Minimal Envelope of set of geometries - Geopandas

Question

To be able to do operations on a set of geometries in a geopandas dataframe, I need to be able to determine whether objects are on the outer "rim" of the set. The set of geometries is as follows:

To do this, I would like to create a polygon that perfectly matches the outer bound of the set of geometrical objects. I first thought about using the convex hull of the set:

convex_hull = Sectioned_geostore_obstacles_geometry.unary_union.convex_hull
convex_hull = geopandas.GeoDataFrame({'geometry': convex_hull, 'convex_hull':[1]})

ax = Sectioned_geostore_obstacles_geometry['Gondola'].plot(color='red')
convex_hull.plot(ax=ax, color='green', alpha=0.5)

which results in

but this isn't quite right since what I am looking for isn't convex. The second idea is to use the envelope:

envelope = Sectioned_geostore_obstacles_geometry.unary_union.envelope
envelope = geopandas.GeoDataFrame({'geometry': envelope, 'convex_hull':[1]})

ax = Sectioned_geostore_obstacles_geometry['Gondola'].plot(color='red')
envelope.plot(ax=ax, color='green', alpha=0.5)

which is

Again, this isn't it. Yet another attempt is to use the cascade_union functionality from shapely:

from shapely.ops import cascaded_union
polygons = list(Sectioned_geostore_obstacles_geometry.Gondola)
boundary = gpd.GeoSeries(cascaded_union(polygons))

which is:

But, this isn't it either as it returns a multipolygon instead of the minimal eveloping polygon. Basically, I need the envelope to shrink to follow the contour of the set of objects.

Any insights would be greatly appreciated.

To test this, I add the following example data:

test_df =  geopandas.GeoSeries([Polygon([(0,0), (2,0), (2,2), (0,2)]),
                              Polygon([(2,2), (4,2), (4,4), (2,4)])])
test_df = geopandas.GeoDataFrame({'geometry': test_df, 'df1':[1,2]})

convex_hull = test_df.unary_union.convex_hull
convex_hull = geopandas.GeoDataFrame({'geometry': convex_hull, 'convex_hull':[1]})

ax1 = test_df['geometry'].plot(color='red')
convex_hull.plot(ax=ax1, color='green', alpha=0.5)

envelope = test_df.unary_union.envelope
envelope = geopandas.GeoDataFrame({'geometry': envelope, 'convex_hull':[1]})

ax2 = test_df['geometry'].plot(color='red')
envelope.plot(ax=ax2, color='green', alpha=0.5)

Answer 1

This was elegantly solved here by determining the concave hull of the set of geometries:

https://gis.stackexchange.com/questions/428882/minimal-enveloping-polygon-to-set-of-geometries

by https://gis.stackexchange.com/users/32958/bera