Error specifying center long for Mercator projection and extents on Python 3.6

[acrosby]

Description

The following code works in Python 2 but fails in Python 3. The commented lines appear to work OK however, so it is really only a minor problem.

Code to reproduce

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

cm_lon=51.15
maplimits = [-26., 128.3, -73.3, -31.1]

ax = plt.axes(projection=ccrs.Mercator(central_longitude=cm_lon))
#ax = plt.axes(projection=ccrs.PlateCarree(central_longitude=cm_lon))
#ax = plt.axes(projection=ccrs.Mercator())
ax.set_extent([maplimits[0], maplimits[1], maplimits[2], maplimits[3]])
plt.show()

Traceback

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/miniconda2/envs/xesmf/lib/python3.6/site-packages/cartopy/mpl/geoaxes.py in set_extent(self, extents, crs)
    686             # the projection extents, so try and give a better error message.
--> 687             x1, y1, x2, y2 = projected.bounds
    688         except ValueError:

ValueError: not enough values to unpack (expected 4, got 0)

During handling of the above exception, another exception occurred:

ValueError                                Traceback (most recent call last)
<ipython-input-1-47cd62a8a14f> in <module>()
     12 ax.add_feature(cartopy.feature.LAND, color='gray')
     13 ax.coastlines(resolution='10m', linewidth=0.25)
---> 14 ax.set_extent([maplimits[0], maplimits[1], maplimits[2], maplimits[3]])
     15 plt.show()

~/miniconda2/envs/xesmf/lib/python3.6/site-packages/cartopy/mpl/geoaxes.py in set_extent(self, extents, crs)
    692                    'y_limits=[{ylim[0]}, {ylim[1]}]).')
    693             raise ValueError(msg.format(xlim=self.projection.x_limits,
--> 694                                         ylim=self.projection.y_limits))
    695         self.set_xlim([x1, x2])
    696         self.set_ylim([y1, y2])

ValueError: Failed to determine the required bounds in projection coordinates. Check that the values provided are within the valid range (x_limits=[-20037508.342789244, -20037508.34278924], y_limits=[-15496570.739723722, 18764656.23138057]).

[dopplershift]

What version of CartoPy are you using? Linux? I can't reproduce here on Python 3.6.5 and CartoPy 0.16.0 from conda-forge on MacOS. Instead (if I add ax.coastlines()), I get the following image:

[acrosby]

I have Python 3.6.6 miniconda running under Ubuntu 18.04 with the following (subset) of packages:

# Name                    Version                   Build  Channel
cartopy                   0.16.0           py36hfa13621_0 
proj4                     5.0.1                h14c3975_0
pyepsg                    0.3.2                    py36_0  
pyproj                    1.9.5.1          py36h7b21b82_1
shapely                   1.6.4            py36h7ef4460_0  
six                       1.11.0                   py36_1  

It sounds like there is some kind of issue with the non- conda-forge repository. I think we have been using conda install -c scitools cartopy, but I see in the documentation that this is no longer recommended/mentioned. Simply running conda install -c conda-forge cartopy does in fact solve the problem...

[ajdawson]

I can reproduce this problem now, it is due to using version 5 of proj (proj4 5.0.1). If you install cartopy from conda defaults you get a version built against proj4 5.0.1, but on conda-forge you get a cartopy built with proj4 4.9.3. You should pin proj4 to a version less than 5 when installing, for a new environment from defaults:

conda create -n my_env_name cartopy "proj4<5"

or to modify an existing install from defaults:

conda install "proj4<5"

Other proj4 5 problems are reported in #1071 and #1045.

[acrosby]

Oh interesting, I'll have to double check that we are indeed having an issue on the Win install that seems to have a lower proj4 major version of 4.

EDIT: Yes it seems the windows installation is OK in this regard, had a different problem unrelated to Cartopy that impacted the test above.