PK nail
A thick-shanked nail with an indentation in the middle of its head, driven into the ground to mark a position precisely.
PK nail: ground marker for survey stations
A PK nail is a heavy-duty nail driven flush into the ground to mark a precise point, typically at a survey station or property corner. The nail itself is roughly 40-50 mm long with a substantial shank, usually 3-4 mm in diameter, and features a shallow circular indentation or dimple stamped into the top of its head. This indentation is the defining feature: it serves as a precise target point for the crosshairs of a surveying instrument when the nail has settled into the soil.
The indentation is critical because it prevents the instrument operator from missing the point by a few millimeters. When a survey crew needs to return to a marked location or when multiple surveyors must occupy the same station, the dimple provides repeatable positioning accurate to within a few millimeters. The nail is driven vertically into the ground with the head sitting just above soil level, or sometimes flush with it if the ground is soft enough.
Materials and placement
PK nails are typically made from galvanized steel or stainless steel to resist corrosion when left in the ground for extended periods. Hard steel versions exist for rocky ground. The nail is usually driven with a hammer or small sledge, and surveyors often place a brass or aluminum witness cap over the nail if the point must remain visible and protected for weeks or months. The witness cap typically has a hole in its center that aligns with the nail's dimple.
The term PK is a legacy abbreviation: it stands for property keeper, referring to the person responsible for maintaining boundary marks. In practice, PK nails are used not only for property corners but for any ground station requiring repeatable, precise positioning, including control points in engineering surveys, construction layout, and topographic surveys.
Common problems include frost heave in frozen ground, which can lift the nail, and settlement in soft soil, which can sink it below the surface or alter its position slightly. Surveyors working in areas where they expect to return to a mark often set PK nails in concrete pads or use them alongside other markers such as iron rods or concrete monuments to provide redundancy.