spad
A nail one or two inches long, of iron, brass, tin, or tinner iron, with a hole through the flattened head, used to mark stations in underground surveying.
spad: surveyor's marker nail for underground work
A spad is a short nail, typically 1 to 2 inches long, driven into rock or timber to mark survey stations deep underground. It has a distinctive flattened head pierced with a hole, allowing surveyors to thread a plumb bob, ranging rod, or sighting string through it. The hole ensures precise vertical or directional alignment from one station to the next, critical for mapping mine passages and calculating depth, grade, and lateral position.
Spads are forged from iron, brass, tin, or tinner iron (a soft iron alloy). Iron spads are cheapest and most common in coal mines. Brass and tin versions resist corrosion better and hold their shape longer in wet conditions, making them preferred in metalliferous mines or deep workings where water seepage is heavy. Tinner iron, used chiefly in Cornish tin mining, offered a middle ground between cost and durability.
Use in underground surveying
A surveyor drives the spad flush or slightly proud into the mine wall or floor at each survey point. The hole receives a plumb bob or the shaft of a ranging pole, which the surveyor sights along to establish a line to the next spad. This chaining method, repeated station by station, builds up the survey network. In shafts and rises, spads are essential anchors for vertical plumbs. Without them, sighting instruments would have no fixed datum to reference, and errors would compound rapidly.
Spads remain in place for months or years, especially in working mines where surveyors must return periodically to re-measure or update plans. Over time they can work loose, corrode, or be damaged by falling debris or timbering work, requiring periodic replacement or verification. A loose or missing spad forces the surveyor to re-establish the entire line, wasting time and introducing risk.
The term "spad" is old and regional; it appears most consistently in British mining records and Cornish mining texts. Its etymology is uncertain, though some sources suggest a connection to the Dutch word for a spike or wedge. In some regions and trades, equivalent devices are called pegs, dogs, or pins, but "spad" remains the standard term in underground surveying vocabulary across the English mining industry.