[R-sig-Geo] sp/rgdal workflows with PROJ >= 6 and GDAL >= 3

[Roger Bivand]

The development version of rgdal on R-Forge is now at rev 894, and is now 
ready for trying out with PROJ6/GDAL3 workflows, and workflows that may 
migrate within 6 months to modern CRS representations. The motivating RFC 
is also updated to cover coordinate operations, the use of prepared 
(pre-searched) coordinate operations, and should be read carefully by 
anyone using rgdal::spTransform(). Note further that rgdal::project() will 
not be adapted for PROJ6, and is effectively deprecated.

I'll be running reverse dependency checks, and may be bugging package 
maintainers. I would really prefer that mainainers of packages using 
spTransform() checked themselves and joined this thread or the associated 
twitter thread: https://twitter.com/RogerBivand/status/1194586193108914177

Be ready for modern PROJ and GDAL, they are already being deployed across 
open source geospatial software, like GRASS, QGIS, pyproj, spatialite etc.

Waiting, hopefully not in vain, for contributions.

Roger

On Wed, 13 Nov 2019, Roger Bivand wrote:

> And this link explains the CDN proposal for grid distribution:
>
> https://www.spatialys.com/en/crowdfunding/
>
> Roger
>
> On Wed, 13 Nov 2019, Roger Bivand wrote:
>
>>  Because PROJ >= 6 and GDAL >= 3 change the way that PROJ strings
>>  (representations of coordinate reference systems) are handled, steps are
>>  being taken to find ways to adapt sp/rgdal workflows. A current proposal
>>  is to store the WKT2_2018 string as a comment to CRS objects as defined in
>>  the sp package.
>>
>>  A draft development-in-progress version of rgdal is available at
>>  https://r-forge.r-project.org/R/?group_id=884, and for sp at
>>  https://github.com/rsbivand/sp (this version of sp requires rgdal >=
>>  1.5-1). This adds the WKT comments to CRS objects on reading vector and
>>  raster data sources, and uses WKT comments if found when writing vector
>>  and raster objects (or at least does as far as I've checked, possibly
>>  fragile).
>>
>>  An RFC with tersely worked cases for using CRS object comments to carry
>>  WKT strings but maintaining full backward compatibility is online at
>>  http://rgdal.r-forge.r-project.org/articles/PROJ6_GDAL3.html.
>>
>>  If you have other ideas or concerns about trying to use this mechanism for
>>  sp CRS objects, please contribute at your earliest convenience.
>>
>>  http://rgdal.r-forge.r-project.org/reference/list_coordOps.html shows the
>>  beginning of the next step, to query transformation operations to find
>>  viable coordinate operation pipelines.
>>
>>  I'm assuming that the previous behaviour (transform without considering
>>  accuracy with whatever is to hand) is not viable going forward, and that
>>  we will need two steps: list coordinate operations between source and
>>  target CRS (using the WKT comments as better specifications than the PROJ
>>  strings), possibly intervene manually to install missing grids, then
>>  undertake the coordinate operation.
>>
>>  The fallback may be simply to choose the least inaccurate available
>>  coordinate operation, but this should be a fallback. This means that all
>>  uses of spTransform() will require intervention.
>>
>>  Is this OK (it is tiresome but modernises workflows once), or is it not OK
>>  (no user intervention is crucial)?
>>
>>  These behaviours may be set in an option, so that package maintainers and
>>  users may delay modernisation, but all are undoubtedly served by rapid
>>  adaptation (GRASS 7.8.1 released yesterday, libspatialite, pyproj, QGIS
>>  development versions all state that they list candidate coordinate
>>  operations).
>>
>>  We cannot ship all the grids, they are very bulky, and probably nobody
>>  needs sub-metre accuracy world-wide. Work in PROJ is starting to create a
>>  content delivery network for trusted download and mechanisms for
>>  registering downloaded grids on user platforms. We would for example not
>>  want Windows users of rgdal and sf to have to download the same grid
>>  twice.
>>
>>  Comments welcome here and at
>>  https://github.com/r-spatial/discuss/issues/28 or
>>  https://github.com/r-spatial/sf/issues/1187
>>
>>  Roger
>> 
>> 
>
>

-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; e-mail: Roger.Bivand using nhh.no
https://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en