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

[Noel Zinn (cc)]

Answers to these questions ...

> But where does it go wrong?
> - Are all parameters passed correctly? Is somewhere a lot of precision 
> lost
> because at a higher level not enough decimals are set?
> - Are the global structs filled correctly by initialization functions?
> - Is the software used as it should be? Take cs2cs: you can do incredible
> things with it, but next to nothing is documented (see its man page).

... require Proj.4 expertise.  I defer to you, Oscar, and to Mikael and to 
Frank.

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: Sunday, October 17, 2010 10:16 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-Badekasdatum 
> shift
> into 7 parameters
> Noel Zinn (cc) ndzinn at comcast.net
> Sun Oct 17 07:44:35 EST 2010
>
> .....
>
> To help me understand whether we agree of not, let me decompose Mikael
> Rittri's problem simply.
>
> A 7-parameter datum transformation (geodetic <=> geodetic) has 3 parts: 
> (1)
> geodetic => geocentric, (2) matrix manipulation on 3D geocentric (ECEF)
> coordinates involving translation, rotation and scale change, and then (3)
> geocentric => geodetic (which is the only inexact piece of the process, 
> but
> you can get any precision you desire).
>
> .......
>
>
>> Let's see. In the problem at hand my Molodenksy-Badekas routine arrives 
>> at
>> an X,Y,Z of:
>> X = 2464278.0897929626; Y = -5783490.396202258; Z = 974642.3859339222;
>
> So, we agree then?  The Proj.4 geocentric => geocentric routine (cs2cs -f
> "%.3f" +proj=geocent +towgs84=0,0,0,5.226,1.238,-2.381,-5.109 +to
> +datum=WGS84 +proj=geocent) has a decimetric difference with your code, my
> code and with Blue Marble.
>
> I have verified that this difference is not due to the difference between
> the single rotation matrix normally used (small angle approximation) and 
> the
> 3 concatenated rotation matrices used by ESRI (for example), which is only
> millimetric in quantity.  Is it something going on under the hood in 
> Proj.4,
> like an embedded call to geodetic <=> geocentric for some reason?  I don't
> know.  We could debate whether 19 centimeters in Y and 12 centimeters in Z
> is significant or not.  For a straight-up matrix manipulation, I think it
> is.
>
> Regards,
> Noel
>
> ---------------------------------
>
> REPLY:
>
> We do agree. Something is somewhere wrong. But what and where?
> If I were to get a bug report stating that in a procedure spanning several
> operations the result departs from the expected value, I would throw this
> report away.
> Every single step should be tested against independent results from 
> others,
> not just the end result of a concatenation of a handful of steps.
>
> Several steps are clear, for instance:
> - The right choice of rotation model is made.
> - BTW: rotation order has no effect if you work with approximations in the
> rotation matrices, like PROJ does.
> - The datum transformation functions of PROJ seem to be correct.
> - At least the core of the geocentric->geodetic conversion function is OK.
>
> But where does it go wrong?
> - Are all parameters passed correctly? Is somewhere a lot of precision 
> lost
> because at a higher level not enough decimals are set?
> - Are the global structs filled correctly by initialization functions?
> - Is the software used as it should be? Take cs2cs: you can do incredible
> things with it, but next to nothing is documented (see its man page).
>
> Oscar van Vlijmen
>
>
>
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