[GIS] Merge overlapping/neighboring polygons – GeoPandas

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I am reading a shapefile and I have extracted a specific row from it that is a Multipolygon. I can extract all the polygons in it with .explode() which works great and I get 130+ polygons.

How can I merge the ones that are next to each other?

I know the 130+ resulting polygons represent two islands, and I want to get each as a polygon. I have looked up how to do it but haven't been able to find anything and this is my first project ever using geopandas and shapely.

gdf=gpd.read_file("comunas.shp").to_crs({'init': 'epsg:4326'})
Rapa_Nui=gdf.loc[gdf["Comuna"]=="Isla de Pascua"]
all_Rapa_Nui=Rapa_Nui.explode()

Best Answer

You can first union all polygons with unary_union:

single_multi_polygon = all_Rapa_Nui.unary_union

This should now be a single MultiPolygon consisting of two polygons for the two islands. And then you can get the polygon parts of this MultiPolygon:

polygons = single_multi_polygon.geoms

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[GIS] Speed up row-wise point in polygon with Geopandas

I would recommend either using the geopandas-cython branch here or pygeos.

If you use pygeos, I would recommend converting the geometries from shapely to the pygeos version first for the best speedups.

[GIS] Shapely polygon union results in strange artifacts of tiny, non-overlapping areas

This is typically a result of the the borders not fitting perfectly one next to another (and this is very easy to get with floating point coordinates).

As an example, I use the world dataset available in geopandas, and take the unary union of the Africa continent:

import geopandas
gdf = geopandas.read_file(geopandas.datasets.get_path('naturalearth_lowres'))
gdf_africa = gdf[gdf['continent'] == 'Africa']
africa = gdf_africa.geometry.unary_union
# only take main continent, disregard Madagascar
africa = list(africa)[1]

You can see on the following plot that also this union is not perfect:

This is a typical problem to encounter when doing unary unions, so now how to solve this:

One possible solution is to take the exterior of this polygon (so disregarding the interior holes) (taken from this answer), and create a new polygon of it:
```
from shapely.geometry import Polygon
africa2 = Polygon(africa.exterior)
```
which gives this result:

which is already much better. There is still a small distortion at the left, because such a 'hole' touched the overall border.
Another solution is to apply a very small buffer to the polygon, and undo it again (taken from this answer)
```
from shapely.geometry import JOIN_STYLE
eps = 0.00001
africa3 = africa.buffer(eps, 1, join_style=JOIN_STYLE.mitre).buffer(-eps, 1, join_style=JOIN_STYLE.mitre)
```
which gives:

In this case, this solution looks better from the figure (no remaining distortion on the left), but this method can slightly change the coordinates (although this is often negligible).

Which of the methods works best for you, probably depends on the actual case.

Best Answer

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[GIS] Speed up row-wise point in polygon with Geopandas

[GIS] Shapely polygon union results in strange artifacts of tiny, non-overlapping areas

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