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

[Sebastiaan Couwenberg]

[Resending without attachments to keep size below threshold]

Debian Bug #813066 [0] revealed a bug in the test handling of the
conversion between RD/NAP and ETRS89 when PROJ.4 4.9.2 is used.

Previous versions of PROJ.4 were unable to transform points outside the
available datum shift grid, as can be seen in the output of the test
script [1] in the file testrdtrans2008-4.9.1 [2].

With version 4.9.2 some points can now be calculated as seen in the
file testrdtrans2008-4.9.2 [3], but for tests 07 to 10 (whose points
"are outside the region where interpolation between either the NLGEO2004
geoid or the RD correction grid points is possible") in the direction
from RD/NAP to ETRS89 the results are entirely incorrect, whereas the
results for those same tests in opposite direction are mostly correct
except test 10 (edge_rd).

The documentation accompanying the rdtrans2008.gsb NTv2-grid and
naptrans2008.gtx VDatum-grid [4] mentions that the points for test 07 to
10 are outside the correction grid and that their "RD coordinates can be
computed, but the output should be handled with care".

With earlier versions there was no usable output, 4.9.2 reports
incorrect values (for some tests).

It looks like the fix for #292 [5][6] is the cause of this change. I'm
not sure whether this change is a bug or a feature.

In de debug logging for 4.9.2 we can see that for tests 07 to 10
pj_apply_vgridshift is no longer called, instead pj_apply_gridshift is
used. For tests 07 to 10 no z value is specified, so this looks like the
intended course of action to not apply the vertical grid shift.

On the other hand, the previous behaviour caused pj_apply_vgridshift to
log correctly that it failed to find a grid shift table for the location.

How should points outside datum shift grids be handled by PROJ.4?

The previous behaviour seems more correct, but with 4.9.2 some
transformations can be done that weren't before.

[0] https://bugs.debian.org/813066
[1]
https://sources.debian.net/src/proj-rdnap/2008-4/debian/patches/add-test.patch/
[2] http://linuxminded.nl/tmp/testrdtrans2008-4.9.1
[3] http://linuxminded.nl/tmp/testrdtrans2008-4.9.2
[4]
https://sources.debian.net/data/non-free/p/proj-rdnap/2008-4/Use%20of%20RDTRANS2008%20and%20NAPTRANS2008.pdf
[5] https://github.com/OSGeo/proj.4/issues/292
[6]
https://github.com/OSGeo/proj.4/commit/5d2af3a89be0898458e4f0fe272affc36642d304

Kind Regards,

Bas

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