Redistricting is the act of arranging all of the census blocks in a jurisdiction into equally populated districts. The unique configurations created by each if these arrangements are termed redistricting plans. A redistricting plan’s specific arrangement of census blocks into districts is recorded by the computer mapping software in a file known as the ‘Census Block to District Equivalency’ file.
Before the widespread use of Geographic Information Systems, or GIS software redistricting plans were documented and recorded into law as written metes & bounds.
Metes and bounds are voluminous written descriptions of the location of district lines. They are often accompanied by paper maps in order to assist with the location of the district lines relative to other geographic features that may be referenced in the written descriptions contained in the metes and bounds.
Since the advent of GIS, ‘Census Block to District Equivalency Files’ also known inter-changeably as assignment files, have replaced metes and bounds as the standard method for precisely documenting and recreating redistricting plans.
Redistricting Plan versus Redistricting Map
“Redistricting Plan” and “Redistricting Map” are terms that are often used interchangeably in redistricting vocabulary. The redistricting plan is the unique configuration of the districts as recorded by the plan’s equivalency file.
The term “Redistricting Map” can actually be used to refer to several related things. It can be used to refer to the redistricting planWhen working with GIS software, “Redistricting Map” can also refer to the collection of spatial files and map layers, including the census blocks, district lines, counties, cities and streets that are displayed as a “map” in the GIS software. “Redistricting Map” can also be used to refer specifically to the map layer/ spatial file/ GIS file, of the redistricting plan’s districts that is created by assigning the census blocks to districts.
The “Maps” that most people are familiar with are images or screen captures of spatial data that are displayed by GIS software a map layer. Most maps exist as static image documents such as, .jpeg, .tiff., .pdf files, or printouts, and may contain additional map layers for reference. In Redistricting, image maps are usually created with GIS software but it is possible to create them with software like Adobe Photoshop. They can only be edited using image editing software. Static image maps are not detailed or precise enough for plan creation, evaluation or documentation.
In these pages, the term “Redistricting Map” refers to the GIS map layers and the associated attribute data that were used to create a “Redistricting Plan”
Map Layers, GIS, and Spatial data files
Map layers, GIS files and spatial files are a set of terms that describe files containing coordinate/ geographic referenced data otherwise known as spatial data. Spatial data files tell GIS software where the district lines and other geographic features should go, while image maps (.pdf, .tiff, .jpeg etc) are screen captures or pictures of spatial files.
“Map layer” is the GIS term for a spatial/ geographic data file as it is displayed by GIS software. They are files that can only be opened and manipulated by GIS software. Map layers may have attribute data associated with them. Examples of spatial or GIS files include ESRI ArcGIS .shp files, Caliper Maptitude .cdf files and Google .kml/.kmz files.
Furthermore, spatial files are GIS package-specific which means that not every GIS program can read and display files created by a different GIS program. To align spatial files from two different sources, the files must use the same coordinate system, which is not always the case; this is why spatial files are not the preferred format for documenting and evaluating a redistricting plan.
Redistricting Plan Equivalency Files
Equivalency files are computer files that contain information about geography used in redistricting. In their simplest form, they are spreadsheets with two columns. For example, one column might contain the census code for every block in the state and a second column might contain the district number to which that census block is assigned in the plan. An equivalency file lets a user move from one unit of analysis to another by showing how these units are related to the districts in the redistricting plan.
In 2001, there were 533,163 census blocks in California. Therefore the 2001 state legislative district equivalency files have 533,163 records, one for each census block in the state. Other columns in the file correspond to whatever kind of plan is being created. For example, there may be a column for State Assembly or Senate District, and a number next to each census block entry inciating the district in which that block is located.
Equivalency files can be saved in any spreadsheet/ data file format and will often be found as .dbf, .xls, .txt, .csv files. Equivalency files can be imported into most standard GIS platforms, and in conjunction with a spatial file of the referenced units of analysis, can be used to recreate the redistricting plan’s lines precisely.
Equivalency files can be opened in a statistical package such as SPSS, SAS, STATA and Excel (as long as the file has less than 60,000 records) and merged to census block data such as the Census Bureau PL94 data and/ or Statewide Database voting and registration data. Using the district assignment field, it is possible (in this example) to tabulate the district totals of the data used into district reports.
TIGER/Line® Shape Files
The boundaries of the census blocks that correspond to the PL94 population counts are published as spatial files in .shp file format by the U.S. Census Bureau. These files are known as TIGER/Line®Shapefiles. TIGER stands for Topologically Integrated Geographic Encoding and Referencing.
The 2010 TIGER/Line Shapefiles that will be used for redistricting with the 2010 census data release are scheduled to be released between December 2010 and February of 2011, state by state. California’s size makes it one of the more difficult states for the Census Bureau to process, thus California TIGER/Line files will most likely be some of the later ones to be released.
The 2010 TIGER/Line Shapefile release will contain map layers of the updated boundaries of the Census Bureau’s statistical tabulation areas (census blocks, block groups, tracts, etc) that correspond to the 2010 PL94 data, and generally, of all the geographical areas for which the census bureau publishes data from the state level down to the census block.