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

[Sebastiaan Couwenberg]

On 02-02-16 15:49, Howard Butler wrote:
> 
>> 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? 

I don't know, but Lennard Huisman <Lennard.Huisman at kadaster.nl> from
Kadaster has been very helpful on the Dutch list with all questions and
issues with the RD/NAP datum shift grids. He may be able to answer that
question.

>> 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.

Thanks for the pushing and merging the fix this quickly. I can confirm
that it fixes the issue for me.

Kind Regards,

Bas

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