All:
I was in the middle of writing when suddenly my e-mail diappeared
from my computer. My search has not failed and I do not find any where !!
If you got it, please ignore it. The COMPLETE message is coming.
Muni.
> -----Original Message-----
> From: Couch, David L.
> Sent: Wednesday, June 21, 2000 8:58 AM
> To: 'Bill Thoen'; [EMAIL PROTECTED]
> Cc: [EMAIL PROTECTED]; [EMAIL PROTECTED]; Kumar, Muneendra
> Subject: RE: (GIS-L) Re: Meaning of "MI" Parameters
>
> 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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