[GRASS-dev] PROJ6 support in GRASS

[Markus Metz]

Several different methods might be available to reproject from one CRS to
another CRS. PROJ6 can select the most appropriate method if a bounding box
is provided. This means that the selected method depends on the bounding
box and that results of a reprojection can differ depending on the provided
bounding box.

In GRASS, this bounding box can be obtained from the current region. That
means if the current region changes, the method and thus the results of
coordinate reprojection might change. This effect can not be underestimated.

With the current pull request #118 the current region is used to help PROJ6
select the best method, and [r|v].import should work as before. Results can
be different compared to PROJ5 or earlier.

IMHO, there is no way around that users become more familiar with details
of coordinate reprojection and need to read the output of [r|v].proj
carefully regarding different methods known to PROJ6.

PROJ6 will not use the best method if any required datum transformation
grid is not available. Users will need to obtain the corresponding grid
themselves, which raises another question: where to put this grid? On
Linux, this would be e.g. /usr/share/proj, but if the user can not write to
/usr/share/proj, the grids need to be saved somewhere in $HOME, and PROJ6
must look at that place. This problem, i.e. the place where PROJ6 should
look for in $HOME is not yet solved AFAICT (proj-6.2.0).

Markus M

On Wed, Sep 4, 2019 at 9:46 PM Markus Metz <markus.metz.giswork at gmail.com>
wrote:

>
>
> On Wed, Sep 4, 2019 at 6:01 PM Markus Neteler <neteler at osgeo.org> wrote:
> >
> > On Wed, Sep 4, 2019 at 4:31 AM Anna Petrášová <kratochanna at gmail.com>
> wrote:
> > > On Tue, Sep 3, 2019 at 4:22 AM Markus Metz <
> markus.metz.giswork at gmail.com> wrote:
> > >>
> > >> Hi all,
> > >>
> > >> there is a new PR for PROJ6 support in GRASS
> > >> https://github.com/OSGeo/grass/pull/118
> > >>
> > >> There are two important changes when using PROJ6:
> > >>
> > >> First, reprojection with v.proj and r.proj is no longer always
> possible without the user making informed decisions. The reason is that
> there can be several different operations available to reproject
> coordinates from one CRS to another CRS. These different operations are
> listed and the user has to provide the appropriate operation with the
> pipeline option, taking care of any axisswap.
> > >
> > > first, thanks for all the work. Second, I don't see how most users are
> supposed to know what to pick. Is there perhaps a way to pick a good
> default? I just can't imagine not having r.import/v.import...
> >
> > I see the same problem: users won't know what to select if defaults
> > are gone with PROJ 6.
>
> We can provide information that enables users to make an informed
> decision. The options listed by PROJ6 are ordered by guessed best choice,
> i.e. the first one is, given the provided information, the best choice. But
> users need to review the list of options.
>
> Reprojection from one CRS to another CRS was never easy. For both raster
> and vector data, some preparation was always needed to decide on
> appropriate settings. PROJ6 provides methods to improve the accuracy of
> reprojected coordinates, depending on the actual use case. The user decides
> (must decide).
> >
> > > Anna
> > >>
> > >>
> > >> Second, axis order is no longer always easting, northing, e.g. for
> EPSG:4326 it is northing, easting, and an axisswap might need to be removed
> from operations provided by PROJ.
> > >>
> > >> There are many more changes (see details in the PR), but these are
> the two most important ones.
> > >>
> > >> Feedback welcome!
> >
> > Thanks for your hard work.
> >
> > I talked to Robert Bivand here at the GeoSTAT 2019 in Münster who gave
> > a talk about the topic:
> >
> > "Not just R-spatial: sustaining open source geospatial software stacks"
> > https://github.com/rsbivand/geostat19_talk
> >
> > You can quick-view the talk in rendered HTML like this (maybe there
> > are better ways):
> >
> http://htmlpreview.github.io/?https://raw.githubusercontent.com/rsbivand/geostat19_talk/master/docs/geostat_bivand_19.html
> >
> > It contains a section about PROJ (6).
>
> Not so random citation:
> "...we need to manipulate the CRS read in with the file to insert our
> choice of how to make the transformation..."
>
> I will try to restrict the number of options based on the current region
> in a modified PR.
>
> Markus M
>
>
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