When a city council, county legislature, or state legislature redistricts itself to reflect changing population, they usually release data in two formats:
- Census Block Equivalency – A list of census blocks in each district, generated by the commerical GIS program (such as Mapitude) used for redistricting.
- Metes and Bounds – A legal description of each district, used in resolving court disputes over district boundaries, and assisting board of elections on where to put voters whose property might be crossed by a Census block
If you planning on making a map, Metes and Bounds won’t be particularly useful. Computers don’t understand english very well, they need numbers and lists. In contrast, the Census Block Equivalency is very useful for mapping things.
Every year, the Census Bureau puts out series of ERSI Shapefiles known as TIGER/Line. You can download TIGER/Line for any state and county in the United States from their website. They provide many different shapefiles and layers such as a Highway, Faces, Edges, and County Subdivision layers, however the one you will be most interested for making district maps is the Tabulation Block (tabblock) layer.
You can use these files in the free program known as Quantum GIS or QGIS. While this tutorial will not explain the ins and outs of QGIS, this should get you started on making redistricting maps.
The Tabulation Block Layer is the file containing all of the Census Blocks for a particular county. A Census Block is the smallest unit of population gathered by Census Block, and consists of all bordering features (bounds) — roads, rivers, shorelines, along with all imaginary lines (metes) — town lines, village lines, other lines drawn for statisitical purposes.
Each Census Block has a number, that is a subdivision of the Census Block, County ID, and State ID that it resides within. For example, the Governor’s Mansion in Albany is located in Census Block 2000 in Census Track 23.00 (zero padded to 002300) in Albany County (Federal Information Processing Standard — FIPS ID: 01) which is in NY State (FIPS ID: 36). County subdivisions are not applied to Census Tract Numbers, as they may in some cases cross county subdivisions, as is the case of smaller districts.
You put those numbers together to get the GEOID — which is the key used for redistricting block lists and most other block-level census data. The Governor’s Mansion is located at a block with a GEOID 360010023002000.
| 36 | 001 | 002300 | 2000 |
| State ID | County ID | Zero Padded Census Tract Number | Census Block Number |
|---|
The block list you get from a redistricting commission typically is in Database Exchange Format (.DBF) or Comma Deliminated Format (.CSV) which are both openable by common spreadsheet applications like Microsoft Excel or OpenOffice Spreadsheet and GIS programs like ArcGIS or Quantum GIS.
This is taken from the LATFOR State Senate Proposed Districts (January 2012) DBF file. It shows you that the Governor resides in Proposed Senate District 44. Across the river in Census Block 4010, Census Tract 524.03, in Rensselear County (FIPS ID 83), NY State (FIPS ID: 36) is located in Proposed Senate District 43.
| 360010023002000 | SD44 |
| 360010021002008 | SD44 |
| 360010021002004 | SD44 |
| 360010021002001 | SD44 |
| 360830524034017 | SD43 |
| 360830524034010 | SD43 |
Download the TIGER/line “Tabulation Block” Shpaefile file for the district you are interested in. You will want the 2010 version. You can download a state-wide tabulation block file, however that is not recommended as the next step will be impossibly slow on most computers. You may also want to open the .CSV or .DBF file in your spreadsheet program and cut out the county you want to speed things up.
You will then want to open up the file in Quantum GIS. You will get a nice map of the county you downloaded, showing all of the Census Blocks.
- From there, go to the Vector -> Join Attributes submenu.
- Make sure that the Target vector layer matches the Tabulation Block Shapefile you wish to join against, then set Target join field to GEOID10 .
- Select click Join dbf table and select the DBF or CSV file you wish to join.
- Change the Join field to BLOCK or whatever the GEOID is titled in your redistricting block file.
- Enter in a location to save the Output Shapefile
- Click okay.
Then wait. A typical county will take 10-40 minutes to join on my 5 year old laptop; your computer may be quicker. If you have a dual processor machine, go on to doing other work in other programs. You will end up with a map that looks like this (stylized for your enjoyment). Each block will be assigned a Senate District (in this example).
Halfway there. Now you need to “dissolve” each Census Block into it’s larger political district. Go to Vector -> Geoprocessing Tools -> Dissolve . Set the Input vector layer to the file you previously joined. Then set the Dissolve field to the field containing the district number — such as DISTRICTID or whatever it is named. Enter a name to safe the file. Click Dissolve.
Outputed will be a Shapefile containing all the political districts in the county you joined and dissolved. This will take 5-20 minutes on my laptop. Other data may exist in that file, such as Census Block number, however at this point that data will be invalid, as only the district number is accurately preserved in such a join. All other data will be picked at random, so delete those columns.
I hope this is helpful. If you just want the Proposed State Senate or State Assembly Districts you can download them from Center for Urban Research. These are the same data, joined using the above process by somebody with a much faster computer. I have also made up a Shapefile containing the Albany County Legislative Districts using this process.