Check with Al Gore, the tracker of missing EMails. He could use some of the
previous comments in one of his invigorating stumped speeches.
----- Original Message -----
From: Kumar, Muneendra <[EMAIL PROTECTED]>
To: Couch, David L. <[EMAIL PROTECTED]>
Cc: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>; 'Bill Thoen'
<[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>; Morrison, Doug <[EMAIL PROTECTED]>;
Wiley, Barbara <[EMAIL PROTECTED]>; Murdoch, David <[EMAIL PROTECTED]>;
McCleary, Dennis <[EMAIL PROTECTED]>; Weigel, Gary R. <[EMAIL PROTECTED]>;
Kumar, Muneendra <[EMAIL PROTECTED]>
Sent: Wednesday, June 21, 2000 10:07 AM
Subject: RE: (GIS-L) Re: Meaning of "MI" Parameters
>
> 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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