FCC Ground Conductivity Map

The U.S. Federal Communications Commission published a Ground Conductivity Map for the continental United States in 1975, and it is still the most current version. No doubt more modern ground conductivity equipment and methods could produce a more detailed and accurate map, but for general purposes, this suffices. When more precise local soil conductivity measurements are required by electric utilities, communications installations, etc., teams of technicians and/or engineers are dispatched to obtain measurements. Many moons ago (early 1990s) while doing a short stint at the Potomac Edison Electric Company in Hagerstown, Maryland, our electric substation guys had some pretty impressive measurement gear that used probes pounded into the ground in multiple locations to gets resistance (conductivity) readings between them. Where the soil conductivity is not sufficient to meet specifications with simple ground rods and/or meshes, chemical agents would be spread around the area to raise conductivity. In extreme cases, tubes were sunk into the ground so that a solution could be poured in at prescribed intervals in order to maintain the specification. Antenna installations have similar ground conductivity requirements in order to assure a sufficient counterpose to assure the radiation patterns are according to expectations.

Federal Communications Commission Ground Conductivity Map, circa 1975

From the FCC Ground Conductivity website:

Figure R3 of 47 CFR 73.190 of the Commission's Rules contains a map of the estimated effective ground conductivity in the United States. This data is used to predict the propagation of AM signals across the United States. A higher ground conductivity indicates better AM propagation characteristics. The map shows that the ground conductivity in the U.S. ranges between 0.5 and 30 millimhos (or millisiemens) per meter. The conductivity of seawater is 5,000 millimhos per meter, resulting in the best propagation of AM signals.

The color map here was printed for the FCC around 1975. Originally the map consisted of two large-print sections (East and West, each approximately 45 by 43 inches.). The sections were scanned in parts, and then the parts joined with photo-stitching software. There may be some artifacts and minor distortion from this process, but on the whole the map here should suffice for most purposes. The map can downloaded in three sizes: TIFF file, 151 MB, PNG image, 17.6 MB, or PNG image, 1.2 MB (pictured) .

The M3 conductivity data is also available as text files. Data for the continental USA is located in the file m3.seq, which also contains updated conductivity data for the Chesapeake Bay and Puget Sound that is not shown on the wall map. Data for Hawaii is contained in the file m3hw.seq. Data for the rest of Region 2 (the Western Hemisphere) is contained in the file r2.seq.

Lastly, the M3 map segments may also be displayed in Google Earth for the continental USA, Canada, and Mexico, via the following KML file: Continental USA/Canada/Mexico M3 KML.

For more information on AM and FM radio broadcasting, please visit the Audio Division website, and the Broadcast Radio Links page.

Posted July 7, 2020