[MetaCRS] Fwd: Re: [Proj] Time dependent coordinate transformations

[Chris Crook]

Hi Martin

Thanks for the pointers.  The ambiguity I was hinting at was not just to do with converting from one coordinate system to another.  It also relates to converting a data set from one coordinate system to another, and then to a third versus converting directly from the first to the third, or for that matter from one to another and then back again.

Depending on how the late binding tables are constructed there would be no assurance that these transformations would give consistent results.  Indeed, I suspect that if mathematically if both these were enforced then that would be equivalent to using an arbitrary pivot coordinate system, though of course there is no requirement that it is encoded this way.  A tree like heirarchy is another working option.

Of course there is a philosophical question as to whether coordinate transformations should give unique or consistent results.  But in many cases this is a practical requirement (eg on the fly transformations in GIS systems)

Cheers
Chris
______________________________________
From: Martin Desruisseaux [martin.desruisseaux at geomatys.fr]
Sent: 09 January 2015 08:34
To: Chris Crook; metacrs at lists.osgeo.org
Subject: Re: [MetaCRS] Fwd: Re: [Proj] Time dependent coordinate transformations

Hello Chris

Le 08/01/15 19:13, Chris Crook a écrit :

While I can see that "late binding" is an alternative to a pivot coordinate system, it seems to invite issues with non-uniqueness, depending on how it is implemented.  Are you able to point me to any references on this please?



The "Early binding" versus "Late binding" approaches are briefly discussed in the Geospatial Integrity of Geoscience Software (GIGS) tests:

http://www.iogp.org/geomatics#2521115-gigs part 1 section 3.4

As mentioned in the above paper, the late-binding approach is a way to resolve (not invite) the non-uniqueness problem. Indeed, my experience in comparing Proj.4 (which in my understanding uses early-binding) with a software implementing the late-binding approach shows slightly different results. Below are some 2-3 years old screenshots I did for GeoAPI (I did not verified if the numbers would be the same today):

Coordinate transformations using Proj.4 (through JNI) 3 years ago:

[cid:part1.03070208.04050207 at geomatys.fr]

Same coordinate transformations using late-binding approach:

[cid:part2.02030309.02090306 at geomatys.fr]

The transformation results are slightly different because there is at least 3 different ways to perform a datum shift between NTF and WGS84: Geocentric translation, Molodensky and Abridged Molodensky. If my memory serves me right, Proj.4 uses Geocentric translation, which is indeed the most accurate transformation method among those 3. But it is not the method mandated by the French mapping agency (IGN), which (if I remember correctly) rather specify that we shall use a Molodensky method. For interoperability with the maps produced by authorities, sometime we have to use the same transformation method that they use, regardless if their method is the most accurate or not.

Note that I verified that forcing the above late-binding implementation to the same transformation method than Proj.4 produced the same results.

The EPSG database contains a table that specify which transformation methods (not only the parameters) to use for various (sourceCRS, targetCRS) pairs. The method from NTF to WGS84 may not be the same than the method from NTF to another CRS. When using that EPSG table, there is no ambiguity. So I think we could summarize the difference between early-binding and late-binding approaches by saying that the late-binding approach uses that table, while the early-binding approach ignores it. Of course the table can not contain all possible (sourceCRS, targetCRS) pairs, so we have to fallback on an early-binding approach if we can not find an entry for our particular pair of CRS (or sometime on a mixed approach, trying to find another pair of CRS which exist in the database).

    Regards,

        Martin



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