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.biv...@nhh.no
https://orcid.org/0000-0003-2392-6140
https://scholar.google.no/citations?user=AWeghB0AAAAJ&hl=en
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