Soil resistivity testing is the single most critical factor in electrical grounding design. This is true when discussing simple electrical design, to dedicated low-resistance grounding systems, or to the far more complex issues involved in Ground Potential Rise (GPR) studies. Electrical interference, causing unwanted signal noise to enter the meter. Metallic objects ‘short-cutting’ the electrical path from probe to probe. The rule of thumb here is that a clearance equal to the pin spacing should be maintained, between the measurement traverse and any parallel buried metallic structures.