Surveying Extension Poles

All three names cover the same tool: a graduated rod on which an instrument marks the height of a point. The wording follows the trade: surveyors say surveying staff, site crews say levelling staff, catalogues list measuring staff or grade rod. Shared features are the E-pattern scale in 1 cm steps, working lengths of 3–7 m and readings good to 1–5 mm. The exception is the profiling staff, which works without any instrument.

Tilting always inflates the reading, because the crosshair meets the graduation higher up a leaning rod. On a 4 m staff a 2° lean costs about 2 mm and a 5° lean well over ten millimetres — far more than the 1–5 mm accuracy of ordinary construction levelling. Measuring staffs therefore carry a circular bubble, centred before every reading. Without one, the staff is rocked towards and away from the instrument and the smallest reading is recorded, since it marks the vertical.

A profiling staff checks the flatness and fall of a finished surface, not the height of a point on site. It is laid onto the slab, screed or sub-base and the gap under its edge is measured — specifications usually put that gap at roughly 2–5 mm under a 2 m straightedge, with no instrument and no second person involved. A levelling staff works the other way round: it stands vertically on the point while an instrument tens of metres away reads the height off it — see staffs for optical levels and staffs for laser levels.

Material follows the surroundings of the measurement. An aluminium staff is stiffer at the same mass — a 5 m model usually weighs 2–3 kg — but it conducts electricity, so it has no place near overhead power lines or railway catenary. A fibreglass staff is non-conductive, which makes it the standard on railway and power-network jobs. In both cases the foot should be metal: a worn or chipped plastic foot shifts the zero of the scale and puts a constant error of 1–2 mm into every reading.