, if a document that lists parameters for converting coordinates in the opposite direction — from WGS 84 to the local datum — then reverse the signs of the shift, rotation, and scale correction parameters.
It's also very important to list the parameters in the correct order. Some documents list the rotation parameters with EZ first, like this: EZ, EY, EX. In those cases, reverse the order of the rotation parameters when defining the custom datum. This is especially easy to overlook when the document uses Greek letters to denote the parameters. If the document lists the parameters in order as , , , then reverse their order in the custom datum definition.
Here's an example of a local datum description (LD-1) as it might appear in a technical article:
LD-1 ellipsoid: International
a 6378388.0 m
f 1 / 297.0
Converting from WGS 84 to LD-1
X 93.5 m
Y 103.5 m
Z 123.3 m
m –2.1 ppm
This datum uses the International ellipsoid, which is number 4 in the ellipsoid table above. The other parameters describe a conversion from WGS 84 to the local datum, so it is required to reverse their signs. No prime meridian is listed, so assume that Greenwich is used. The custom datum definition in MapInfo would look like this:
9999, 4, -93.5, -103.5, -123.3, 0.25, -0.11, -0.07, 2.1, 0
Insert this string of numbers in place of the datum number in any line in the MAPINFOW.PRJ file. For example, define the following coordinate systems using this custom datum: (NOTE: Strings must be entered on a single line).
”Longitude / Latitude (LD-1)”, 1, 9999, 4, -93.5, -103.5, -123.3, 0.25, -0.11, -0.07, 2.1, 0
”UTM Zone 30 (LD-1)”, 8, 9999, 4, -93.5, -103.5, -123.3, 0.25, -0.11, -0.07, 2.1, 0, 7, -3, 0, 0.9996, 500000, 0
- The first example is simply the Name of the new coordinate system followed by its Projection number (1) for Longitude\Latitude and then 9999 for a custom datum followed by the datum definition parameters.
- The second example lists the Name, its Projection number (8) for Transverse Mercator, 9999 for a custom datum, the required datum parameters (see above), then the coordinate system components for a UTM projection (Units, Origin Long., Origin Lat., Scale factor, etc.). See Appendix F for UTM components.
Here's another sample local datum description, called LD-2 this time:
LD-2 ellipsoid: Krassovsky
a 6378245.0 m
f 1 / 298.3
Converting from LD-2 to WGS 84
u +24 m
v –123 m
w –94 m
m +1.1 · 10-6
This datum uses the Krassovsky ellipsoid, which is number 3 in the ellipsoid table above. It is not required to reverse the signs of the parameters since they describe a conversion from the local datum to WGS 84. However, the rotation parameters are listed with or Z first, so we must reverse the rotation parameters order in the custom datum definition:
9999, 3, 24, -123, -94, -0.02, 0.25, 0.13, 0.0000011, 0
Here's a final example, LD-3, that provides only the ellipsoid and shift parameters:
LD-3 ellipsoid: Clarke 1880
a 6378249.145 m
f 1 / 293.465
WGS 84 minus local datum LD-3
dX –7 m
dY 36 m
dZ 225 m
This datum uses the Clarke 1880 ellipsoid, which is number 6 in the ellipsoid table above. It is not required to reverse the signs of the parameters or worry about the order of the rotation parameters (since they aren't present). In this case, use datum number 999 instead of 9999 in the custom datum definition. These two definitions are equivalent, and use either one as the second example will ignore the zeros for the scale factor and rotation parameters which are not used here:
999, 6, -7, 36, 225
9999, 6, -7, 36, 225, 0, 0, 0, 0, 0
As with the other custom datum definitions, insert one of these custom datum definitions in place of the datum number after the name and projection number in the proper MAPINFOW.PRJ line, followed by the coordinate system components necessary for this projection from the Appendix, as in these examples:
"Longitude / Latitude (LD-3)”, 1, 999, 6, -7, 36, 225
”UTM Zone 30 (LD-3)”, 8, 999, 6, -7, 36, 225, 7, -3, 0, 0.9996, 500000, 0
When converting coordinates from one datum to another, MapInfo 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's 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.
Beginning with MapInfo 4.1.2, 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 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:
If this entry is set to zero, then the Molodensky conversion method will be used instead of NADCON. If using the 16-bit version of MapInfo, add this line to the [MAPINFO COMMON] section in MAPINFO.INI to turn off the NADCON conversion:
Last Modified: 06/09/2005 11:36:44 AM