Caliper Mapping and Transportation Glossary

DEFINITION

What is a Splitline Algorithm?

The Splitline Algorithm is a mathematical approach to redistricting that aims to eliminate the element of choice from the redistricting process. Districts are drawn by rigorously applying a recursive algorithm that finds the shortest line that divides a given area into two equal parts.

The algorithm is as follows for dividing a state into N districts:

  1. Start with the boundary outline of the state.
  2. Let A = [N/2], B=[N/2]
  3. Among all possible dividing lines that split the state into two parts with population ratio A:B, choose the shortest. (Notes: since the Earth is round, when we say "line" we more precisely mean "great circle." If there is an exact length-tie for "shortest" then break that tie by using the line closest to North-South orientation, and if it's still a tie, then use the Westernmost of the tied dividing lines. "Length" means distance between the two furthest-apart points on the line, that both lie within the district being split.)
  4. We now have two hemi-states, each to contain a specified number (namely A and B) of districts. Handle them recursively via the same splitting procedure.

Advantages:
- Limits the ability to redistrict in a partisan fashion

Disadvantages:
- Cannot take into account Communities of Interest
- Will most likely not create Minority Majority districts.

These issues make it unsuitable for the majority of states since consideration of Communities of Interest is necessary.

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