Interpreting mtfcc and geo_id
And how those values relate to US Census GeoID and FIPS Codes
OVERVIEW
BallotReady gathers and exports geospatial data of political districts of varying sizes (ie. states, counties, municipalities, districts, and subdistricts) across the country. These geospatial exports have some unique attributes that can usually be related to US census GeoID and FIPS codes, explained below. In some cases, mtfcc values will not have any corresponding entry in the census file; those cases are also explained below.
DEFINITIONS
HOW TO USE MTFCC AND GEO_ID
The first two digits of any geo_id correspond to the state-level FIPS code for the state to which it belongs. For example, every geofence for anywhere in Texas starts with 48, because that's the state-level FIPS code for Texas.
Mtfcc and geo_id fields should be treated as pairs. Meaning that there could be more than one record in the census file with the same geo_id, but the mtfcc value identifies the type of census entity. BallotReady datasets should be joined to the census file on both the mtfcc and geo_id.
mtfcc values that start with X will not have any corresponding entry in the census file. These mtfcc/geo_id pairs are for custom boundaries that BallotReady collected, that are not available via the census. Note that there's not one clear explanation about how to use the custom mtfcc values.
One example for how we create the custom values is that city council subdistricts will have an mtfcc = X0001, and the first 7 digits of the geo_id will match back to the US Census GeoID for that place. Take Austin City Council - District 2, which has mtfcc = X0001 and geo_id = 480500000002. Those first 7 digits (4805000) would match the US Census code for Austin, TX (mtfcc = G4110, geo_id = 4805000). The last 5 digits in our custom geo_id is used to designate the district within the city, this one being District 2.
The geo_id values for school board and municipal entities do not cleanly map back to counties, but just the state level reference. You can see how those geo_ids are calculated by the Census in
the table on this page.
Tracking geofences over time
Depending on the scope of your export, there can be multiple geofences for the same mtfcc/geo_id pair that are distinguished by the valid_from and valid_to fields. That's how we track how the boundaries for a given political jurisdiction can change over time (due to redistricting, annexations, etc.).
