(geopandas) How to output longitude/latitude scale correctly in Mercator projetion?

Question

Longitude and latitude scale next to axes are not correct in a Mercator map created with geopandas (Python3).

I made a Mercator-projecrioned map with geopandas like this.

import geopandas as gpd
import matplotlib.pyplot as plt

world = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres'))
world = world[(world.name != "Antarctica") & (world.name != "Fr. S. Antarctic Lands")]
world = world.to_crs("EPSG:3395")

ax = world.plot()
ax.set_title("Mercator")

plt.show()

The map itself was correctly output, but longitude/latitude scales next to axes were incorrect as in images below.

Mercator_map

Longitude is supposed to range -180 to 180, and latitude -90 to 90. However, scales in the image don't do so.

How could I get correctly output scales?

Answer 1

EPSG:3395 is a mercator (cylindrical) projection with units of metres. You need a cylindrical projection in units of degrees (which is called a Plate Carree or Equidistant Cylindrical projection). Use EPSG:4326 - For example see https://geopandas.readthedocs.io/en/latest/docs/user_guide/projections.html

More info on EPSG:4326 here: https://epsg.io/4326

More info on EPSG:3395 here: https://epsg.io/3395

Discussion of Mercator compared to Plate Carree (aka Platte Carre) https://idvux.wordpress.com/2007/06/06/mercator-vs-well-not-mercator-platte-carre/

Answer 2

To draw geographic graticule (lines of meridian and parallels of latitude), and label them with values in degrees you can use cartopy gridlines() function. Here is the modified code and the output plot.

cartopy.gridlines document

import geopandas as gpd
import matplotlib.pyplot as plt
import numpy as np
import cartopy.crs as ccrs

myproj = ccrs.Mercator()
pcproj = ccrs.PlateCarree()

fig = plt.figure(figsize=(16, 7))
extent =[-179,179,  -78, 85]  #lonmin, lonmax, latmin, latmax
ax = plt.axes(projection=myproj )
ax.set_extent(extent, crs=pcproj)

lon_grid = np.arange(-180, 180, 30)
lat_grid = np.arange(-75, 83, 15)
gl = ax.gridlines(draw_labels=True,
                  xlocs=lon_grid, ylocs=lat_grid,
                  x_inline=False, y_inline=False,
                  color='k', linestyle='dotted')

world = gpd.read_file(gpd.datasets.get_path('naturalearth_lowres'))
antarctica = world[world.name == "Antarctica"].to_crs("EPSG:3395")  #just in case

world = world[(world.name != "Antarctica") & (world.name != "Fr. S. Antarctic Lands")]
world = world.to_crs("EPSG:3395")

ax = world.plot(ax=ax, edgecolor='k', lw=0.3)

# if you prefer to plot `Antarctica`
if True:
    antarctica.plot(ax=ax, color='lightgray')

ax.set_title("Mercator")

plt.show()