Methodology: digital radio map
Most of the information for the digital radio map comes from the FCC's CDBS database as of April 13, 2010. FM contours come from a separate FCC file accessed Jan. 30, 2010. Information about station owners and radio format comes from iBiquity (as of April 22, 2010); Arbitron market information comes from AllAccess.com (as of May, 13, 2010) and is reproduced here with permission. ZIP code information is from ESRI's 2009 file. Some station formats have been expanded from the abbreviations used by iBiquity and, in other cases, edited for clarity.
FM station markers are placed at the transmitter location identified in the FCC's radio contour files. This location may vary greatly from city identified as the "community served" on FCC filings. The service contours (the circular service areas that appear on the map) are based on licensed operating conditions, and may not reflect day-to-day transmission changes. According to the FCC: "Distances to FM service contours are generated using the effective radiated power in a given direction, and the radial HAAT in that direction, with the FCC's F(50,50) propagation curves. See FM Propagation Curves and Antenna Height Above Average Terrain (HAAT) Calculations for additional information on this process." FM contour shapes have been simplified to minimize download time.
AM signal strength varies greatly by time of day and other factors. Consequently, the FCC does not calculate station range the way they do for FM stations. In the map, we've considered AM stations within 50 miles of a particular ZIP code to be available in that ZIP code. Most AM station markers are placed at the strongest daytime transmitting antenna; in cases where FCC data is missing or ambiguous, the markers have been placed on the "community served".
The US Postal service assigns some ZIP codes to single locations; these ZIP codes are not included. ZIP code outlines used in this analysis are from ESRI. Address lookups are performed by Google's geocoding and reverse geocoding services; for best results, use a complete street address.