[GIS] Python Shapely – filling small gaps between multi polygons

buffergeopandaspolygonpythonshapely

I have a set of rectangular polygons, which currently form a directional path.
The library I am using is Python Shapely / GeoPandas.

The issue I am having is as the path changes direction, there are small gaps and overlaps between the polygons.

I could iterate through all my polygon objects and calculate the angle in between each intersecting point and generate a new set of polygons but I figure there is a much better way.

I am fairly close – using the shapely buffer function, as seen in the following code snippet. I am able to remove the gaps, but it ends up merging my polygons into 1x. But I require them to remain 2x seperate polygons.

polygons = MultiPolygon([Polygon([(0, 0.5), (0, 2.5), (1, 2.5),(1,0.5)]),Polygon([(0, 0), (1, 1), (2,0),(1,-1)])])
polygons = polygons.buffer(2).buffer(-2)

Best Answer

Here one code that can help you.

from shapely.geometry import MultiPolygon, Polygon
import matplotlib.pyplot as plt
import geopandas as gpd
polygons = MultiPolygon([Polygon([(0, 0.5), (0, 2.5), (1, 2.5),(1,0.5)]),Polygon([(0, 0), (1, 1), (2,0),(1,-1)])])

polygons = gpd.GeoSeries(polygons)

polygons_hull = polygons.convex_hull

plt.plot(*polygons_hull.exterior.xy)

Related Solutions

[GIS] Using spatial index to intersect points with polygon, when points and polygon have same minimum bounding box

I try to explain with an image. If you do not agree that this strategy works I will delete my answer. However, I will continue to use it in my own work.

I took Texas as an example and I split it into 1 by 1 degree pieces. The grid that shows behind corresponds with the R-Tree index despite at the boundaries where the real R-Tree BBOXes are smaller.

In the Texas case a point never hits more than one R-Tree box. In case of adjacent polygons there may be more hits, but never more than a handful. Next thing to do is to make an Intersects test with a true geometry of the corresponding piece. If point is inside that piece it is inside Texas as well, and in the USA, North America etc.

[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

Related Solutions

[GIS] Using spatial index to intersect points with polygon, when points and polygon have same minimum bounding box

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

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