Construct PolygonArray or MultiPolygonArray from Exterior Coordinates

[philipc2]

Closes #122

Overview

• Introduces the PolygonArray.from_exterior_coords() and MultiPolygonArray.from_exterior_coords() class methods, which take-in an list/array of exterior coordinates equivalent to the following which are derived from Shapely Polygons/MultiPolygons

spatialpandas/spatialpandas/geometry/polygon.py

Lines 40 to 42 in dd941f2

	if shape.exterior is not None:
	exterior = np.asarray(shape.exterior.coords).ravel()
	polygon_coords = [exterior]

spatialpandas/spatialpandas/geometry/multipolygon.py

Lines 40 to 48 in dd941f2

	if isinstance(shape, sg.MultiPolygon):
	multipolygon = []
	for polygon in shape.geoms:
	polygon_coords = Polygon._shapely_to_coordinates(polygon)
	multipolygon.append(polygon_coords)
	return multipolygon
	elif isinstance(shape, sg.Polygon):
	return [Polygon._shapely_to_coordinates(shape)]

Usage

In UXarray, we use SpatialPandas paired with Datashader to visualize geoscience data that resides on unstructured grids as Polygons. Our unstructured grid is represented as a set of connectivity and coordinate arrays, which we would like to avoid needing to convert to Shapely, because we can derive the exterior of each Polygon quickly using the connectivity and coordinate information.

Performance Comparison

Initial benchmarks, comparing across n_polygon & n_vertices

All benchmarks are run on an NCAR Casper HPC Node, with 4 CPU Threads and 128gb of Memory

Test 1

(40,000,000 Closed Polygons with 7 total verticies)

Method / # of Polygons	400,000	4,000,000	40,000,000
Construct with Exterior Coord Array	1.02s	9.74s	97.0s
Create Shapely Polygons + Construct with Shapely Polygons	16.2s	159s	1560s
Only Construct with Shapely Polygons	15.2s	149s	1500s

Roughly a 16x speedup compared to using Shapely

[philipc2]

Depending on any initial feedback from the developers here, I'd be willing to continue implementing this to support Polygons with holes, MultiPolygons, etc.

While this PR focusses on Polygons specifically, I believe other geometries could benefit from this type of construction support.

[hoxbro]

The logic added seems simple enough not to add a lot of tests, but it would still be nice with one for both polygon and multi-polygon.

Maybe also some tests that handle edge cases like wrong input shape/size.

[hoxbro]

Thank you for the PR! 👍