ARN ANSTRUMENTS alidade survey instruments Height Gauge

The alidade is the part of a theodolite that rotates around the vertical axis, and that bears the horizontal axis around which the telescope (or visor, in early telescope-less instruments) turns up or down. These can be used to read off an angle on a scale that is engraved around the outer edge (or limb) of the instrument. Alidades of this form are found on astrolabes, mariner's astrolabes and similar instruments. Alidade D has vanes without any openings. The earliest alidades consisted of a bar, rod or similar component with a vane on each end. Each vane (also called a pinnule or pinule) has a hole, slot or other indicator through which one can view a distant object. There may also be a pointer or pointers on the alidade to indicate a position on a scale.