python – Creating a Polygon with a Hole in Shapely

geometrymulti-polygonpolygonpythonshapely

So I have two polygons: (i) An outer polygon, and (ii) an inner Polygon representing the 'hole'. I'm trying to merge these two to give a single Polygon, with the hole.

For easy illustration, the outer Polygon looks like:

And the (ii) Hole looks like:

Both these Polygons are Shapely Polygons. To create a Polygon with the hole, I researched a lot through this exchange and although I found limited resources, I tried the following:

#x: Shapely Polygon Type - The outer Polygon
#y: Shapely Polygon Type - The 'hole'
w=geometry.Polygon(x, holes=[y])

I still seem to be getting the same issue, the hole is not represented. Any tips?

Best Answer

If starting with two Polygon objects then you can use the object.difference() method:

with_hole = outer.difference(inner)

Related Solutions

Python Shapely – Creating Polygons with Holes

I think the only issue here is that you need to pass the coordinates to the Polygon constructor, not the list of coordinates. So in trial one, instead of

#Trial 1
newPolygon = Polygon(polygonExteriors, polygonInteriors)

do:

#Trial 1
newPolygon = Polygon(polygonExteriors, [[pt for pt in inner.coords] for inner in polygonInteriors])

As I don't have all of your code, here is an example to demonstrate the solution, which returns the same polygon as inputPolygon:

from shapely.geometry import Polygon

# Example polygon with two holes
inputPolygon = Polygon(((0,0),(10,0),(10,10),(0,10)), (((1,3),(5,3),(5,1),(1,1)), ((9,9),(9,8),(8,8),(8,9))))

polygonExterior = inputPolygon.exterior
polygonInteriors = []
for i in range(len(inputPolygon.interiors)):
    # do the stuff with your polygons
    polygonInteriors.append(inputPolygon.interiors[i])

newPolygon = Polygon(polygonExterior, [[pt for pt in inner.coords] for inner in polygonInteriors])

Python – How to Create a Shapely Polygon with a Hole

A polygon can have multiple holes in it, so the holes argument is a list of list of coordinate tuples:

>>> exterior = [(2, 8), (7, 8), (7, 3), (2, 3), (2, 8)]
>>> interior = [(4, 7), (4, 5), (6, 5), (6, 7), (4, 7)]

This fails:

>>> pgn = Polygon(exterior, holes=interior)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/rowlings/.local/lib/python2.7/site-packages/shapely/geometry/polygon.py", line 240, in __init__
    ret = geos_polygon_from_py(shell, holes)
  File "/home/rowlings/.local/lib/python2.7/site-packages/shapely/geometry/polygon.py", line 506, in geos_polygon_from_py
    N = exemplar._ndim
AttributeError: 'tuple' object has no attribute '_ndim'

because interior isn't a list of interior polygons, its just one. So make it a list with one member:

>>> pgn = Polygon(exterior, holes=[interior])
>>> pgn
<shapely.geometry.polygon.Polygon object at 0x7fb3639bc250>

Docs:

The second is an optional unordered sequence of ring-like sequences specifying the interior boundaries or “holes” of the feature.

The "source code" links are great working examples of the shapely API, eg this:

ext = [(0, 0), (0, 2), (2, 2), (2, 0), (0, 0)]
int = [(1, 0), (0.5, 0.5), (1, 1), (1.5, 0.5), (1, 0)][::-1]
polygon = Polygon(ext, [int])

in https://shapely.readthedocs.io/en/latest/code/polygon.py

Best Answer

Related Solutions

Python Shapely – Creating Polygons with Holes

Python – How to Create a Shapely Polygon with a Hole

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