The API accepts calls using a geographic FIPS code. These codes are available recursively through the API but if you have an application that produces a geographic coordinate point, here are ways you can convert that to a FIPS code:

1. See the Census Geocoder and API
   
2. See the TIGERWeb Mapping Web Service

These relationship files are provided as a tool to help data users compare the universe of Census geographies from the 1990, 2000 and 2010 Censuses. From these files, data users may determine how geographies from one Census relate to those from the prior Census.