[Proj] Using Proj.4 to compress a 10-parameter Molodensky-Badekasdatum shift into 7 parameters

[Noel Zinn (cc)]

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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