If I have an 8 ft deep ground rod and want to test its effectiveness, can I simply measure the resistance between the local power outlet's ground and the earth ground in question? Will this give me any useful data? I measure about 6.5 Ohms between the utility ground and the isolated earth ground. I'm thinking the resistance between the two grounds should be (0+(the accumulative resistance (as determined by its resistivity or resistance per unit length) of the conductive material between the point of contact with my probe and the point of dissipation to ground)). Is this accurate? Thanks for the insights!