检查LinearRing是否包含点在Shapely中始终返回False

Question

This is variation of Shapely contains(point) always gives False , not a duplicate

I want to determine if a point inside a certain figure created of many points, but to start simple I do this test, which fails. I define a 2x2 square, and the first point should be inside, the second outside, but both calls return False

import unittest
from shapely.geometry import LineString,Point,LinearRing
class TestTools(unittest.TestCase):
    def test_isInside(self):
        points = [
            [0,0],
            [2,0],
            [2,2],
            [0,2]
        ]
        ring=LinearRing(points)
        print(ring)
        print(Point(1,2))
        self.assertEqual(ring.contains(Point(1,1)),True) 
        self.assertEqual(ring.contains(Point(3,3)),False)

The output is

LINEARRING (0 0, 2 0, 2 2, 0 2, 0 0)
POINT (1 2)
======================================================================
FAIL: test_isInside (__main__.TestTools)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test_tools.py", line 17, in test_isInside
    self.assertEqual(ring.contains(Point(1,1)),True)
AssertionError: False != True

So obviously I'm doing something (simple?) wrong.

Answer 1

According to the Shapely documentation there are three fundamental types of geometric objects: points, curves, and surfaces. The LinearRing is one of the implementations of the curve type.

Citing the docs:

A Curve has an interior set consisting of the infinitely many points along its length (imagine a Point dragged in space), a boundary set consisting of its two end points, and an exterior set of all other points. A Curve has a topological dimension of 1.

And here is what it says about object.contains(other) method:

Returns True if no points of other lie in the exterior of the object and at least one point of the interior of other lies in the interior of object .

So, in your case the points Point(1, 1) , Point(3, 3) don't lie on the curve LinearRing([[0, 0], [2, 0], [2, 2], [0, 2]]) but outside it, in the exterior, hence your tests return False .

In order to check if the points are enclosed by the LinearRing , in your simple case, you could construct polygons from those rings and do the same check:

>>> Polygon(ring).contains(Point(1, 1))
True

>>> Polygon(ring).contains(Point(3, 3))
False