Difference between two-point, three-point, and four-point Ground Testing?

Literally, the number of points of contact with the soil. More specifically, these commonly used terms refer to dead earth, fall of potential, and Wenner method tests, respectively. In the dead earth method, contact is made at just two points: the ground electrode under test and a convenient reference ground, such as the water pipe system or a metal fence post.

In the fall of potential method, a genuine ground tester makes contact via the test electrode, plus the current and potential probes. With the Wenner method, no ground electrode is involved, but rather the independent electrical properties of the soil itself can be measured using a four-probe setup and a recognized standard procedure.