[Proj] PROJ.4 4.9.2 and transformation of points outside datum shift grid

Howard Butler howard at hobu.co
Tue Feb 2 09:49:48 EST 2016

> On Feb 2, 2016, at 4:52 AM, Even Rouault <even.rouault at spatialys.com> wrote:
> Looking at the grid (downloaded from 
> https://github.com/NaturalGIS/ntv2_transformations_grids_and_sample_data/blob/master/nl/naptrans2008.gtx 
> ), it appears it doesn't use the official null value -88.88880f  but various big 
> negative values like -2147479936, -2147479808 and several others as well.

Is there a way to find out why these grids are doing their own thing for nodata? 

> Checking >1000 or <-1000 is somewhat arbitrary, but I'm not aware of vgrids 
> that have such big offsets (at least on Earth), so it is likely safe. The 
> original ticket https://github.com/OSGeo/proj.4/issues/271 doesn't report that 
> they can cause problems in practice.
> Howard ?

I saw the patch in 271 and the link to the GTX doc and reasoned that the code didn't seem to be following the spec.  That's why I applied it.

I think we should implement it with support for both, as you've pushed.

I plan to work with you at the Paris Code Sprint [1] to push out a release of Proj 4.9.3. There are already ~15 issues closed [2] for that upcoming release. We can knock out a few more and make a release candidate by the end of that week.

[1] https://wiki.osgeo.org/wiki/Paris_Code_Sprint_2016
[2] https://github.com/OSGeo/proj.4/issues?q=milestone%3A4.9.3+is%3Aclosed


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