Create Cartopy projection from pyproj.Proj

[fmaussion]

Maybe I missed it, but since the issues #153 and #710 are still open I thought I would ask this too:

is it possible to create a cartopy projection from a pyproj.Proj instance?

[dopplershift]

No, but I'd love to have it too.

[fmaussion]

I've come up with a quick n dirty solution here

There should be a more generic way to do it (both pyproj and cartopy use the proj4 engine) but I don't know cartopy well enough to help here...

[graziano-giuliani]

I know that this is crude, but...
proposed_pj4.txt

[pelson]

Just to be clear on this, I am against using a proj4 string as a constructor of a "Proj4Projection" - the solution I'd be looking for is to construct a proper cartopy Projection instance from the proj4 string. For example:

>>> projection = ccrs.from_proj('+ellps=WGS84 +proj=robin +lon_0=40 +no_defs')
>>> print(projection)
Robinson(central_longitude=40)

[djhoese]

I'd be willing to work on this if everyone can agree on how it should be done. To do the above behavior where from_proj returns the proper CRS projection class it would probably be easiest if each individual class had a class attribute mapping keyword arguments to PROJ.4 parameters.

I was thinking the from_proj could try to find a matching CRS.Projection class and if it can't it could fall back to a generic PROJ4Projection type class. I noticed that the _EPSGProjection class is already almost a generic PROJ.4 class.

[fmaussion]

I cannot comment on the details here, but I can only emphasize again that a proj4 string -> cartopy converter would be extremely useful...

[pelson]

I'd be willing to work on this if everyone can agree on how it should be done. To do the above behavior where from_proj returns the proper CRS projection class it would probably be easiest if each individual class had a class attribute mapping keyword arguments to PROJ.4 parameters.

In general, I'm completely OK with that (and potentially even automatically adding those kwargs through to the __init__), but kwargs is not 1:1 proj4 arguments. For example https://github.com/SciTools/cartopy/blob/master/lib/cartopy/crs.py#L1277 'lon_0', 180 + pole_longitude. There will have to be special cases, and I'd like those special cases to belong to the respective classes, rather than having if crs == Robinson type conditions in a function.

[djhoese]

Without thinking about it too much maybe a from_proj classmethod on each CRS class? Then a global utility function from_proj that calls the individual from_proj methods.