[Proj] Using Proj.4 to compress a 10-parameter Molodensky-Badekasdatum shift into 7 parameters
Noel Zinn (cc)
ndzinn at comcast.net
Sat Oct 16 09:42:42 EST 2010
Thanks for working this through. Comments follow.
Oscar van Vlijmen wrote:
> I have no difficulties reproducing the EPSG example exactly with my own
> Molodensky-Badekas routines or with Mikaels procedure around an M-B
> corrected set of dX,dY,dZ for a 7-parameter datum shift.
Nor do I. They agree at the evaluation point (maybe not elsewhere).
> Please note that M-B parameters are usually stated as for a coordinate
> frame
> rotation model, whereas PROJ4 uses a position vector rotation model.
> One shouldn't always blame PROJ4.
Understood. Mikael made that clear. That's not the issue here.
> I haven't tested the geocentric -> geodetic code from PROJ4's geocent.c,
> but
> it certainly looks good.
This may be the issue. A quote from Mikael Rittri follows:
> i) Make a 7-parameter "something" by temporarily replacing
> the question marks by zeros. It is not really a datum
> shift, but we might call it a 7-parameter compressor.
> Use it in a cs2cs command like this
>
> >cs2cs -f "%.3f" +proj=geocent +towgs84=0,0,0,5.226,1.238,-2.381,-5.109
> +to +datum=WGS84 +proj=geocent
>
> As input, use the evaluation point eX,eY,eZ of the M-B datum shift:
>
> 2464351.59 -5783466.61 974809.81
>
> We get the output
>
> 2464278.090 -5783490.207 974643.507
I contend that the correct output (delivered by my Matlab scripts and by
Blue Marble) is
=> 2464278.090 -5783490.396 974642.386 (to the same precision)
If my assertion is correct, then Proj.4 has a slight problem. It's not
blame, just a request for an independent verification of this particular
step (i) and (if verified) another possible improvement for Proj.4.
Noel
Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)
--------------------------------------------------
From: "OvV_HN" <ovv at hetnet.nl>
Sent: Saturday, October 16, 2010 7:56 AM
To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>
Subject: [Proj] Using Proj.4 to compress a 10-parameter
Molodensky-Badekasdatum shift into 7 parameters
> In reply to:
>
> [Proj] Using Proj.4 to compress a 10-parameter Molodensky-Badekas datum
> shift into 7 parameters
> Noel Zinn (cc) ndzinn at comcast.net
> Fri Oct 15 13:57:24 EST 2010
>
> Your provision of a worked example "compressing" a M-B transformation into
> a
> 7-parameter transformation is a real contribution. Thanks ... and thanks
> to
> Melita Kennedy, too.
>
> I've coded this example in Matlab and agree with your final results
>> 66d4'54.705"W 9d34'49.001"N 180.499
>
> Nevertheless, I disagree with your first intermediate computation
>> 2464278.090 -5783490.207 974643.507
>
> My intermediate results are
> => 2464278.08979296 -5783490.39620226 974642.385933922
>
> etc...
>
> REPLY:
>
> I have no difficulties reproducing the EPSG example exactly with my own
> Molodensky-Badekas routines or with Mikaels procedure around an M-B
> corrected set of dX,dY,dZ for a 7-parameter datum shift.
> Please note that M-B parameters are usually stated as for a coordinate
> frame
> rotation model, whereas PROJ4 uses a position vector rotation model.
> One shouldn't always blame PROJ4.
> I haven't tested the geocentric -> geodetic code from PROJ4's geocent.c,
> but
> it certainly looks good.
>
> Oscar van Vlijmen
>
>
>
>
>
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