Theodolite

Introduction Horizontal angles can be measured by a compass. But to get precise readings and to measure horizontal and vertical angles another instrument called theodolite is used. It can be used for laying off horizontal angles, locating points on line, prolong- ing survey lines, establishing grades, determining differences in elevation, setting out curves, etc.

There are two types of adjustment necessary, these being the station or temporary adjustments and the permanent adjustments.

Temporary Adjustments of Theodolite

Setting: The instrument to be set over the station, centering by a plumb bob and approximate levelling with the help of tripod legs.

Levelling: Done using leveling screws or foot screws to make the vertical axis truly vertical.

Elimination of parallax: Parallax is a condition arising when the image formed by the objective is not in the plane of the cross hairs. It is done in two steps:

1. Focusing the eye-piece: For the distinct vision of the cross hairs.
2. Focusing the objective: The focusing screw is turned till the image appears clear and sharp and is in the plane of crosshairs.

Permanent Adjustments of Theodolite

Adjustment of plate level: To make the axis of the plate bubble perpendicular to the vertical axis when the bubble is central.

Adjustment of line of sight: Line of sight should coincide with the optical axis of the telescope.

Adjustment of horizontal axis: Horizontal axis should be perpendicular to the vertical axis. The spire test is done.

Adjustment of altitude level and vertical index frame: Clip and tangent screws are used for adjusting vertical frame and leveling of altitude.

Error in Theodolite

Temporary Error:- Centering, leveling, and making images and reticles together are the temporary adjustment in the total station.

Permanent Error:- Permanent adjustment of total station consists of.

1. Horizontal collimation error
2. Vertical collimation error
3. Tilting axis error
4. Compensator index error
5. Eccentricity error
6. Circle graduation error
7. Collimation of laser pointer

Errors eliminated:

1. Errors due to the eccentricity of verniers and centers are eliminated by taking both vernier readings.
2. Errors due to in adjustment of the line of collimation and the trunnion axis are eliminated by taking both face readings.
3. Error due to inaccurate graduations is eliminated by taking the readings at different parts of the circle.
4. Errors due to the inaccurate bisection of the object, eccentric centering, etc., maybe to some extent counter-balanced in different observations.
5. Errors due to slip, displacement of station signals and want of verticality of the vertical axis, etc., are not eliminated since they are all cumulative.