A specialized survey nail made of iron, brass, or tin with a flattened head and a small hole. It is driven into wooden plugs in the roof of an underground mine or tunnel. Surveyors hang a plumb bob string through the hole to establish an exact station point over the instrument.
Heard on the job
Drive a spad into that timber overhead so we can hang a string and set up the transit.
Insider tip
In underground work, stations are almost always on the roof rather than the floor to prevent them from being destroyed by heavy equipment.