A Comparison of Precinct and District Voting Data Using Persistent Homology to Identify Gerrymandering in North Carolina

• URL: http://arxiv.org/abs/2506.13997v2
• Date: Tue, 24 Jun 2025 23:54:43 GMT
• Title: A Comparison of Precinct and District Voting Data Using Persistent Homology to Identify Gerrymandering in North Carolina
• Authors: Ananya Shah,
• Abstract summary: We present an extension of Feng and Porter's 2019 paper on the use of the level-set method for the construction of a filtered simplicial complex from geospatial election data.<n>Comparing the persistent homologies of Democratic voting areas on the precinct and district level shows when areas have been 'cracked' or 'packed' for partisan gain.<n>This research presents a novel application of topological data analysis in evaluating gerrymandering.
• Score: 0.0
• License: http://creativecommons.org/licenses/by/4.0/
• Abstract: We present an extension of Feng and Porter's 2019 paper on the use of the level-set method for the construction of a filtered simplicial complex from geospatial election data. Precincts are regarded to be too small to be gerrymandered, allowing us to identify discrepancies between precinct and district level voting data to quantify gerrymandering in the United States. Comparing the persistent homologies of Democratic voting areas on the precinct and district level shows when areas have been 'cracked' or 'packed' for partisan gain. This analysis was done for North Carolina House of Representatives elections (2012 to 2024). North Carolina has been redistricted 4 times in the past 10 years, whereas most states redistrict decennially, allowing us to understand how and when redistricted maps deviate from precinct-level voting data, and when gerrymandering occurs. Comparing persistence barcodes at the precinct and district levels (using the bottleneck distance) shows that precinct-level voting patterns do not significantly fluctuate biannually, while district level patterns do, suggesting that shifts are likely a result of redistricting rather than voter behavior, providing strong evidence of gerrymandering. North Carolina election data was collected from the public domain. Composite shapefiles were created using QGIS and R, and rasterized using Python. The level-set method was employed to generate filtered simplicial complexes. Persistence barcodes were produced using GUDHI and PHAT libraries. Additionally, we compare our results with traditional measures such as Polsby-Popper and Reock scores (gerrymandering identification measures). This research presents a novel application of topological data analysis in evaluating gerrymandering.

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