Boundary splitting with a small Globe

[kadrlica]

Description

Thanks for all the hard work on this nice tool.

I've started to transfer over some code that used basemap for plotting astronomical sky maps. Since I want a spherical Globe with no direct connection to physical scale of the Earth, I thought it would make sense to use a unit sphere:

globe = ccrs.Globe(ellipse=None,semimajor_axis=1.0,semiminor_axis=1.0)

This seems to work for the most part; however, when I try to draw a LineString that cross the global map boundary, it fails to break properly at the map boundary for some latitudes. I'm guessing that this is some numerical resolution issue, since this issues disappears as the radius of the globe is increased (and appears to be gone completely at radius = 1e5). My guess is that there is some implicit assumption of the globe size hidden somewhere, but I'm not sure where this can be found (or if I can somehow increase the resolution from the api directly).

Code to reproduce

import matplotlib.pyplot as plt
import numpy as np

import cartopy.crs as ccrs
from shapely.geometry.polygon import LineString

fig = plt.figure(figsize=(12,4))
plt.subplots_adjust(left=0.01,right=0.99,wspace=0.01)

radii = [1.0,1e4,1e5]
lon = np.linspace(0,360,100)
lat = 60*np.sin(np.radians(lon))

for i,radius in enumerate(radii):
    globe=ccrs.Globe(ellipse=None,semimajor_axis=radius,semiminor_axis=radius)
    projection=ccrs.Mollweide(globe=globe)
    geodetic = ccrs.Geodetic(globe=globe)

    ax = fig.add_subplot(1, len(radii), i+1, projection=projection)
    plt.title('radius = %g'%radius)

    for delta in np.linspace(-90,90,15):
        line = LineString(list(zip(lon+delta,lat)))
        ax.add_geometries([line],crs=geodetic,facecolor='none',edgecolor='k')

Partial environment definition

Operating system

Mac OS X 10.15.4

Cartopy version

0.18.0

conda list

...
python                    3.7.6           h90870a6_5_cpython    conda-forge
cartopy                   0.18.0           py37h08e9697_0    conda-forge
shapely                   1.7.0            py37hfcf0db4_3    conda-forge
...

[QuLogic]

For Mollweide, I think this may be because of the fixed threshold:

cartopy/lib/cartopy/crs.py

Lines 1805 to 1807 in 60ae9a6

	@property
	def threshold(self):
	return 1e5

Unfortunately, not very many projections scale the threshold with the globe.