user3747260 user3747260 - 1 month ago 16
Python Question

shapely is_valid for polygons in 3D

I'm trying to validate some polygons that are on planes with

is_valid
, but I get
Too few points in geometry component at or near point
for polygons where the z is not constant.

Is there a way to validate these other polygons?

Here's an example:

from shapely.geometry import Polygon


poly1 = Polygon([(0,0), (1,1), (1,0)])
print(poly1.is_valid)
# True

# z=1
poly2 = Polygon([(0,0,1), (1,1,1), (1,0,1)])
print(poly2.is_valid)
# True

# x=1
poly3 = Polygon([(1,0,0), (1,1,1), (1,1,0)])
print(poly3.is_valid)
# Too few points in geometry component at or near point 1 0 0
# False

Answer

The problem is that shapely in fact ignores the z coordinate. So, as far as shapely can tell you are building a polygon with the points [(1,0),(1,1), (1,1)] that aren't enough to build a polygon.

See this other SO question for more information: python-polygon-does-not-close-shapely.

IMHO, shapely shouldn't allow three dimension coordinates, because it brings this kind of confusions.