A wooden or metal pole with a pointed, iron-shod bottom for driving into the ground and a flat or socketed top. Surveyors mount a compass or light transit on it to take bearings in rough terrain where setting up a tripod is impractical. It allows for rapid setups in dense woods or steep slopes.
Heard on the job
Drive the Jacob's staff deep into the mud so the compass doesn't wobble while I read the bearing.
Insider tip
While largely replaced by tripods for precision work, it remains useful for quick reconnaissance or rough boundary retracement in heavy brush.