Mark I am aware of soil resistivity variations around the country. Thanks for bringing it up as it is worth talking about As you know the NEC does not specify an impedance for earth ground other than to say must be supplemented by 1 additional rod if the impedance is above 25 Ohm's. You are also aware of the SPARKY JOKE: Drive Two Rods, Collect a Check, and Call it Done. The irony is the joke is true because that is default. No Sparky bothers with driving 1-rod then trying to prove it to the inspector, because the chances of it being 25 ohm's or less is astronomical. So you drive two rods and its done right? OK with that said on average a single rod will yield around 500 ohm's. However when it comes to lightning the actual impedance of a driven Rod is irrelevant and meaningless. The reason is because of the impedance of SINGLE GROUND CONDUCTOR at frequencies associated with lightning. For example If I were to give you a spec to drive 15 rods and keep driving them until you reach 5 Ohm's as tested with a 3 point Dead Fall of Potential Meggar. I then ask you to run a 750 MCM from the disconnect to the nearest rod underneath the disconnect box. And let's say that piece of 750 MCM is 10 feet in length once you complete the circuit. Would you say at that point we have an excellent ground? Most would and do. But I am here to tell you it is worthless as a lightning and RF ground, and just about worthless as an Electrical Ground. Are you surprised by that statement? Let's just start with the Electrical portion. Let's say I take a fault somewhere between the meter and main panel directly to earth. From the disconnect we have dual 200 amp fuses on a single phase 240/120 service. What happens if say L1 goes to earth? Nothing right? With 120 volts imposed across 5 ohm's we get 60 amps of current through a 200 amp fuse. Fuse does not see a fault with 60 amps, it needs to see at least 1200 amps before it will operate in less than 1 second. OK yeah I know the fault will likely happen in a raceway which is bonded and get a few thousand amps of current, the fuse operates and clears the fault. But Earth had nothing to do with it, as no current ever flowed through earth and not needed. OK now let's get real with lightning and RF. What is the impedance of a 10 foot long of 750 MCM cable? You cannot answer that without more information right? You need a frequency to determine that. So here are some answers at various frequencies for a single 750 MCM copper conductor. Will heck it could be silver, makes no difference. 1 Mhz = 26 Ohm's 10 Mhz = 260 Ohm's 100 Mhz = 2.6 K Ohm's OK for lightning the K/D current rise time equates to more about or greater than 100 Mhz. Now answer this question What is the total series impedance of a 5 ohm earth ground in series with a 2.6 K Ohm's conductor equal? 2.6 K Ohm's right. Last comment after you pounded those 15 ground rods into the ground all day you measured the impedance at power frequencies of DC up to 300 Hz. Actual RF and lightning frequencies are considerable higher.