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.

import matplotlib.pyplot as plt

import cartopy.crs as ccrs


def sample_data(shape=(73, 145)):
    """Return ``lons``, ``lats`` and ``data`` of some fake data."""
    import numpy as np

    nlats, nlons = shape
    lats = np.linspace(-np.pi / 2, np.pi / 2, nlats)
    lons = np.linspace(0, 2 * np.pi, nlons)
    lons, lats = np.meshgrid(lons, lats)
    wave = 0.75 * (np.sin(2 * lats) ** 8) * np.cos(4 * lons)
    mean = 0.5 * np.cos(2 * lats) * ((np.sin(2 * lats)) ** 2 + 2)

    lats = np.rad2deg(lats)
    lons = np.rad2deg(lons)
    data = wave + mean

    return lons, lats, data


def main():

    # Use the same sample data as the waves example, 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())
    ax1.set_title("transform_first=False")
    ax2 = fig.add_subplot(2, 1, 2, projection=ccrs.EckertIII())
    ax2.set_title("transform_first=True")

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

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

        # And black line contours.
        ax.contour(x, y, z, levels=filled_c.levels,
                   colors=['black'],
                   transform=ccrs.PlateCarree(),
                   transform_first=transform_first)

    plt.show()


if __name__ == '__main__':
    main()