Neutral redistricting criteria

Redistricting criteria typically divide into categories based on the source of the information used for the criteria. Criteria based demographic and geographic information are generally neutral. Use of political data for redistricting is the traditional means to gerrymander. However, there are neutral political criteria as well. Compliance with the Voting Rights Act generally may require demographic, geographic and political data.

Demographic Criteria

Demographic criteria include population equality and minority representation. Communities of interest can also be a demographic criterion.

Demographic criteria can be derived from census data. The Census releases a data set called Redistricting Data (PL 94-171). This is limited data set in the range of information but it is tabulated at a very fine granularity. The 2010 Redistricting Data Summary File included the total population, voting age population (VAP) 18 and older, population by race, population by Hispanic identification, total housing units, and housing units by occupancy.

Contiguity

The most important geographic condition for districts is that they be contiguous. In its simplest form this means that all areas in the district touch the same area, that is there are no separate pieces that make up the district. The idea of dividing a large area into smaller contiguous pieces allows redistricting to be translated into a problem in graph theory.

Some states place restrictions on contiguity. The most common restriction is to bar point contiguity. Point contiguity exists when an area of the district touches the rest of the district at only a single point. Two same-colored squares that are adjacent on a diagonal of a checkerboard are point contiguous.

States can also require that it be convenient to travel within a district. This requires knowledge of the roads in the state. For example there can be a requirement that all areas of the district be reachable from each other by road. Roads can be limited to public roads, all season roads, and can include ferries. Use of major highways to link larger areas of a district can also encourage districts that represent communities of interest.

Geographic Integrity

A common geographic condition is to preserve the natural political boundaries such as counties, cities, and townships. This requirement for political integrity can take a number of forms. A general requirement for integrity suggests that a map should seek to avoid unnecessary chops of a political unit. A more specific requirement for integrity is to mandate that certain political units like counties be preserved first, then preserving other units if a county must be split. This creates a hierarchy for integrity among the geographic units.

If a state requires integrity of geographic units the the county is the most common one to be preserved. If a state is divided into counties every resident is in a county, and is likely to recognize in which county they reside. States use county structure to provide many services, especially election control and tabulation. The combination of universal coverage and election services make counties the natural geographic unit to preserve.

In some cases hierarchy can be strictly maintained. For example, many states divide counties into townships, towns, cities or boroughs so that every area in the county is in one of the county subdivisions. For those states one can impose a requirement that if a county must be chopped, then the county subdivisions should not be chopped unless necessary. In states that lack defined county subdivisions, there are other units like voter tabulation districts that can be used.

The hierarchy can also extend larger than the county. Metropolitan areas are defined in terms of contiguous groups of one or more counties.

In many states cities and villages pose a problem. Not all residents are in incorporated municipalities, they can span county and township lines, and are not always recognized as county subdivisions. If municipal integrity is important then it requires balancing the municipal integrity with county integrity.