Definition

Noun: 1. A surveying instrument: A device used in surveying, typically in conjunction with a plane table, for sighting distant objects, measuring angles, and drawing accurate lines of direction on a map or chart. 2. The upper part of a theodolite: The movable component of a theodolite that includes the telescope, scales, and other sighting mechanisms used for precise angular measurement.

Usage

The word "alidad" (also spelled "alidade") is a technical term used primarily in the fields of surveying, cartography, and navigation. It refers to the sighting apparatus of an instrument.

Examples

• As a surveying instrument:
  • The surveyor used an alidad with his plane table to map the coastline.
  • Accurate plotting requires a stable plane table and a precise alidad.
• As part of a theodolite:
  • He carefully rotated the alidad of the theodolite to measure the horizontal angle between the two points.
  • The vernier scale on the alidad allows for very fine angular readings.

Advanced Usage

• "Peep-sight alidad": A simple alidad with sighting vanes (peep sights) instead of a telescope, used for less precise work.
• "Telescopic alidad": An alidad equipped with a telescope for long-distance and more accurate sighting.

Variants and Related Words

• Alidade: The more common alternate spelling of "alidad".
• Sight rule: Another, less common, term for a simple alidad used on a plane table.

Synonyms

• Sighting device
• Index arm (in the context of certain instruments)

Related Terms (Not Synonyms)

• Plane table: The drawing board used with an alidad.
• Theodolite: A precision instrument for measuring angles, of which the alidad is a key component.
• Transit: A type of theodolite; its rotating telescope assembly is an alidad.

Noun

1. surveying instrument used with a plane table for drawing lines of sight on a distant object and for measuring angles
2. surveying instrument consisting of the upper movable part of a theodolite including the telescope and its attachments