Figure R3 of 47 CFR 73.190 of the FCC'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. Larger maps for the entire country are available at http://www.fcc.gov/mmb/asd/m3/m3.html.
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