cartopy
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pcolormesh no longer wraps around
Description
pcolormesh no longer draws the "other half" of gridpoints that have their center at the edge of the maps - see the example and figure. This may very well be an 'indended' consequence of #1646 but I thought I report it anyway.
This used to work in older versions of cartopy/ matplotlib (e.g. cartopy=0.19 and matplotlib=3.4)
Code to reproduce
import cartopy.crs as ccrs
import matplotlib.pyplot as plt
import numpy as np
lon = np.arange(-180, 151, 30)
lat = np.arange(75, -76, -30)
LON, LAT = np.meshgrid(lon, lat)
lon_edges = np.arange(-195, 181, 30)
lat_edges = np.arange(90, -91, -30)
data = np.ones(shape=lat.shape + lon.shape)
f, axs = plt.subplots(1, 2, subplot_kw=dict(projection=ccrs.PlateCarree()))
f.subplots_adjust(wspace=0.05)
# 1 - using edges
ax = axs[0]
ax.pcolormesh(
lon_edges,
lat_edges,
data,
transform=ccrs.PlateCarree(),
cmap="Reds",
ec="0.3",
lw=0.5,
)
ax.set_title("using edges")
# 2 - using centers
ax = axs[1]
ax.pcolormesh(
lon,
lat,
data,
transform=ccrs.PlateCarree(),
cmap="Reds",
ec="0.3",
lw=0.5,
)
ax.set_title("using centers")
for ax in axs:
ax.plot(LON.flatten(), LAT.flatten(), "o", ms=1)
ax.coastlines()
ax.set_global()

Full environment definition
Operating system
Linux
Cartopy version
0.20.2
matplotlib.version 3.5.1
conda list
# packages in environment at /home/mathause/.conda/envs/regionmask-docs:
#
# Name Version Build Channel
_libgcc_mutex 0.1 conda_forge conda-forge
_openmp_mutex 4.5 1_gnu conda-forge
affine 2.3.0 py_0 conda-forge
aiohttp 3.8.1 py310h6acc77f_0 conda-forge
aiosignal 1.2.0 pyhd8ed1ab_0 conda-forge
alabaster 0.7.12 py_0 conda-forge
alsa-lib 1.2.3 h516909a_0 conda-forge
appdirs 1.4.4 pyh9f0ad1d_0 conda-forge
argon2-cffi 21.3.0 pyhd8ed1ab_0 conda-forge
argon2-cffi-bindings 21.2.0 py310h6acc77f_1 conda-forge
asttokens 2.0.5 pyhd8ed1ab_0 conda-forge
async-timeout 4.0.2 pyhd8ed1ab_0 conda-forge
attrs 21.4.0 pyhd8ed1ab_0 conda-forge
babel 2.9.1 pyh44b312d_0 conda-forge
backcall 0.2.0 pyh9f0ad1d_0 conda-forge
backports 1.0 py_2 conda-forge
backports.functools_lru_cache 1.6.4 pyhd8ed1ab_0 conda-forge
black 21.12b0 pyhd8ed1ab_0 conda-forge
bleach 4.1.0 pyhd8ed1ab_0 conda-forge
blosc 1.21.0 h9c3ff4c_0 conda-forge
bokeh 2.4.2 py310hff52083_0 conda-forge
boost-cpp 1.74.0 h359cf19_5 conda-forge
branca 0.4.2 pyhd8ed1ab_0 conda-forge
brotli 1.0.9 h7f98852_6 conda-forge
brotli-bin 1.0.9 h7f98852_6 conda-forge
brotlipy 0.7.0 py310h6acc77f_1003 conda-forge
bzip2 1.0.8 h7f98852_4 conda-forge
c-ares 1.18.1 h7f98852_0 conda-forge
ca-certificates 2021.10.8 ha878542_0 conda-forge
cairo 1.16.0 ha00ac49_1009 conda-forge
cartopy 0.20.2 py310he9c7799_3 conda-forge
certifi 2021.10.8 py310hff52083_1 conda-forge
cffi 1.15.0 py310h0fdd8cc_0 conda-forge
cfitsio 4.0.0 h9a35b8e_0 conda-forge
cftime 1.5.2 py310h96516ba_0 conda-forge
charset-normalizer 2.0.10 pyhd8ed1ab_0 conda-forge
click 8.0.3 py310hff52083_1 conda-forge
click-plugins 1.1.1 py_0 conda-forge
cligj 0.7.2 pyhd8ed1ab_1 conda-forge
cloudpickle 2.0.0 pyhd8ed1ab_0 conda-forge
colorama 0.4.4 pyh9f0ad1d_0 conda-forge
cryptography 36.0.1 py310h685ca39_0 conda-forge
curl 7.81.0 h2574ce0_0 conda-forge
cycler 0.11.0 pyhd8ed1ab_0 conda-forge
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dask 2022.1.0 pyhd8ed1ab_0 conda-forge
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libnsl 2.0.0 h7f98852_0 conda-forge
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regionmask 0.8.0.post1.dev24+g17855bf.d20220125 dev_0 <develop>
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pip list
Interestingly if you do lon + 30 or lon_edge + 30 it seems to work.
Thanks for the analysis @mathause! This appears to be due to the pyproj upgrade actually and wrapping the coordinates over 180, but not less than -180. I put a fix in for that in #1998
Thank you for the quick fix! Feels like I keep throwing "edge" cases at you.
This appears to be due to the pyproj upgrade actually and wrapping the coordinates over 180, but not less than -180.
This also gives me an idea how to create a work around for now (use 0..360 instead of -180..180).
(Definitely not the same problem but if you try different crs with the above code (e.g.EckertIII or Robinson) you will also see some issues with the border cell)
import cartopy.crs as ccrs
import matplotlib.pyplot as plt
import numpy as np
lon = np.arange(-180, 151, 30)
lat = np.arange(75, -76, -30)
data = np.ones(shape=lat.shape + lon.shape)
f, ax = plt.subplots(1, 1, subplot_kw=dict(projection=ccrs.EckertIII()))
ax.pcolormesh(
lon_edges,
lat_edges,
data,
transform=ccrs.PlateCarree(),
cmap="Reds",
ec="0.3",
lw=0.5,
)
ax.coastlines()
ax.set_global()

I think that is to be expected here and I'm not sure how easy it would be to fix. I think what is happening is that the pcolormesh is QuadMesh, so everything is drawn in straight lines. So, your cells from 60-90 are all vertical. Except for the "wrapped" cells, which get drawn as a PolyCollection, and thus there is interpolation along the path that happens. This interpolation causes an extra point in the middle of the path, which is no longer a straight line. Thus, when the QuadMesh straight edges meet up with the PolyCollection interpolated edges, there can be a disconnect between the two.
Thanks, makes sense. Just something I saw trying out different projections.