We consider the problem of political redistricting: given the locations of people in a geographical area (e.g. a US state), the goal is to decompose the area into subareas, called districts, so that the populations of the districts are as close as possible and the districts are ``compact'' and ``contiguous,'' to use the terms referred to in most US state constitutions and/or US Supreme Court rulings.
We study a method that outputs a solution in which each district is the intersection of a convex polygon with the geographical area. The average number of sides per polygon is less than six. The polygons tend to be quite compact. Every two districts differ in population by at most one (so we call the solution balanced).
In fact, the solution is a centroidal power diagram: each polygon has an associated center in ℝ³ such that
* the projection of the center onto the plane z = 0 is the centroid of the locations of people assigned to the polygon, and
* for each person assigned to that polygon, the polygon's center is closest among all centers. The polygons are convex because they are the intersections of 3D Voronoi cells with the plane.
The solution is, in a well-defined sense, a locally optimal solution to the problem of choosing centers in the plane and choosing an assignment of people to those 2-d centers so as to minimize the sum of squared distances subject to the assignment being balanced.
* A practical problem with this approach is that, in real-world redistricting, exact locations of people are unknown. Instead, the input consists of polygons (census blocks) and associated populations. A real redistricting must not split census blocks. We therefore propose a second phase that perturbs the solution slightly so it does not split census blocks. In our experiments, the second phase achieves this while preserving perfect population balance. The district polygons are no longer convex at the fine scale because their boundaries must follow the boundaries of census blocks, but at a coarse scale they preserve the shape of the original polygons.
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The impact of highly compact algorithmic redistricting on the rural-versus-urban balance
It is commonly believed that, in congressional and state legislature elections in the United States, rural voters have an inherent political advantage over urban voters. We study this hypothesis using an idealized redistricting method, balanced centroidal power diagrams, that achieves essentially perfect population balance while optimizing a principled measure of compactness. We find that, using this method, the degree to which rural or urban voters have a political advantage depends on the number of districts and the population density of urban areas. Moreover, we find that the political advantage in any case tends to be dramatically less than that afforded by district plans used in the real world, including district plans drawn by presumably neutral parties such as the courts. One possible explanation is suggested by the following discovery: modifying centroidal power diagrams to prefer placing boundaries along city boundaries significantly increases the advantage rural voters have over urban voters.
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- Award ID(s):
- 1841954
- NSF-PAR ID:
- 10217105
- Date Published:
- Journal Name:
- Proceedings of the 28th International Conference on Advances in Geographic Information Systems
- Page Range / eLocation ID:
- 397 to 400
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1145/3397536.3422249
