Equal Earth Projection

The Equal Earth map projection is a matplotlib add-on based on the projection developed by Bojan Šavrič (@BojanSavric), Tom Patterson and Bernhard Jenny:

The Equal Earth map projection is a new equal-area pseudocylindrical projection for world maps. It is inspired by the widely used Robinson projection, but unlike the Robinson projection, retains the relative size of areas. The projection equations are simple to implement and fast to evaluate. Continental outlines are shown in a visually pleasing and balanced way.

• https://doi.org/10.1080/13658816.2018.1504949
• https://www.researchgate.net/publication/326879978_The_Equal_Earth_map_projection

where:

• λ and φ are the longitude and the latitude, respectively
• θ is a parametric latitude
• A1 = 1.340264
• A2 = -0.081106
• A3 = 0.000893
• A4 = 0.003796

Full Documentation

Full documentation is at: https://dneuman.github.io/EqualEarth/

Requirements

shapefile (from pyshp) is required to read the map data. This is available from Anaconda, but must be installed first, from the command line::

>>>conda install shapefile

Usage

Importing the module causes the Equal Earth projection to be registered with Matplotlib so that it can be used when creating a subplot:

>>>import matplotlib.pyplot as plt
>>>import EqualEarth
>>>longs = [-110, 100, 100, -110]
>>>lats = [40, 40, -40, 40]
>>>fig = plt.figure('Equal Earth Projection')
>>>ax = fig.add_subplot(111, projection='equal_earth')
>>>ax.plot(longs, lats)
>>>plt.grid(True)
>>>plt.show()

Note that the default behaviour is to take all data in degrees. If radians are preferred, use the rad=True optional keyword in fig.add_subplot(), ie:

>>>ax = fig.add_subplot(111, projection='equal_earth', rad=True)

All plots must be done in radians at this point.

New in This Version (2.0)

DrawCoastlines: World map data from Natural Earth <https://www.naturalearthdata.com>_ will download into the maps folder in the same directory as the Equal Earth module. This is 500kb on disk, but is downloaded in .zip format and unzipped automatically. Other maps can be used if you supply the shape files. Once the axes is set up, you can draw the continents::

    >>>ax.DrawCoastlines(facecolor='grey', edgecolor='none')

Great Circle (geodesic) lines: Navigation lines can be plotted using the shortest path on the globe. These lines take plot keywords and wrap around if necessary::

    >>>pts = np.array([[-150, 45], [150, 45]])
    >>>ax.plot_geodesic(pts, 'b:', linewidth=1, alpha=.8)

Tissot Indicatrix of Distortion: Circles can be plotted at regular intervals on the projection to show map distortion at various places::

    >>>ax.DrawTissot(width=10.)

Plot Note

Lines drawn by ax.plot() method are clipped by the projection if any portions are outside it due to points being greater than +/- 180° in longitude. If you want to show lines wrapping around, they must be drawn twice. The second time will require the outside points put back into the correct range, but with their connecting points now outside the projection.

Future Work

Ideally, this projection should be in Cartopy, where there are far more features available. I will be doing what I can to make this happen.

Sources

Based on code from:

• https://matplotlib.org/gallery/misc/custom_projection.html

as well as code from @mbostock:

• https://beta.observablehq.com/@mbostock/equal-earth-projection

@Author: Dan Neuman (@dan613)