Expose PROJ parameter +gamma for Oblique Mercator about cartopy HOT 4 OPEN

I think it'd be fine to just plumb in the gamma parameter.

from cartopy.

Seems reasonable to expose that keyword argument if you'd find it helpful.
https://proj.org/en/9.3/operations/projections/omerc.html

@trexfeathers added this in #2096 so they might have thoughts/opinions on it as well.

from cartopy.

@trexfeathers added this in #2096 so they might have thoughts/opinions on it as well.

I implemented this based on some example files from CORDEX, which use the basic parameters that you see in the Cartopy class. As @Jeitan says, it seems fairly obvious how things would work if the 'alternative' Proj parameters were used, would just need a bit more logic inside the class.

Any decisions about class design were NOT made from a strongly knowledgeable/opinionated position - I was just seeing what worked. If you need to undo anything to expand the functionality, go ahead 👍

from cartopy.

@greglucas @trexfeathers Yay, thank you! I'm glad you are considering it!

One thing I have noticed is that there does appear to be a funny branch cut in PROJ (for the Oblique Mercator specifically), or perhaps I'm mis-reading the documentation. The docs specify limits on gamma but only when not also providing alpha. When I set gamma to 0 but alpha to anything with abs() > 90, it does a weird fliperoo. To get a smooth rotation, say in the counterclockwise direction by specifying positive alpha, from 85 to 90 to 95, I have to set alpha to 85, 90, -95 and gamma to 0, 0, 180. The same thing happens in northern and southern hemisphere. I don't know if Cartopy wants to deal with it, though, since it's a PROJ phenomenon.

To me personally, I'd be fine just passing through gamma, just figured I'd mention it.

from cartopy.