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

[Martin Desruisseaux]

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:*


*Same coordinate transformations using late-binding approach:*

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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