[Proj] WGS84 to ED50

Clifford J Mugnier cjmce at lsu.edu
Mon Nov 19 17:43:39 EST 2007

I have been reading this thread with some amusement, as datum shifts are only a compromise to begin with, and attempting to achieve better than 1-2 meters with even a 7-parameter transformation is a totally unreasonable expectation.  The original data for the classical datum is not that good!
Matching another transformation is a reasonable expectation, but it could be (likely), that the "client" is attempting to match actual observations with Trimble/Applanix gear working directly in WGS84 rather than a Trimble/Applanix algorithm!  European Datum 50 (ED50) was "cooked up" in 1950 by Geodesists at the U.S. Army Map Service after WWII using geodetic surveys of Spain and Portugal performed over the prior 50-75 years!  That stuff wasn't that good!
Try reading my columns on the Grids and Datums of Spain and Portugal at:  www.ASPRS.org/GRIDS  and you'll get a better appreciation of the complexity of the problem being sought with a mere 7-parameter transformation.  (Spain is July, 2000, and Portugal is April, 2002)
Combining Applanix and Trimble gear is cutting-edge geodetic equipment commonly used for Real Time Kinneatic (RTK) Aerial Photography for hair-splitting accuracy.  That stuff is working directly in WGS84 Broadcast Ephemeris and is then re-adjusted in post-processed mode with the Precise Ephemeris a couple weeks later.
That's NOT going to "match" data that's 75-100 years old!
PROJ4 is a cartographic tool and is not a geodetic panacea.
Clifford J. Mugnier, C.P., C.M.S.
LSU Student ASCE Chapter Faculty Advisor
National Director (2006-2008),
Photogrammetric Applications Division
American Society for Photogrammetry and Remote Sensing 
Chief of Geodesy,
Department of Civil Engineering 
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http://www.asprs.org/resources/GRIDS/ <https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=https://email.lsu.edu/exchweb/bin/redir.asp?URL=http://www.asprs.org/resources/GRIDS/> 


From: proj-bounces at lists.maptools.org on behalf of Roger Oberholtzer
Sent: Mon 19-Nov-07 16:24
To: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] WGS84 to ED50

On Mon, 2007-11-19 at 13:57 -0800, Hamish wrote:

> > The Applanix provides this information on their calculations:
> ...
> >  Coordinate transformation from WGS84 to mapping frame datum
> >     dX = 125.098545; dY = 76.000054; dZ = 156.198703; f =
> > 0.999991695369;
> >     R1 = 0.000000000000; R2 = 0.000000000000; R3 = -0.000005473550;
> The above R3,f values makes me hesitate a little, but it may be that Applanix
> and Trimble are both preforming a less precise 3-term transform, while PROJ is
> doing a more precise 7-term transform. That would certainly explain a 1-4m
> error.

My understanding is that Applanix is doing a 7 parameter transform. What
I am unsure of is how to get proj to do the same. If it is the default,
then I am curious how the 7 parameters are specified. Are these the
values in the +towgs84 option? I have another library for this that is
using 7 config values for its 7 parameter transform, and I would expect
to see similar values in proj.

> Rather than trust either software or method implicitly, it would be better to
> judge the results based on published test points from the Spainish and/or
> Portuguese government mapping office, and double check the validity of the
> 7-terms params from an official document too. Depending on national convention,
> the +- signs of the terms may or may not be switched.

I have been trying to do just that. The person I am doing this for in
Madrid has provided the test values I am trying to duplicate. Anyone
know of an official source of test values for Spain?

Roger Oberholtzer

OPQ Systems / Ramböll RST
Ramböll Sverige AB
Kapellgränd 7
P.O. Box 4205
SE-102 65 Stockholm, Sweden

Tel: Int +46 8-615 60 20
Fax: Int +46 8-31 42 23

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