All:
I should point out to everyone that the Bursa-Wolf transformation procedure
is not recommended for local datums (Muneendra Kumar, personal
communication). This is due to a couple of different problems when
estimating the parameters themselves. First, the parameters tend to be
highly correlated when solved simultaneously. Solving the rotations
separately from the translations helps to reduce the correlation. Kumar
wrote a paper on how to do this.
Secondly, there is the geometric problem of using Cartesian coordinates.
When computing WGS84 Cartesian coordinates, the EGM96 geoid model should be
included. That is the ellipsoid height is equal to the sum of the
orthometric height (approximately equal to the height above the MSL) and
the geoid height. The ellipsoid height is what must be used when computing
Cartesian coordinates. This is straight forward for WGS84 values: compute
the EGM96 geoid and add it to the elevation and use the result for the
Cartesian conversion.
But what about local datums such as ED50? The geoid is still required when
computing Cartesian coordinates. If the local geoid is ignored, the
systematic error can be significant for "some" applications. For the vast
majority of applications, I would suggest using the Molodensky equations as
per NIMA Technical Report 8350.2. They are easy to use. Those of you who
are interested might see the Ohio State Report by Badenkas. The publication
date is around 1970. He details the use of 7-parameter solutions for local
datums.
Whatever the case, a zeroed out Scale factor should not be used. This too
could be a fairly significant error.
Take care!,
Dave
----------------------------------------------
David Couch,
Gateway Operations Engineer, NIMA
314-260-5073
[EMAIL PROTECTED]
> -----Original Message-----
> From: Bill Thoen [SMTP:[EMAIL PROTECTED]]
> Sent: Tuesday, June 20, 2000 2:54 PM
> To: [EMAIL PROTECTED]
> Cc: [EMAIL PROTECTED]; [EMAIL PROTECTED]
> Subject: (GIS-L) Re: Meaning of "MI" Parameters
>
> ---------------------------------------------------------------------
>
>
>
> 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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