Theodolite construction principle; micrometer

A theodolite is an instrument for measuring horizontal and vertical angles. A telescope that can turn about a vertical axis and a horizontal axis is sighted on a point, and the angular movement is read on graduated circles.

Construction

        eyepiece   telescope   objective
         \______________/
            |    ||  vertical circle
         ---+----++---  horizontal axis
         |  standards (A-frame)  |
         ======= upper plate (verniers) ======
         ======= lower plate (graduated circle) ===
              levelling head, 3 foot screws
                   tripod

• Levelling head with three foot screws, resting on the tripod.
• Lower plate carrying the graduated horizontal circle (0 to 360 degrees), with a lower clamp and tangent screw.
• Upper plate carrying the verniers (or micrometer), plate levels, and the standards; upper clamp and tangent screw.
• Telescope (internal focusing) with cross hairs, fitted to the horizontal axis, with altitude level.
• Vertical circle fixed to the telescope, with its vernier on the T-frame.

Principle

The telescope turns in a horizontal plane about the vertical axis and in a vertical plane about the horizontal axis. The difference of the circle readings when the telescope is sighted on two points gives the angle between them.

Function of the micrometer screw (optical theodolite)

The micrometer screw turns an optical plane-parallel plate which shifts the image of the circle graduation. It is turned until the index mark falls exactly on the nearest main graduation. The graduated drum on the screw gives the fraction of the smallest division (minutes and seconds). It replaces the vernier, with a least count of up to 1 second or less.

Uses of a theodolite

Measuring horizontal and vertical angles; setting out angles; ranging a line; prolonging a line; measuring distances by stadia tacheometry; finding heights; running traverse and triangulation; setting out buildings, roads and bridges.