Geo-Coding vs. ZIP Codes: What's the Difference?

August 15, 2012 by Crescerance

One critical feature that a Grassroots System should have is the ability to assign people to their legislative districts based on their home address. Supporters vote where they live, and when they vote they are a constituent of a legislator. Knowing which district your supporters live in gives you the ability to look at a legislator and instantly know how many of your advocates are the legislator’s constituents. Having this information allows you to better engage your elected officials because you can illustrate to them how strong the support for your organization is within their district.

Legislative districts are determined in each state every ten years using the decennial census data. District lines are drawn according to demographic data based on race statistics of the population. The process utilizes a geography measurement called a census block. A census block is bounded by streets and resembles a city block. In turn, it is part of a group of blocks, which are part of a census tract, which is part of a county. The district drawing process can therefore neatly fit into counties or cities depending on how the elected officials drawing the lines decide they want the lines drawn. When the redistricting process is done, each state produces map and files illustrating the legislative district boundaries. One of the files they create is called the Block Equivalency File and it shows all the census blocks that make up each district within each state.

The challenge in relating your supporters to their legislative districts is how accurately determine an address’ census block. This process is called geographic encoding or “geo-coding”. The geo-coding process takes an address and matches it to the master file of all street addresses and their associated census blocks. The census bureau has such a file that they use to conduct the census and several private companies also maintain this data for address verification purposes.

So what happens if instead of geocoding, your grassroots system uses ZIP codes on the address of your supporters to match up people to their legislators? Does postal geography fit into the scheme for creating legislative districts? Unfortunately, the answer here is no. Using ZIP codes to match to legislative district can result in inaccurate information because zip codes define mailing geography and have nothing do to with census geography. Zip codes are drawn to help the postal service deliver mail efficiently. So ZIP code geographic areas can cross legislative district boundaries. If you have an address where the ZIP crosses two districts, you don’t have an easy way to determine which district the address should be assigned to. Even 9 digit ZIP codes have this problem. While these ZIP codes go down to the street segment level, covering a handful of addresses along a street, they may still span two or more legislative boundaries because they are not defined with legislative districts in mind.

Below we see a map of Georgia. The large colored areas indicate the legislative districts, while the smaller areas represent ZIP Code geography. As you can see in the cut-away, some ZIP codes span multiple legislative districts. If you have an advocate that lives in that ZIP code there is no way for you to know which district they actually live in.

How does your Grassroots System assign your supporters to their districts? If you are using ZIP codes instead of census block data you probably do have inaccurate assignments!