[gdal-dev] Relation between PROJ8, GDAL, and national grids

[Juan Pedro Pérez Alcántara]

Even, thanks for replying, and Javier, thanks for the advice about the list
etiquette. I will not change the subject in such events.

I'm sure it's the only PROJ installation because I'm building the libraries
in a Docker container. I've also conducted some more tests from zero, I've
found out I've been using some old tests I wrote some time ago that were
outdated and faulty, sorry about that. What follows is the detailed actions
I've taken.

With the IGN (Spanish National Geographic Institute) geodesic calculator I
performed a EPSG 23030 (ED50 UTM 30N) to EPSG 25830 (ETRS89 UTM 30N) shift:

235205.243 4142110.093  >  235094 4141906

I take this as the target result.

I set PROJ_LIB to /usr/local/share/proj and configured PROJ with a simple
./configure, no options.

Using cs2cs:

cs2cs EPSG:23030 EPSG:25830 > 235094.09 4141905.02

which I suppose is a 7 parameters shift, which is great, but not perfect,
can do better with the grid. Thus:

cs2cs +proj=utm +zone=30 +ellps=intl +units=m +no_defs
+nadgrids=/usr/local/share/proj/es_ign_SPED2ETV2.tif +to EPSG:25830 >
235094.00 4141906.00

which nails it. Again, my fault assuming PROJ was using the grid by default.

Then I compiled GDAL with another flat ./configure, it found a PROJ >= 6.
Using gdaltransform:

gdaltransform -s_srs EPSG:23030 -t_srs EPSG:25830 > 235094.087184815
4141905.01683498

which is consistent with cs2cs. Using:

gdaltransform -s_srs "+proj=utm +zone=30 +ellps=intl +units=m +no_defs
+nadgrids=/usr/local/share/proj/es_ign_SPED2ETV2.tif" -t_srs EPSG:25830 >
235094.000297496 4141905.99963473

which is consistent with cs2cs too.

Then, having solved this part of the issue, I would like very much if you
can advice me on how to "tweak" default definitions of the SRS to include
the use of the grid by default, which would be not absolutely necessary for
my application but a nice to have.

Also, just out of curiosity, why are PROJ and GDAL yielding slightly
different outputs (albeit perfectly ok both)? Is GDAL doing its own
calculations besides PROJ? Forgive my lack of deep understanding of the
GDAL inner architecture for I am an end user and not a developer. Just
curious.


Best regards,

---

Juan Pedro Pérez Alcántara

jp.perez.alcantara at gmail.com


On Sat, 27 Mar 2021 at 21:32, Even Rouault <even.rouault at spatialys.com>
wrote:

> Juan Pedro,
>
>
> >
> > I'm at a loss trying to understand the relationship between PROJ8,
> > GDAL, and how to use TIFF national grids. So far my findings are:
> >
> > - PROJ8 is installed and cs2cs uses the grid to transform, performing
> > an extremely accurate shift;
> >
> > - GDAL, compile on the above PROJ8, when using
> > gdaltransform, yields less accurate
>
> There's no fundamental reason for transforms with gdaltransform being
> less accurate than with cs2cs. You have to be much more specific with
> the exact gdaltransform invokation you are trying. And are you really
> sure that you use the PROJ version you think ? (check with ldd
> libgdal.so on Linux)
>
> You can also set the PROJ_DEBUG=2 and CPL_DEBUG=ON environment variables
> and look at the traces.
>
> Even
>
>
> --
> http://www.spatialys.com
> My software is free, but my time generally not.
>
>
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