Redistricting can be carried out using manual techniques - colour markers, paper maps, and calculators - or using sophisticated computers and GIS software. The technology employed does not change the task of the redistricters but it does change how the process is conducted and expands the information and options available to decision-makers when a final redistricting plan must be chosen. If GIS is to be used for drawing district lines, the first change in the process is the need to compile an electronic database.
Collecting the Necessary Information
Redistricting requires the collection of several different types of information. The two essential pieces of information are population data and maps. The population data, which may be in the form of census enumeration data or voter registration data, provide the only means of creating districts that are relatively equal in population. The population data must be associated with specific geographic areas and must be as accurate and up-to-date as possible. Maps are needed to ensure that only contiguous geographic population units are assigned to districts.
If the potential political impact of proposed redistricting plans is desired, then a third piece of information is necessary as well: political data. Political data may consist of statistics on the political party affiliation of electors, if available, as well as election results - tabulations of votes for candidates and ballot measures from previous elections by voting area. Including political data in the redistricting database allows line drawers to produce a political profile of proposed districts and to predict, to some degree, the partisan implications of a redistricting plan.
Election results can easily be entered into the redistricting database if they are reported for the same geographic unit as the population data. This will likely be the case when the population units for redistricting are based on voter registration data. If, however, the population units are based on a census enumeration, the geographic units for population and political data may not be the same. In that case, census geography and election geography may have to be matched in some manner to create geographic units that can be associated with both population and political data.
Population Data
Population data is essential to the redistricting process. Redistricting operates by combining discrete geographically-based units of population to produce districts of relatively equal population. The population data can be either total population counts based on a census enumeration or voter registration data; the geographic population units will therefore reflect either census geography or election geography (these units of geography are often, in fact, the same).
Traditionally the necessary population data has been collected and tabulated by hand. Recently, however, many countries have begun to computerize this data. If GIS software is to be used, population data (regardless of whether it is census enumeration or voter registration data) must be available in electronic form. Furthermore, each reported population count must be associated with a single unit of geography.
Maps
Maps are essential in the redistricting process, but obtaining adequate maps can be one of the most challenging obstacles facing redistricters. While standard maps, such as road maps, identify geographic features and the boundaries of administrative areas such as counties, cities and towns, they may not provide sufficient detail for drawing electoral districts.
If redistricting is based on a census count of the population, then the maps must show the boundaries of the census geographic units for which population statistics are available. If redistricting is based on the number of registered voters, then the maps must show the boundaries of the voting areas for which voter registration statistics are available. The boundaries of the existing electoral districts are usually needed as well as a beginning point for drawing new district lines.
In order to use GIS software maps must be digitised and available in electronic form. Some redistricters have access to computerised maps because their governments have supported the development of uniform, digitised maps of the entire country and have chosen to make these maps available to redistricters. Digitised maps may be available from other sources, for example: local governments may have compiled digitised maps for transportation, health and other service planning or for land and resource management purposes; utility companies may have created digitised maps for service provision, network management or telecommunications purposes; or private companies may have digitised maps for fleet management, retail site location or for general marketing purposes.
If boundary authorities have access only to paper maps, digitised computer maps of the country can be created, but only at an enormous cost. Of course these detailed computer maps can be used for many purposes other than redistricting and therefore may be worth the time, effort and expense.
Synthesising the Information
If GIS software is to be used to redistrict, a redistricting database must be prepared once the necessary information has been collected. In an electronic redistricting database, spatial data is related to non-spatial information such as population and demographic data. The spatial data must include the boundaries of the geographic units to be used as the basic building blocks for creating the districts. Population data must be related to each geographic unit. This data may be total population or the number of registered voters for each unit. Additional demographic data, such as population counts by race, ethnicity, religion or language-use for each unit may also be included if useful for redistricting. If the redistricting database is to include political information, then votes for candidates from previous elections should be entered for each geographic unit.
Choosing Between Census and Election Geography
Quite often census and election geography are one and the same and choosing between the two is not an issue. In some countries, however, census geography and election geography do not coincide and choices may have to be made. If voter registration data is to be used to create electoral districts, then election geography (for example, polling areas) may be employed as the basic building block for drawing district lines. This geography may correspond to the geographic units for which census data is reported, but it may not. If, on the other hand, census data is to be used, then the geographic units for which census data is reported is usually employed as the basic building blocks for redistricting.
If both census and election geography is available, and the two do not coincide, then the choice of which to use as the basic building blocks for redistricting often depends on who is making the decision. Election officials often prefer that election geography be used as the building blocks for redistricting, since they wish to avoid changing the existing system of voting areas as much as possible. Political parties and incumbent representatives may also wish to use election geography to create redistricting plans because political information is more readily obtainable for proposed district plans if the districts follow voting area boundaries. Regardless of which geographic unit is selected, if both political and census demographic data are desired, and election geography and the population geography do not correspond with one another, they will have to be matched. Matching census geography and election geography can be quite challenging.
Once a decision is made as to which unit of geography will serve as the basic building block for redistricting, a database can be created. This database is composed of spatial data reflecting the geographic units chosen as the basic building blocks, as well as other geographic information of importance such as administrative boundaries (for example, cities, towns, villages), and tabular data associated with these geographic units (for example, census counts, previous election results).
Drawing District Boundaries
Once a database has been prepared, the next step in the redistricting process is the formation of districts. The line drawers create a redistricting plan either by assigning geographic units to districts for the first time or by moving geographic units in an existing plan from one district to another. A redistricting plan is complete when all geographic units in a given territory are assigned to a district and all districts in the plan meet the predetermined redistricting criteria. The geographic units to be assigned may be as large as counties or cities and towns, or they may be smaller units of geography such as census blocks or voting areas.
If the line drawers are to create a new redistricting plan by modifying districts in an already existing plan, their tasks may be relatively straightforward. This is especially true if a decision has been made that districts should be modified only as much as necessary to meet equal population standards. If there is no existing redistricting plan to modify or if the number of districts in an existing plan is to change substantially, however, the line drawers' tasks become more challenging.
The line drawing process is an interactive one; the redistricters assign a piece of geography and then determine how the inclusion of that particular geographic unit affects the size and composition of the district. Although aggregating the population totals for the districts after each new assignment is a straightforward process, it can be a tedious one as well. And as geographic units continue to be shifted, this process is repeated over and over again. A plan is completed and ready for evaluation when all geographic units have been assigned to a district.
Although this process has traditionally been done manually with an adding machine or a hand-held calculator, computers and GIS software can automate this process so that each time an assignment is made, the resulting map, population counts, and even political election results, are re-tabulated and displayed immediately on the computer screen. This allows redistricters to access the result of their assignments immediately. Redistricting plans can be created considerably faster and more efficiently. And the results can be accessed much more accurately.
