Contour transform options

This example demonstrates the difference between transforming the points before/after generating the contours. It uses the transform_first keyword argument to indicate that Cartopy should transform the points before calling the contouring algorithm, which can have a significant impact on speed (it is much faster to transform points than it is to transform patches). This does have a negative impact on the wrapped coordinates as one can see in the second axes that the data does not extend to the full global extent.

transform_first=False, transform_first=True
import as ccrs
import matplotlib.pyplot as plt

from waves import sample_data

def main():

    # Use the waves example to provide some sample data, but make it
    # more dependent on y for more interesting contours.
    x, y, z = sample_data((20, 40))
    z = z * -1.5 * y

    # Setup a global EckertIII map with faint coastlines.
    fig = plt.figure()
    ax1 = fig.add_subplot(2, 1, 1, projection=ccrs.EckertIII())
    ax2 = fig.add_subplot(2, 1, 2, projection=ccrs.EckertIII())

    for ax, transform_first in zip([ax1, ax2], [False, True]):
        ax.coastlines('110m', alpha=0.1)

        # Add colourful filled contours.
        filled_c = ax.contourf(x, y, z, transform=ccrs.PlateCarree(),

        # And black line contours.
        ax.contour(x, y, z, levels=filled_c.levels,

if __name__ == '__main__':

Total running time of the script: ( 0 minutes 0.614 seconds)

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