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 https://www.fcc.gov/media/radio/m3-ground-conductivity-map.
