On Tue, 26 May 2015, Daniel Kelley wrote:
Roger,
I plan to make the next version of oce use rgdal::project() to do its
work. The idea of using an external PROJ.4 is fine in my own work, but
oce has a lot of users who are unaccustomed to building and installing
external libraries. Also, many work at institutes that do not permit
this kind of system modification. So, for oce I had to make a choice of
including the C sources or using another package.
Dan,
(while this might seem very specialised, it's actually an example of a
typical package development problem, that of accessing facilities in one
package from another).
I suggest using rgdal::rawTransform() instead of rgdal::project(), because
it calls the PROJ.4 pj_transform, rather than pj_fwd or pj_inv, and so
permitting datum transformation. rgdal::rawTransform() was contributed to
rgdal by Robert Hijmans to make it easier to write code in the raster
package using rgdal facilities; it is in rgdal/R/wrappers.R. Look at how
Robert handles the import from the rgdal NAMESPACE into raster.
Roger
PS. I'm afraid that there isn't an easy solution to the wrap-around on the
back of the globe - most of the fixes involve breaking geometries on the
"other" side to work around the problem.
At first I felt the proj4 R package to be a natural candidate. However,
it produces an error when any point in a lon-lat vector is “offscale”,
instead of inserting NA for the projected value. Indexing over a world
coastline is too slow for practical work.
Over the weekend I set up a test version of oce to use rgdal::project(),
and it’s early days but things seem to be working well.
I need to study the “sp” package in more detail. Maybe I’ll find the
solution to a problem “oce” has when lines in a coastal polygon “cross
over” the edge of the earth. This produces spurious domain-crossing
lines when e.g. lon_0 is set to put the Pacific at the centre (a common
choice of oceanographers).
Thanks VERY much for your detailed and helpful answer. And thanks for
your work on “rgdal” and “sp”, two very fine packages.
— Dan.
PS. glad to see more software going to github.
Dan Kelley
Oceanography Department
Dalhousie University
Halifax, NS, Canada
On May 26, 2015, at 4:14 PM, Roger Bivand <roger.biv...@nhh.no> wrote:
Dan,
I'm sorry to be late to the party (one is offline occasionally), but all the
advice given so far is at best only partially correct.
Use of PROJ.4 is crucially conditional on access to PROJ_LIB a shell variable
setting the address of metadata files not included in the C source. PROJ.4
without this access is very limited, not least because the EPSG listing of
projections is not available, not to mention a small file of default values
that recently floored rgdal and other packages using its PROJ.4 facilities
(PROJ.4 4.9.1 omitted the file by mistake).
At present, oce does not ship with PROJ_LIB at all. I agree that it does not
need to ship with the PROJ.4 C source, and should, like rgdal and proj4, use
the system PROJ.4 (or the CRAN PROJ.4). Only rgdal checks in ./configure for
settings related to PROJ_LIB. Using PROJ.4 externally makes sense because of
version shifting and missed bug fixes (there were some bad bugs before the
aborted 4.9.0 release), and crucially updated metadata files. Using a single
PROJ.4 on a platform avoids different applications seeing possibly different
metadata.
My guess would be that anything you might need to do can be done using function
stubs exported by rgdal for use in raster (the package). You are free to choose
not to use sp objects, but you'll find that the high-level support for these in
rgdal is robust and well-motivated.
If you go for the proj4 rather than rgdal solution, do make sure that the CRAN
Windows and OSX binaries are built with the metadata PROJ_LIB copied to the
binary packages.
Again, apologies for not jumping in at the right time.
Roger
PS. Please also note that PROJ.4 is moving to
https://github.com/OSGeo/proj.4
--
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: roger.biv...@nhh.no
--
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: roger.biv...@nhh.no
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