Fix invalid polygon in Shapely

python geometry shapely
jpcgt picture jpcgt · Dec 30, 2013 · Viewed 25.1k times · Source

Shapely defines a Polygon as invalid if any of its segments intersect, including segments that are colinear. Many software packages will create a region or area with a "cutout" as shown here which has colinear segments:

enter image description here

>>> pp = Polygon([(0,0), (0,3), (3,3), (3,0), (2,0), 
                  (2,2), (1,2), (1,1), (2,1), (2,0), (0,0)])
>>> pp.is_valid
WARNING:shapely.geos:Self-intersection at or near point 2 0
False

Naturally, the cutout can be implemented natively in Shapely, or this same geometry can be implemented as two valid polygons, but if I only have the list of points shown above, is there an easy to "fix" this (create valid geometry from this list of points)?

Answer

jpcgt picture jpcgt · Jan 1, 2014

I found a solution that works for the specific case given:

>>> pp2 = pp.buffer(0)
>>> pp2.is_valid
True
>>> pp2.exterior.coords[:]
[(0.0, 0.0), (0.0, 3.0), (3.0, 3.0), (3.0, 0.0), (2.0, 0.0), (0.0, 0.0)]
>>> pp2.interiors[0].coords[:]
[(2.0, 1.0), (2.0, 2.0), (1.0, 2.0), (1.0, 1.0), (2.0, 1.0)]

Related questions

How to do a scatter plot with empty circles in Python?

In Python, with Matplotlib, how can a scatter plot with empty circles be plotted? The goal is to draw empty circles around some of the colored disks already plotted by scatter(), so as to highlight them, ideally without having to …

Read More
How can I check if two segments intersect?

How can I check if 2 segments intersect? I've the following data: Segment1 [ {x1,y1}, {x2,y2} ] Segment2 [ {x1,y1}, {x2,y2} ] I need to write a small algorithm in Python to detect if the 2 lines are intersecting.

Read More
How can you determine a point is between two other points on a line segment?

Let's say you have a two dimensional plane with 2 points (called a and b) on it represented by an x integer and a y integer for each point. How can you determine if another point c is on the line …

Read More