Pitney Bowes MapXtreme - Manual (Page 585)

Appendix H: Elements of a Coordinate System
Custom Datums
MapXtreme v7.1 592 Developer Guide
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, you can use the parameters
exactly as they appear in the document. However, if you have a document that lists parameters for
converting coordinates in the opposite direction — from WGS 84 to the local datum — then you must
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, you must reverse the order of the
rotation parameters when defining the custom datum. This is especially easy to overlook when your
document uses Greek letters to denote the parameters. If the document lists the parameters in order
as w, y, e, then you must reverse their order in the custom datum definition.
Here’s an example of a local datum description (we’ll call it LD-2) as it might appear in a technical
article:
LD-2 ellipsoid: Krassovsky
Converting from LD-2 to WGS 84
This datum uses the Krassovsky ellipsoid, which is number 3 in the ellipsoid table above. We do not
need 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 w first, so we must reverse their order
in the custom datum definition:
9999, 3, 24, -123, -94, -0.02, 0.25, 0.13, 1.1, 0
Here’s a final example, LD-3, that provides only the ellipsoid and shift parameters:
LD-3 ellipsoid: Clarke 1880
a 6378245.0 m
f 1 / 298.3
u +24 m
v –123 m
w –94 m
w +0.13
y +0.25
e -0.02
m +1.1 Ю 10-6