This is just starting to percolate into the news. The FCC last friday approved release of google’s TV white spaces database which documents what unused TV channels may be used for wireless internet access type services in which areas. You can browse a visual representation in google maps:
On this page, you can download these databases that include US, Canadian, and Mexican TV Sations, TV translators, Broadcast Auxillary SErvice Links, PLMRS/CMRS base station operations, Multi-channel Video Programming Distrbutor (MVPD) receive sits, Temporary BAS links, Low Power Auxillary Devices, Fixed TV-band white space devices.
These databases might be more accurate than the usual FCC transmitter and tower databases for parts of the spectrum since if you aren’t in this database someone is likely to start transmitting on top of you.
Another reminder that unused spectrum is up for grabs.