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Paired rails and a T-shaped staff controlling a gradient.
Also known as: Boning rod, Traveller staff, Sight rail survey
A sight rail is a crossbar set at a calculated height, and a traveller a T-shaped staff of fixed length; sighting from one rail to the next across the traveller controls a trench's gradient by eye.
Controls a gradient by eye, the trench being correct when the traveller's top aligns with the two rails.
Contexts: Maps
Requires no instrument at all once the rails are set, which is why the method survived the arrival of the level and is still used in drainage work.
Contexts: Maps
Two horizontal rails on posts at either end of a run, with a T-shaped staff standing in the excavation between them.
A fuller structured visual breakdown is not yet documented for this symbol.