It hard to say what tolerances are to be employed. Measuring resistances in different seasons it could help in a way. For more information see British Standard 7430/2011 ch. 9.3 Soil resistivity or:
Indian Standard IS : 3043 -1987(Reaffirmed 2006) SECTION 2 CONNECTIONS TO EARTH or:
IEEE 81/1984 IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Ground System ch.8.2.1.6 Interpretation of the Results.
If you look at IEEE 80, it has a graph on how ground impedance can change as temperature changes. Ice is a bad conductor and in place like Iowa, some utilities pound ground rods 30 ft into the ground to make sure they have a low impedance ground path. The frost depth in Iowa is about 3 ft I believe.
