Cartopy + Matplotlib (contourf) - Map Overriding data

Question

I'm trying to do a Contour Plot having the Global Map in background. Having in mind that my data have LON and LAT values, I decided to use Cartopy with MatplotLib.

The problem is that I can plot my data and the map perfectly when separated, but when I try to integrate the data with the map the Cartopy map override my data plot.

This is my code:

ax = plt.axes(projection=cartopy.crs.PlateCarree())

v = np.linspace(0, 80, 25, endpoint=True)
cp = plt.contourf(matrixLon, matrixLat, matrixTec, v, transform=cartopy.crs.PlateCarree())
plt.colorbar(cp)

ax.add_feature(cartopy.feature.LAND)
ax.add_feature(cartopy.feature.OCEAN)
ax.add_feature(cartopy.feature.COASTLINE)
ax.add_feature(cartopy.feature.BORDERS, linestyle=':')
ax.set_extent([-85, -30, -60, 15])

plt.title('TEC Map')
plt.show()

Plots:

Plotting only Data

Plotting only the map

It is strange because I think that the logical is the data override the map (and maybe I have to try a transparent color scale) but not the other way around.

Can someone help me with this issue?

Answer 1

Here is the working code that you may try and learn.

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

# prep some data for contourf plot
# extents: upper-right of the map
x = np.linspace(-65, -30, 30)
y = np.linspace(-30, 15, 30)
matrixLon, matrixLat = np.meshgrid(x, y)
matrixTec = 10*np.sin(matrixLon**2 + matrixLat**2)/(matrixLon**2 + matrixLat**2)

ax = plt.axes(projection=cartopy.crs.PlateCarree())

# prep increasing values of v covering values of Z (matrixTec)
v = np.arange(-0.15, 0.15, 0.025)

# plot with appropriate parameters
# zorder: put the filled-contour on top
# alpha: set transparency to allow some visibility of graphics below
cp = plt.contourf(matrixLon, matrixLat, matrixTec, v, \
                  transform=cartopy.crs.PlateCarree(), \
                  zorder=2, \
                  alpha=0.65, \
                  cmap=plt.cm.copper)
plt.colorbar(cp)

ax.add_feature(cartopy.feature.LAND)
ax.add_feature(cartopy.feature.OCEAN)
ax.add_feature(cartopy.feature.COASTLINE)
ax.add_feature(cartopy.feature.BORDERS, linestyle=':')
ax.set_extent([-85, -30, -60, 15])

plt.title('TEC Map')
plt.show()

The essence is the use of zorder and alpha in plt.contourf() that can be set to show or hide some features on the map.