As far as I can tell, the Accumeter is a 2-electrode "ground ohmmeter". Hard to get repeatable readings in anything other than ideal conditions (this limitation was pointed out even in the eval story they publish on the website).
If you're serious about resistivity measurement, you need a minimum of a 3 or 4 wire system such as are used in geophysical exploration. The 3-wire has limited capability, the 4-wire is industry standard. With 3 and 4 wire systems you're measuring the resistivity of the ground itself and not the resistivity of your electrode contact.
The irony of it is, the Accumeter with a little more engineering effort having almost no impact on manufacturing cost, could have been a real ground resistivity system instead of the county-fair style corny and nearly useless gadget it actually is.
[EDIT] just checked out the GPL-200. It's the same sort of gadget. Both gadgets are described as glorified ohmmeters and the advertising for neither gives any indication of Wenner or Schlumberger array capability. For the same price tag, it could have been the real thing, what a shame!
NOTE: All resistivity exploration systems of any kind require mapping and interpretation, even the Accumeter advertising illustrates that fact. It's not beep and dig like a metal detector.
