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The FCC has .seq files that show ground conductivity which it makes available for download. Unfortunately, there doesn't appear to be any documentation that comes with the files, or even headers.

I can make an educated guess that there are two longitudes/latitudes for each record, and the last two numbers represent perhaps a range of ground conductivity. What is the meaning of each field? Is there anywhere I can learn more?

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    $\begingroup$ Working on it... it looks like it's a vector file for the contour map data... the second through fifth fields are lat(N)/lon(W)/lat(N)/lon(W) for a line segment, sixth and seventh fields are the conductivities on either side of the segment, in mS/m. But there's more to figure out, like what the "9999" records mean (9998 appears to be the end-of-file marker), whether the jumps in the first column value are significant, and a few other things. $\endgroup$ – hobbs - KC2G Jun 27 '19 at 13:49
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The figures on that map are approximate.

They represent estimated ground conductivity in millimhos per meter. And it varies with (among other things) how wet or dry the soil is.

FCC CONUS ground conductivity map

I live near a line where on one side it is 15, and on the other side it is 8. Driving across that line, the conductivity does not suddenly jump to the other number. Rather, it changes gradually. Having said that, there are some places in the US where there is a drastic change between the soil types, which often are visible.

Much more details can be found here on the Topband Reflector. In one of those threads, K2AV has a method for laying a dipole on the ground to measure it.

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    $\begingroup$ The map data is presumably encoded in the .seq files linked to from the web page given in the question. The question is about how to interpret the .seq files, which are text files containing seven columns of numbers with no header row. $\endgroup$ – rclocher3 Jul 28 '19 at 4:35

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