Clifford,
I'm certainly no expert on Geodesy, but I think you didn't get
all the parameters from whoever asked you for help. Seems to me
that all the factors you claim are missing are, in fact, in the
MapInfo projection file. Perhaps a scale factor of zero is what
you mean by a missing (or at least "truncated") value, but it
*is* there.
Adding comments to the the Luxembourg (Bursa-Wolf) parameters
record listed in mapinfow.prj (see appendix H and I in the latest
MapInfo User's Guide) I see:
"LuxGrid (Bursa-Wolf)",
8, {projection 8 = Transverse Mercator/Gauss Kruger}
9999, {means "custom datum"}
4, {ellipsoid: 4=International 1924, a=6378388.0,
1/f=297.0}
-185.836, {shift dX, in meters}
13.479, {shift dY, in meters}
-14.527, {shift dZ, in meters}
0.441203, {rotation Ex, in arc-seconds}
3.027399, {rotation Ey, in arc-seconds}
-2.607685, {rotation Ez, in arc-seconds}
0, {scale correction factor m, in parts per 10^6}
0, {longitude of prime meridian}
7, {units 7 = meters}
6.166666667, {origin Longitude}
49.83333333, {origin Latitude}
1, {scale factor}
80000, {false easting}
100000 {false northing}
The MI book doesn't say what in which order the scaling, rotation
and translation are done, but I assume the order for performing
the transform is scale, rotate and then translate since these
ellipsoids have a common center.
According to the manual, the "Simplified Bursa Wolf" is
"truncated" because that particular transform assumes that only
an ellipsoid and shift parameters are needed.
So what should the scale factor be for Luxembourg (Bursa-Wolf)
transformation? And are the parameters for the "International
1924" datum the same as the "European Datum 1950" you cite below?
If not, I need to correct my settings. Also to satisfy my
curiosity, I'd like to try out a couple of coordinate transforms
using MapInfo's parameter file (set up properly) against whatever
you use to see how "grossly inaccurate" MapInfo is. I'm not
convinced that "computer jockeys" are entirely ignorant
knowlessmen.
Regards,
- Bill Thoen
P.S. Good flame, full marks for rebarbative epithets!
[EMAIL PROTECTED] wrote:
> The parameters you sent are an example of a MI projection file for
> Luxembourg, and it is rather complicated. Bursa-Wolf is the name of a
> mathematical model for performing transformations from one specific Datum
> to another specific Datum. In this case, it refers to the transformation
> from European Datum 1950 to the World Geodetic System 1984 (WGS84) used by
> GPS satellites. The parameters are in meters and are earth-centered in
> the Geocentric Coordinate System and refer to a dX,dY,dZ translation at the
> center of the earth AFTER a three-parameter rotation RZ,Ry,Rx (in radians
> or arc seconds) and an overall scalar (dimensionless) multiplication is
> performed FIRST.
>
> What is remarkable is that the "MI example file" is grossly incorrect!
> That is indicated by the fact that only six of the seven parameters for a
> Bursa-Wolf Transformation are given, (the Scale Factor is not even given),
> and for the "(Simplified Bursa-Wolf)" it is an obvious truncation!!
>
> This example file is an excellent lesson of what happens when programmers
> and mathematicians try to play with Geodesy - they obtain the usual result;
> a complete screw-up! Apparently unknown to some "computer jockeys," the
> solution for the seven parameters of a Bursa-Wolf model or a
> Molodensky-Basdekas model always involves a solution for the rotations and
> the scalar FIRST - THEN the translation parameters are solved. That means
> that the rotation parameters (and scale factor) may NEVER BE TRUNCATED!!!
> To the neophyte (read "computer jock"), the rotation angles look so small
> that they appear to be insignificant. However to one that works with the
> math of the actual parameter solutions, one knows that the tiny rotations
> are at the center of the earth! When you examine the effect at the surface
> of the ellipsoid - the magnitude is quite substantial! Same thing goes for
> the scale factor expressed as so many parts per million. Tiny numbers with
> negative exponents, but when you consider the semi-major axis is
> 6,378+ kilometers; the effect is again quite substantial! Transformations
> are affected by such blunders and resultant data will be grossly incorrect.
>
> Therefore, in the "example MapInfo file for simplified Bursa-Wolf
> parameters," the only thing simple is the simpleton that truncated the
> critical parameters!
>
> In regard to your local problem of implementing such data for your country,
> I shall be happy to assist you in determining the appropriate parameters if
> you send your data to me via e-mail. I will then solve for a 3-parameter
> model, a 4-parameter model, and a 7-parameter model and provide you with
> the accuracy results for each model solved.
>
> Consider looking at some of my past columns on Grids and Datums of
> different countries at:
>
> http://www.ASPRS.org/resources.html
>
> Please let me know if I may be of further assistance, and consider joining
> the American Society for Photogrammetry and Remote Sensing (ASPRS).
>
> Good luck!
>
> Clifford J. Mugnier ([EMAIL PROTECTED])
> Surveying, Geodesy, and Photogrammetry
> LOUISIANA STATE UNIVERSITY
> 12408 CEBA Building
> Baton Rouge, Louisiana 70803
> Voice and Facsimilie: (225) 388 - 8536
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