Add map scales, compasses, and missing basemap features

[lukelbd]

ProPlot should integrate with existing complex basemap features. For power users, this can be done by calling methods on ax.projection (which is the Basemap instance associated with the axes). But if we wanted to make things easier for users (in the spirit of ProPlot) we could add options to ProjectionAxes._format_apply as follows:

def _format_apply(
         ...
         etopo=None, bluemarble=None,
         mapscale=None, mapscale_kw=None,
         wmsimage=None, wmsimage_kw=None,
         warpimage=None, warpimage_kw=None,
         tissot=None, tissot_kw=None,
         patch_kw=None, **kwargs,
         ):
         """
         ...
         bluemarble : optional
             For basemap axes only. Draws the blue marble image.
         etopo : optional
             For basemap axes only. Draws topography data.
         mapscale, mapscale_kw : optional
             For basemap axes only. Draws a map scale.
         tissot, tissot_kw : optional
             For basemap axes only. Draws Tissot indicatrixes.
         warpimage, warpimage_kw : optional
             For basemap axes only. Adds arbitrary images to the background.
             See `~mpl_toolkits.basemap.Basemap.warpimage`.
         wmsimage, wmsimage_kw : optional
             For basemap axes only. Downloads and plots images using the WMS protocol.
             See `~mpl_toolkits.basemap.Basemap.warpimage`.
         ...
   """

Cartopy is missing several of these features, but distance scales could be implemented manually as discussed in this stackoverflow thread and this cartopy Github issue. This could be added to ProPlot ProjectionAxes, or to the cartopy project directly.

It would also be really interesting if we could add compasses to plots that show the Northward direction. This is not available in basemap or cartopy.

I'm not really motivated to address these proposals anytime soon but if someone else is, I think this is very doable for new contributors.

[knaaptime]

depending on your goals, contextily may also be worth considering here

[stefraynaud]

I would suggest to implement it directly within Cartopy if it is already in Basemap.
I have an old compass working with basemap that I may be useful here or directly in cartopy, but I don't have time to adapt it. Here is the code: https://github.com/VACUMM/vacumm/blob/5d72cfd5314d9ba33d955c7c374903b8880d3c2c/lib/python/vacumm/misc/core_plot.py#L7637

[lukelbd]

I agree that these features belong in cartopy, not proplot. Also, I think all of these features (especially wmsimage, bluemarble, warpimage, and tissot) are specialized enough that they probably don't belong in GeoAxes.format() -- instead they should only be accessible via the standard cartopy interface.

For adding different backgrounds, the examples on this page or the GeoAxes.background_img() method could be used. For adding "tissot" indicators, there is apparently a GeoAxes.tissot() function. For adding map scales, we can monitor SciTools/cartopy#1728 or use the stackoverflow solution I posted above. For adding compasses, someone can start a new thread or use Stephane's solution posted above. And if you are a basemap holdout, you can simply use the basemap methods with e.g. ax.projection.drawmapscale() and ax.projection.tissot().