Oblique Mercator class

[trexfeathers]

Rationale

Iris users are requesting (SciTools/iris#4830) support for the Oblique Mercator projection, in line with the CF Conventions. This will allow them to quickly and easily plot data from NetCDF files that use this projection.

• Oblique Mercator in CF Conventions
• Oblique Mercator in PROJ

(Code largely copied from TransverseMercator, with appropriate modifications).

Implications

New feature, no other expected implications. Closes #1618.

To-do:

• Tests
• An entry in docs/source/reference/projections.rst
• Resolve the coastline 'folding' bug
  (Oblique Mercator class #2096 (comment))
• Confirm whether the x_limits and y_limits settings are appropriate
  (Oblique Mercator class #2096 (comment))

Usage example

from cartopy import crs as ccrs
from matplotlib import pyplot as plt

projection = ccrs.ObliqueMercator()
ax = plt.axes(projection=projection)
ax.coastlines()
ax.gridlines()
plt.show()

[trexfeathers]

I need help!

Solution: f9b99ca

Running with the following arguments causes some weird coastline 'folding', which I haven't seen anywhere else in Cartopy. Any ideas for cause and solution are welcome.

from cartopy import crs as ccrs
from matplotlib import pyplot as plt

projection = ccrs.ObliqueMercator(
    central_longitude=-59,
    central_latitude=-22,
    false_easting=-25000,
    false_northing=-25000,
    scale_factor=1,
    azimuth=90,
)

ax = plt.axes(projection=projection)
ax.coastlines()
ax.gridlines()
plt.show()

[dopplershift]

I think setting more appropriate values for x_limits and y_limits might help automatically block out the folding? Essentially those values are the valid ranges for those coordinates in projected space.

[trexfeathers]

I think setting more appropriate values for x_limits and y_limits might help automatically block out the folding? Essentially those values are the valid ranges for those coordinates in projected space.

Thanks @dopplershift, we did some experiments but the limits made no difference to the overlapping coastlines. Reasonably happy with the current iteration, having solved both confusions.

[trexfeathers]

@SciTools/cartopy-devs any ideas what I've done to cause the docs failure?

[rcomer]

@trexfeathers I think it wasn’t you pydata/pydata-sphinx-theme#1226

[trexfeathers]

@trexfeathers I think it wasn’t you pydata/pydata-sphinx-theme#1226

✔

[bjlittle]

@trexfeathers Thanks for this awesome contribution, much appreciated.

Just some super minor comments to service, and then we can bank this new CRS loveliness 😄

[bjlittle]

@trexfeathers Thanks again... now it's time to oblique my mercator!

Lovely contribution, cheers 🍻

[QuLogic]

ObliqueMercator is not in the projections list because you forgot to run make projection.

[bjlittle]

@QuLogic Thanks, see #2146 👍

[trexfeathers]

Forgot to say: thanks to @ESadek-MO and @scottrobinson02 for their help on this project ❤