Polygon intersection error in Shapely: "shapely.geos.TopologicalError: The operation 'GEOSIntersection_r' produced a null geometry"

Nik picture Nik · Oct 25, 2012 · Viewed 17.1k times · Source

I have been trying to debug this problem but unable to do so. I am trying to find the intersection of two Polygon objects. It works most of the time but for the following case, it raises the following exception:

P1 area: 13.125721955
P2 area: 1.0
Traceback (most recent call last):
File "geom2d.py", line 235, in <module>
print p1.intersection(p2)
File "/usr/local/lib/python2.7/dist-packages/shapely/geometry/base.py", line 334, in     intersection
return geom_factory(self.impl['intersection'](self, other))
  File "/usr/local/lib/python2.7/dist-packages/shapely/topology.py", line 47, in __call__
    "The operation '%s' produced a null geometry. Likely cause is invalidity of the geometry %s" % (self.fn.__name__, repr(this)))
shapely.geos.TopologicalError: The operation 'GEOSIntersection_r' produced a null     geometry. Likely cause is invalidity of the geometry <shapely.geometry.polygon.Polygon      object at 0x8e5ad6c>

The code is as follows.

from shapely.geometry import Point,Polygon,MultiPolygon

poly1 = [(35.0041000000000011, -88.1954999999999956), (34.9917999999999978,         -85.6068000000000069), (32.8404000000000025, -85.1756000000000029), (32.2593000000000032, -84.8927000000000049), (32.1535000000000011, -85.0341999999999985), (31.7946999999999989, -85.1358000000000033), (31.5199999999999996, -85.0438000000000045), (31.3384000000000000, -85.0836000000000041), (31.2092999999999989, -85.1069999999999993), (31.0023000000000017, -84.9943999999999988), (30.9953000000000003, -87.6008999999999958), (30.9422999999999995, -87.5926000000000045), (30.8538999999999994, -87.6256000000000057), (30.6744999999999983, -87.4072000000000031), (30.4404000000000003, -87.3687999999999931), (30.1463000000000001, -87.5240000000000009), (30.1545999999999985, -88.3863999999999947), (31.8938999999999986, -88.4742999999999995), (34.8937999999999988, -88.1020999999999930), (34.9478999999999971, -88.1721000000000004), (34.9106999999999985, -88.1461000000000041)]
poly2 = [(34.7998910000000024, -88.2202139999999986), (34.7998910000000024,  -87.2202139999999986), (35.7998910000000024, -87.2202139999999986), (35.7998910000000024, -88.2202139999999986)]

p1 = Polygon(poly1)
p2 = Polygon(poly2)
print 'P1 area:',p1.area
print 'P2 area:',p2.area
print p1.intersection(p2)

Since it prints the areas of the two polygons, I assume that the polygons are formed correctly. I also (somehow) printed the first polygon to make sure that it is indeed a simple polygon.

Could anyone please explain why I am getting this exception?

Edit: I printed p1.is_valid and it turns out to be False. There is some explanation here. Search for the string is_valid. It says that

A valid Polygon may not possess any overlapping exterior or interior rings.

Could someone please explain what this means and if there is a possible work-around? BTW, I also noticed that if I remove the last coordinate from poly1, the whole thing works. Perhaps the whole list of coordinates makes the polygon complex.

Answer

subnivean picture subnivean · Dec 30, 2012

As previously pointed out, p1 is not valid. On plotting it, I noticed a little 'bowtie' at the lower right. I assume you don't need this in your polygon; if not, you can try Shapely's buffer(0) trick (documented in the Shapely Manual) to fix that:

In [382]: p1.is_valid
Out[382]: False

In [383]: p1 = p1.buffer(0)

In [384]: p1.is_valid
Out[384]: True

buffer(0) has the following effect:

Before:

enter image description here

After:

enter image description here

And you can now do this:

print p1.intersection(p2)
POLYGON ((34.9396324323625151 -88.1614025927056559, 34.8937999999999988 -88.1020999999999930, 34.7998910000000024 -88.1137513649788247, 34.7998910000000024 -87.2202139999999986, 34.9994660069532983 -87.2202139999999986, 35.0041000000000011 -88.1954999999999956, 34.9396324323625151 -88.1614025927056559))

Note that I have had some instances (with areas that looked more like "birds' nests" than simple bowties) where this didn't work; check to make sure you get back a single Polygon object and not a MultiPolygon one.