["python","pandas","geopandas","shapely"]
Make a union of polygons in GeoPandas, or Shapely (into a single geometry)

I am trying to find the union of two polygons in GeoPandas and output a single geometry that encompasses points from both polygons as its vertices. The geopandas.overlay function gives me polygons for each individual union but I would like a single polygon. For context, I'm using this to combine two administrative areas together into a single area (i.e. include a town district within a country). The following example is from the geopandas website and illustrates what I'd like: from matplotlib import pyplot as plt import geopandas as gpd from shapely.geometry import Polygon polys1 = gpd.GeoSeries([Polygon([(0,0), (2,0), (2,2), (0,2)]), Polygon([(2,2), (4,2), (4,4), (2,4)])]) polys2 = gpd.GeoSeries([Polygon([(1,1), (3,1), (3,3), (1,3)]), Polygon([(3,3), (5,3), (5,5), (3,5)])]) df1 = gpd.GeoDataFrame({'geometry': polys1, 'df1':[1,2]}) df2 = gpd.GeoDataFrame({'geometry': polys2, 'df2':[1,2]}) res_union = gpd.overlay(df1, df2, how='union') res_union.plot() None of the output geometries are what I was expected, which is the following: poly_union = gpd.GeoSeries([Polygon([(0,0), (0,2), (1,2), (1,3), \ (2,3), (2,4), (3, 4), (3, 5), (5, 5), (5, 3), (4, 3), (4, 2), \ (3,2), (3,1), (2, 1), (2, 0), (0, 0)])]) poly_union.plot(color = 'red') plt.show() Firstly, how do I output the above polygon (poly_union) from the input polygons (df1, df2) using GeoPandas or shapely? Secondly, what is the correct nomenclature associated with the geometry (poly_union) that I'm trying to find? I would call it a 'union' but every example I find that refers to 'unions' does not output this geometry. Note: This example does not seem to output a single polygon either: poly1 = df1['geometry']; poly2 = df2['geometry'] mergedpoly = poly1.union(poly2) mergedpoly.plot()
Note: cascaded_union mentioned in the answer below is superceded by unary_union if GEOS 3.2+ is used - this allows unions on different geometry types, not only polygons. To check your version, >>> import shapely.geos >>> shapely.geos.geos_version (3, 5, 1) From the question/answer here, it seems this is called a cascaded_union within shapely: from shapely.ops import cascaded_union polygons = [poly1[0], poly1[1], poly2[0], poly2[1]] boundary = gpd.GeoSeries(cascaded_union(polygons)) boundary.plot(color = 'red') plt.show()