sp/rgdal workflows with PROJ >= 6 and GDAL >= 3

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