Recovery of 0.7% Redistricting Error
The temporary 2022 districts in VAN were created using shapefiles from MN LCC GIS and addresses from VAN, which are geocoded by the DNC. 99.3% of addresses were correctly mapped to their new districts. Due to the limitations of geocoding statewide addresses, 0.7% of addresses were mapped to an incorrect district.
We found that these errors tended to happen in the following areas:
Houses that were located less than one block from an HD border
Houses in new developments, particularly in outer ring suburbs and exurbs
11% of erroneous mappings were from less-than-perfect geocodes. We ran every delegate record through geocod.io’s geocode scoring system. Any codes marked under 100% certainty were examined by our Redistricting team and matched to the appropriate district by cross-referencing addresses from VAN with LCC GIS maps and Google maps.
79% of errors came from geocodes that were scored with 100% accuracy but were a less accurate type of geocode (for example, some geocodes use the center of a street between blocks 1600 and 1500 to approximate the location of house number 1550) or an incorrectly applied geocode. To find these, we sorted the delegate list by CD, SD, HD, city, and ZIP. That way, mismatched addresses stuck out to us as we were scrolling through the list. We corrected them by cross-referencing with LCC GIS and the US Census geocodes.
10% of errors were caught manually. To ensure that we caught all errors, we went line-by-line through 86% of the addresses on the delegate list, checking them against US Census data. We hand-checked every address in CD’s 1, 2, 4 and 5 as well as every address in the 73 MN ZIP codes that cross 4 or more HD boundaries.
While doing these checks, we found that the DNC’s geocodes were often more accurate than those from the Census. For example, the DNC often provided rooftop geocodes in cases where the Census could only provide the center of a ZIP code.
22% only changed HD (no convention change)
78% changed SD convention
27% changed CD convention