Definition

1. Noun:
   • A surveying instrument used with a plane table: An alidade is a ruler-like device with a sighting mechanism, used for drawing lines of sight on a distant object and for measuring angles when placed on a plane table for mapping.
   • The upper movable part of a theodolite: In a theodolite, the alidade is the rotating upper section that includes the telescope, scales, and other attachments for precise angular measurement.

Usage Examples

• Noun:
  • The surveyor carefully aligned the alidade on the plane table to sight the distant tower.
  • To measure the horizontal angle, he rotated the theodolite's alidade until the crosshairs were centered on the target.

Advanced Usage

• "Peep-sight alidade": A simple alidade with a narrow slit or peephole for sighting, as opposed to one with a telescopic sight.
  • For a quick, rough survey, he used a simple peep-sight alidade.

Variants and Related Words

• Alidad (noun): An alternative spelling of alidade.
• Telescopic alidade (noun phrase): An alidade equipped with a small telescope for greater accuracy in sighting.
  • The telescopic alidade allowed for much more precise measurements over long distances.

Synonyms

• Sight rule: A term sometimes used interchangeably with alidade in the context of plane table surveying.
• Index arm: In some contexts, the rotating part of an instrument that carries the sighting mechanism.

Related Terms (Not Phrasal Verbs or Idioms)

• Plane table (noun phrase): The drawing board on which the alidade is used.
• Theodolite (noun): A precision optical instrument for measuring angles, of which the alidade is a key component.

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