Pitney Bowes MapXtreme - Manual (Page 582)

Appendix H: Elements of a Coordinate System
Custom Datums
MapXtreme v7.1 589 Developer Guide
Defining Custom Datums
Most coordinate systems use one of Pitney Bowes Software’s predefined datums, listed in this
appendix. If you need to use a datum that isn’t in the list, and you know what the datum’s
mathematical parameters are, then you can define the coordinate system using a custom datum.
MapInfo uses the following information to define a datum:
• 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
next table.
• 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. You may also see them denoted
by DX, DY, and DZ, 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. You may also see
them denoted by eX, eY, and eZ, or by e, y, and w.
• 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 k.
• 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 you use the NTF datum in a coordinate system, all
longitudes in that coordinate system are relative to Paris instead of Greenwich.
You can define a custom datum in any coordinate system definition. 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, you can use datum number 999
instead of 9999, to simplify the definition:
999, EllipsoidNumber, dX, dY, dZ
The ellipsoid number must be chosen from the following list. Currently, there is no way to define a
custom ellipsoid. If you need to use an ellipsoid that does not appear on this list, please notify
MapInfo Technical Support so that we can add your ellipsoid to a future MapInfo release.
Number Ellipsoid a 1/f
9 Airy 1930 6377563.396 299.3249646
13 Airy 1930 (modified for Ireland 1965) 6377340.189 299.3249646
51 ATS7 77 6378135.0 298.257
2 Australian 6378160.0 298.25