When working with maps in Python, the cartopy library is a powerful tool that allows us to create and manipulate geospatial data. One common task is to add a rectangle to a map to select a specific subspace. In this article, we will explore three different ways to achieve this using Python and cartopy.

Option 1: Using matplotlib’s Rectangle

The first option is to use the Rectangle class from the matplotlib library, which is integrated with cartopy. This approach involves creating a Rectangle object with the desired dimensions and coordinates, and then adding it to the map using the ax.add_patch() method.

import cartopy.crs as ccrs
import matplotlib.pyplot as plt
from matplotlib.patches import Rectangle

# Create a map using cartopy
fig = plt.figure(figsize=(10, 10))
ax = fig.add_subplot(1, 1, 1, projection=ccrs.PlateCarree())

# Add a rectangle to the map
rect = Rectangle((lon1, lat1), width, height, edgecolor='red', facecolor='none')
ax.add_patch(rect)

# Customize the map
ax.coastlines()
ax.gridlines()

# Show the map
plt.show()

This approach is straightforward and allows for customization of the rectangle’s appearance. However, it requires importing the matplotlib library and using its Rectangle class, which may not be ideal if you are only working with cartopy.

Option 2: Using cartopy’s GeoAxes

The second option is to use cartopy’s GeoAxes class, which provides a more cartopy-centric solution. This approach involves creating a GeoAxes object and using its add_geometries() method to add a rectangle as a Shapely Polygon.

import cartopy.crs as ccrs
import matplotlib.pyplot as plt
from cartopy.mpl.geoaxes import GeoAxes
from shapely.geometry import Polygon

# Create a map using cartopy
fig = plt.figure(figsize=(10, 10))
ax = fig.add_subplot(1, 1, 1, projection=ccrs.PlateCarree())

# Add a rectangle to the map
rect = Polygon([(lon1, lat1), (lon2, lat1), (lon2, lat2), (lon1, lat2)])
ax.add_geometries([rect], ccrs.PlateCarree(), facecolor='none', edgecolor='red')

# Customize the map
ax.coastlines()
ax.gridlines()

# Show the map
plt.show()

This approach leverages cartopy’s GeoAxes class and the Shapely library to create and add the rectangle as a Polygon. It provides a more cartopy-centric solution and avoids the need to import matplotlib. However, it requires an additional dependency (Shapely) and may be slightly more complex for beginners.

Option 3: Using cartopy’s PathPatch

The third option is to use cartopy’s PathPatch class, which allows us to add a rectangle as a matplotlib PathPatch object. This approach involves creating a PathPatch object with the desired dimensions and coordinates, and then adding it to the map using the ax.add_patch() method.

import cartopy.crs as ccrs
import matplotlib.pyplot as plt
from matplotlib.patches import PathPatch
from matplotlib.path import Path

# Create a map using cartopy
fig = plt.figure(figsize=(10, 10))
ax = fig.add_subplot(1, 1, 1, projection=ccrs.PlateCarree())

# Add a rectangle to the map
vertices = [(lon1, lat1), (lon2, lat1), (lon2, lat2), (lon1, lat2), (lon1, lat1)]
codes = [Path.MOVETO, Path.LINETO, Path.LINETO, Path.LINETO, Path.CLOSEPOLY]
path = Path(vertices, codes)
rect = PathPatch(path, edgecolor='red', facecolor='none')
ax.add_patch(rect)

# Customize the map
ax.coastlines()
ax.gridlines()

# Show the map
plt.show()

This approach uses cartopy’s PathPatch class to add the rectangle as a matplotlib PathPatch object. It provides flexibility in defining the rectangle’s shape and allows for customization. However, it also requires importing the matplotlib library and using its Path and PathPatch classes.

After exploring these three options, it is clear that the second option, using cartopy’s GeoAxes, is the most suitable for adding a rectangle to a map made with cartopy in Python. It provides a cartopy-centric solution without the need for additional dependencies or importing the matplotlib library. It strikes a good balance between simplicity and customization, making it the recommended approach for this task.