There are more accurate methods to calculate ground conductivity, but
what's the point? The skin depth in soil is on the order of 10 or 20 feet,
depending on the frequency and soil quality. This means that substantial
current is flowing down to a few times this depth. Certainly where I live,
and I'd bet that in most locations, the conductivity is far from uniform.
So in order to know the conductivity of the soil which is carrying
current, you'd need to measure it down to several tens of feet.

Once you had that data, what would you do with it? Currently available
modeling programs assume homogeneous ground to an infinite depth. So you'd
have to choose some single value from among your measurements if your
objective is to get better accuracy from a program. But there's no
evidence that a homogeneous ground with any single value of conductivity
will behave the same as a stratified ground.

So having even an extremely accurate measure of surface conductivity at a
particular radio frequency (and it does vary with frequency) still gives
you much too little information to build even a crudely accurate model of
the actual ground in which the current is flowing.

Roy Lewallen, W7EL

Certainly, all valid points. I was more interested in actually doing
precise measurements, but considered it might improve my model accuracy. I
even thought of digging a hole to see how the soil varied. Doubt I would
have dug down 20 or 30 ft. Most of the ground here is clay, and then
probably bedrock, at this elevation of just over 4,000 ft ASL. Ansoft's
HFSS, or CST, could probably handle an accurate, stratified, ground model.

Frank