K9YC said:
> The FCC map is ancient -- I'm guessing late '40s-'50s.
Good guess, Jim. N6LF just sent me an old IRE article from 1954 that
explains the derivation of the map. It's intended to predict propagation
for AM broadcast signals over broad areas, not accurately indicate local
conductivity. The first item shows why it's not suitable for ham use:
"The following characteristics were considered desirable and
incorporated into this conductivity map.
1. The indicated conductivity should not change at
less than 10 mile intervals."
This will download the article:
https://www.dropbox.com/scl/fi/igy9czdbhhlh1y24jhrn0/An-Effective-Ground-Conductivity-Map-for-Continental-United-States.pdf?rlkey=l4fwymiq0g721d25p24sdu0mg&dl=1
Brian
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