I guess from JLMCDANIEL´s explanation above it does indeed make it difficult to know what part of this impedance is in fact the ground since the phase wiring is included. My house is in fact spread out over 2 buildings so there are some larger than usual distances involved.
I was just trying to get an estimate because lightning hit near the house the other day and 3 of my breakers went off. Also some damage to some minor electronics. Luckily computer was isolated.
Excelent and informative discussion here.