I’ve been working with the Shapely python package in python. This is a short guide for how to merge polygons in python.

This guide is based on “cascaded_union.py” in the shapely examples.

Find out how to install shapely in python here.

How to Merge Polygons in Python

Download the script for this guide here.

In a most simple sense we can just do something like this to merge polygons:

from shapely.geometry import Polygon
from shapely.ops import cascaded_union

polygon1 = Polygon([(0, 0), (5, 3), (5, 0)])
polygon2 = Polygon([(0, 0), (3, 10), (3, 0)])

polygons = [polygon1, polygon2]

u = cascaded_union(polygons)

This will produce a merged polygon “u”.

Let’s build on this to plot the original polygons, merge them, and then plot the merged polygon, too.

First, plot the original polygons:

# plot these two polygons separately
fig = plt.figure(1, figsize=SIZE, dpi=90)
ax = fig.add_subplot(111)
poly1patch = PolygonPatch(polygon1, fc=BLUE, ec=BLUE, alpha=0.5, zorder=2)
poly2patch = PolygonPatch(polygon2, ec=BLUE, alpha=0.5, zorder=2)
ax.add_patch(poly1patch)
ax.add_patch(poly2patch)
xrange = [-2, 12]
yrange = [-2, 8]
ax.set_xlim(*xrange)
ax.set_xticks(range(*xrange) + [xrange[-1]])
ax.set_ylim(*yrange)
ax.set_yticks(range(*yrange) + [yrange[-1]])
ax.set_aspect(1)

Which gives us this output:

Now merge the two polygons and plot them:

# Make the merged polygons
u = cascaded_union(polygons)

# Make new figure for the merged polygon
fig2 = plt.figure(2, figsize=SIZE, dpi=90)
ax2 = fig2.add_subplot(111)
patch2b = PolygonPatch(u, fc=BLUE, ec=BLUE, alpha=1, zorder=2)
ax2.add_patch(patch2b)

xrange = [-2, 12]
yrange = [-2, 8]
ax2.set_xlim(*xrange)
ax2.set_xticks(range(*xrange) + [xrange[-1]])
ax2.set_ylim(*yrange)
ax2.set_yticks(range(*yrange) + [yrange[-1]])
ax2.set_aspect(1)

fig2.show(2)

Which gives us the output:

Get the python file for this guide here.

How to Merge Polygons in Python

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