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Norm Explains Molodensky
vs. 3 Parameter
The Molodensky Transformation is widely used to convert geographic coordinates
from one datum to another. Unfortunately, Casual Cartographers have no way to
determine the Molodensky parameters for a specific situation. If some
geodesist hasn't already computed for you, you're out of luck. This program
addresses this problem.
While the Molodensky Transformation is a widely used transformation technique,
it is not the only one. One of the other techniques which is rarely used is the Three
Parameter Transformation. While the Three Parameter Transformation is not generaly
considered to be quite as good as the Molodensky, we mention it here for two reasons.
First, computing the parameters necessary is rather easy to accomplish. Second, for
a small region of geography, the Three Parameter Transformation parameters can be a good
approximation of the Molodensky.
Therefore, we suggest that you use the accompanying program to compute the Three
Parameter Transformation parameters, and use them as Molodensky parameters in programs
such as Tralaine. We believe that the results will be quite good, especially if the
geographic region which is being covered is small, i.e. not more than several degrees.
Certainly, the results will be better than doing nothing at all, which is often the
only alternative.
A control point is a point at which you happen to know what the geographic
coordinates are ( and optionally the ellipsoidal height) in the (as yet) undefined datum
and a defined datum (usually WGS84). The term registration point is also used to
refer to such a point. If you have at least one control point and know the
ellipsoids upon which the undefined and reference datum are based, you can use this
program to calculate the Three Parameter Transformation parameters (delta X, delta Y, and
delta Z) necessary to create a definition for the undefined datum.
If you have more than one control point, you can expect to get slightly
different results for each point. In this case, you may wish to use an average of
all control points to create the new datum definition.
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August 1999
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