What is a theodolite?
A theodolite is an optical or electronic surveying instrument used to measure horizontal and vertical angles with great precision. It consists of a telescope that can rotate both horizontally and vertically, two finely graduated circles for reading the angles, and spirit vials for levelling the instrument. It sits on a tripod and is centred exactly over a survey point with a plumb bob or optical plumb.
Uses
- Setting out a building: building lines, right angles and the grid lines of foundations and columns are set out from fixed survey points.
- Checking verticality: sighting up a facade, column or lift shaft shows whether it is plumb.
- Roads and infrastructure: fixing the direction and gradient of roads, pipelines and quays.
- Land surveying: triangulation, where positions are calculated from measured angles.
Theodolite, level and total station
- A level only measures differences in height along a horizontal plane, usually with a levelling staff.
- A theodolite measures angles, but not distances.
- A total station is an electronic theodolite with a built-in distance meter and data memory. On most building sites the total station has largely replaced the classic theodolite.
| Level | Measures only differences in height along a horizontal plane, usually with a levelling staff |
|---|---|
| Theodolite | Measures angles, but not distances |
| Total station | Electronic theodolite with a built-in distance meter and data memory |
Tips
- Check that the instrument is perfectly level before use. A small tilt causes a noticeable error over a long distance.
- Measure each angle in both telescope positions (face left and face right) and take the average. Many instrument errors then cancel each other out.
- Have the instrument calibrated regularly. For setting out that structural elements depend on, bring in a professional setting-out engineer or land surveyor.
Related terms
- Survey point
- Plumb bob
- Plumb
- Building line
- Spirit level
- Setting-out peg
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