Shapely Convex Hull area gives radically different results from scipy.ConvexHull area?

Question

I must be missing something incredibly obvious, but there seems to be a huge difference in the area computed by scipy.ConvexHull.area compared to shapely.convex_hull.area. I get

Scipy Convex Hull Area: 457761.9061526276, Shapely Convex Hull Area: 13192154623.86528

from shapely.geometry import MultiPoint
from scipy.spatial import ConvexHull
import json


def convex_hull_compare():
    with open("./test_points.json", 'r') as f:
        points = json.load(f)
        print(points)
        hull = ConvexHull(points)
        ch_area = MultiPoint(points).convex_hull.area

        print(
            f"Scipy Convex Hull Area: {hull.area}, Shapely Convex Hull Area: {ch_area}")
        # Scipy Convex Hull Area: 457761.9061526276, Shapely Convex Hull Area: 13192154623.86528


if __name__ == "__main__":
    convex_hull_compare()

I've uploaded the test_points.json on a Github Gist here and written a minimal version of the code for easy reproduction.

If you'd like to answer the question and possibly explain why volume is area (and what area corresponds to with 2D points), I'll gladly accept your answer. — Lieu Zheng Hong, Feb 17 '20 at 13:32

score 1 · Accepted Answer · answered Feb 17 '20 at 13:29

A helpful commenter linked the following question: the gist of it is that for 2D points, we want ConvexHull.volume, not ConvexHull.area.

This indeed gives us the right answer, but this is really confusing because the documentation doesn't mention this at all.

from shapely.geometry import MultiPoint
from scipy.spatial import ConvexHull
import json


def convex_hull_compare():
    with open("./test_points.json", 'r') as f:
        points = json.load(f)
        print(points)
        hull = ConvexHull(points)
        ch_area = MultiPoint(points).convex_hull.area

        print(
            f"Scipy Convex Hull Area: {hull.area}, Shapely Convex Hull Area: {ch_area}")
        # Scipy Convex Hull Area: 457761.9061526276, Shapely Convex Hull Area: 13192154623.86528

        print(
            f"Scipy Convex Hull Area: {hull.volume}, Shapely Convex Hull Area: {ch_area}")
        #Scipy Convex Hull Area: 13192154623.865295, Shapely Convex Hull Area: 13192154623.86528


if __name__ == "__main__":
    convex_hull_compare()

The documentation does explain it: see my answer here https://stackoverflow.com/a/65660582/1048456 — Vic, Jan 11 '21 at 02:22

Shapely Convex Hull area gives radically different results from scipy.ConvexHull area?

1 Answers1