Alidade

An alidade is a sighting device that rotates around a measuring instrument’s center to establish a direction or measure an angle. It may use sights, a telescope, or a pointer.

Sighting rule: A straightedge fitted with sights for aligning an instrument with a distant target. Its paired sights are the simplest form of alidade.
Plane-table surveying: A field-mapping method in which observations and plotted lines are made directly on a drawing board. An alidade supplies the sightline used to plot directions from the table’s location.
Dioptra: An ancient Greek surveying and astronomical instrument for measuring angles and alignments. Its rotating sighting components are among the alidade’s early predecessors.
Compass: An instrument that indicates direction, usually by aligning with Earth’s magnetic field. A compass indicates a reference direction; an alidade sights a chosen target and reads its angle.
Telescope: An optical instrument that gathers light to make distant objects appear clearer or larger. Telescopic alidades replace open sights when precise targeting is needed.
Topographic surveying: Surveying that measures and maps terrain features and elevations. Surveyors use alidades to establish bearings and measure angles between mapped features.
Groma: A Roman surveying instrument used to lay out straight lines and right angles. Its sighting arms show an earlier, simpler approach to directional alignment.
Clinometer: An instrument for measuring angles of slope, elevation, or inclination. A clinometer measures inclination rather than providing the alidade’s general rotating sightline.
Vernier scale: A graduated scale that reads fractions of a division on a main scale. Many alidades use it to read angles more finely than the main graduation permits.
Triangulation: A method of locating points by measuring angles from known positions. Alidades provide the directions that form the measured triangles.

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GraphometerBroader topic: The graphometer’s sights serve as alidades, aligning the instrument with survey points.