Contourf not plotting negative contours in certain cases about cartopy HOT 2 OPEN

Thanks for looking into this @rcomer. I will pass your recommendation on. I played around with your try_plot function and got some interesting results. The resultant contours are extremely sensitive to that central value.

try_plot(-4.44e-16)

try_plot(0)

try_plot(-5e-16)

try_plot(-4e-16)

try_plot(-4.5e-16)

I also added lev0 to the data (after casting it to float64) which would align the 'zeros' in the data with the middle level, and this resulted in 5 identical plots. I guess this would indicate a floating point conversion and/or equality error somewhere?

from cartopy.

Thanks for the report and the clear reproducer @dsroberts. This seems reminiscent of #2224 which I ultimately solved by locating the specific problem level and analyzing that. I had some trouble locating the problem level in this case as it seems to be very specific to the slightly off-zero value generated by arange. But now I have this:

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import numpy as np
import netCDF4

ds = netCDF4.Dataset('dataset.nc','r')

def try_plot(lev0):
    ax = plt.axes(projection=ccrs.PlateCarree(central_longitude=1))
    cf = ax.contourf(np.array(ds['lon']), np.array(ds['lat']), np.array(ds['a']), transform=ccrs.PlateCarree(), levels=[lev0, lev0+0.2], extend='both')
    plt.colorbar(cf)
    plt.show()


try_plot(-4.44e-16)
try_plot(0)

The slightly negative value give this (bad!)

Whereas zero give this (good!)

A workaround for your researcher might be to use levels=np.linspace(-2, 2, 21), which gives me this:

from cartopy.