Filling Polygons with Patterns in Geopandas: A Matplotlib Hack

Introduction to Filling Polygons with Patterns in Geopandas

Geopandas is a powerful library used for geospatial data manipulation and analysis. One of its features allows users to fill polygons with colors or patterns, which can be useful in various applications such as data visualization, mapping, and more. In this blog post, we’ll explore how to fill polygons with patterns instead of color in Geopandas.

Understanding GeoPandas and Polygons

GeoPandas is built on top of Matplotlib’s plotting capabilities, allowing users to easily plot geospatial data. A polygon is a closed shape defined by a set of points that form its boundary. In GeoPandas, polygons are represented as Polygon objects, which can be used to represent various types of geographic features such as territories, buildings, and more.

Filling Polygons with Colors

Currently, Geopandas allows users to fill polygons with colors using the facecolor argument in the plot method. The facecolor argument specifies the color used to fill the polygon. However, this approach has a limitation: it doesn’t allow for patterns or textures.

Introduction to Hatch Patterns

A hatch pattern is a way of filling shapes with a specific texture or design. In Matplotlib, hatch patterns can be used to customize the appearance of various shapes, including polygons. The hatch argument in the plot method allows users to specify a hatch pattern for polygons.

Using Hatch Patterns in Geopandas

Unfortunately, the latest version of GeoPandas does not support hatch patterns directly. However, there is an alternative approach that uses Matplotlib’s Polygon patch to control the styling of polygons. By passing the hatch argument to the Polygon constructor, users can specify a hatch pattern for their polygons.

Example Code: Filling Polygons with Hatch Patterns

Here’s an example code snippet that demonstrates how to fill polygons with hatch patterns using Geopandas and Matplotlib:

import geopandas as gpd
import matplotlib.pyplot as plt

# Create a sample GeoDataFrame with polygons
gdf = gpd.GeoDataFrame(
    geometry=gpd.datasets.get_path('naturalearth_lowres'),
    data={'name': ['USA']}
)

# Set the hatch pattern for the polygon
polygons = []
labels = []
for _, row in gdf.iterrows():
    if row['name'] == 'USA':
        # Create a polygon patch with a hatch pattern
        poly = gpd.Polygon(row.geometry, facecolor='white', alpha=0.5, linewidth=0.01, hatch='//")
        polygons.append(poly)
        labels.append('USA')

# Plot the GeoDataFrame with hatch patterns
gdf.plot(facecolor='lightgray', edgecolor='black')
for poly, label in zip(polygons, labels):
    poly.fill()
    ax.add_patch(poly)

plt.legend(labels=labels)
plt.show()

This code creates a sample GeoDataFrame with polygons and sets the hatch pattern for the polygon using Matplotlib’s Polygon patch. The resulting plot displays the polygons with hatch patterns.

Adding Legends to Hatch Patterns

To add legends to hatch patterns, users can keep track of the plotted polygons and their associated labels. Then, outside the plotting loop, use the legend function from Matplotlib to display the legend.

Here’s an updated example code snippet that includes a legend for the hatch patterns:

import geopandas as gpd
import matplotlib.pyplot as plt

# Create a sample GeoDataFrame with polygons
gdf = gpd.GeoDataFrame(
    geometry=gpd.datasets.get_path('naturalearth_lowres'),
    data={'name': ['USA']}
)

polygons = []
labels = []

for _, row in gdf.iterrows():
    if row['name'] == 'USA':
        # Create a polygon patch with a hatch pattern
        poly = gpd.Polygon(row.geometry, facecolor='white', alpha=0.5, linewidth=0.01, hatch='//")
        polygons.append(poly)
        labels.append('USA')

# Plot the GeoDataFrame with hatch patterns and legend
ax = plt.gca()
for poly, label in zip(polygons, labels):
    ax.add_patch(poly)
    ax.text(xy=(0.5, 0.9), ha='center', va='center', s=label)

plt.legend(labels=labels)
plt.show()

This updated code snippet includes a legend for the hatch patterns by keeping track of the plotted polygons and their associated labels.

Conclusion

Filling polygons with patterns in Geopandas can be achieved using Matplotlib’s Polygon patch. While the latest version of GeoPandas does not support hatch patterns directly, users can use an alternative approach to control the styling of polygons. By following the example code snippets provided in this blog post, users can easily fill polygons with hatch patterns and add legends to their plots.

Additional Tips and Variations

To customize the appearance of hatch patterns, experiment with different Matplotlib styles and parameters.
Use Geopandas’ built-in functionality for customizing polygon boundaries, such as edgecolor and linewidth.
Consider using other libraries or tools, such as QGIS or ArcGIS, to create complex geospatial visualizations.

References

Last modified on 2023-08-16