Theodolite
A theodolite measures horizontal and vertical angles precisely, but not distance. It is the ancestor of the total station and remains in use for angle-only tasks such as verticality checks, machine alignment and teaching, where adding distance measurement would be unnecessary cost.
Angles only, done well - the instrument that still teaches surveying and still checks whether a column is plumb.
ALSO CALLED: digital theodolite · optical theodolite · transit theodolite · angle measuring instrument
Typical specifications
Class-level figures. Individual models vary, so treat these as the range to check a quotation against rather than a guarantee.
- What it measures
- Horizontal and vertical angles
- Typical angular accuracy
- 2" to 20" depending on grade
- Distance measurement
- None - taped, stadia or computed separately
- Typical magnification
- 26× to 30×
- Display
- Digital readout on modern units; optical micrometer on older ones
- Common use today
- Verticality, alignment, angle traverse, teaching labs
How a theodolite works
01
Centre and level
The instrument is set precisely over the station point with an optical or laser plummet and levelled on the plate bubble.
02
Zero on a reference
The horizontal circle is set to zero - or to a known bearing - on a chosen reference direction so all subsequent angles are relative to something meaningful.
03
Observe on both faces
Every angle is read on face left and face right. Averaging the two cancels collimation and index errors, which is why single-face work is never accepted for precise results.
04
Book and compute
Angles are meaned, checked against the closing condition, and combined with separately measured distances to compute coordinates.
What it is used for
- Verticality and plumb checks on columns and towers
- Machine and structural alignment
- Angle traverse where distances are taped
- Setting out straight lines and right angles
- Deflection and tilt observation
- Teaching surveying practice in colleges
Who uses it
- Engineering colleges and polytechnics
- Steel fabrication and erection teams
- Industrial maintenance engineers
- Small contractors
- Survey training institutes
How to choose a theodolite
Digital or optical
A digital theodolite removes micrometer reading error and is easier to teach on. Optical instruments are cheaper and still common in lab sets where the reading exercise is itself the lesson.
Angular accuracy grade
20" is adequate for teaching and rough alignment. Verticality checks on tall structures and machine alignment want 2"–5".
Laser or optical plummet
A laser plummet speeds up centring noticeably, especially for students. Optical plummets need no power and never drift out of adjustment unnoticed.
Whether you actually want a total station
If distances will be needed at all, a basic total station usually costs less than a theodolite plus the workaround for measuring distance. Buy the theodolite only when angles are genuinely all you need.
What it costs
Theodolites are among the cheapest survey instruments, and institutional lab purchases are usually specified as sets with tripods and ranging rods. Price is driven by angular accuracy grade and whether the readout is digital. To confirm: theodolite price bands and lab set contents
Theodolite vs the alternatives
Theodolite vs Total station
A total station is a theodolite with distance measurement and onboard computation. For any survey producing coordinates, it is the correct instrument; the theodolite survives where only angles matter.
Read about total stationWhat Axisline does with a theodolite
FAQ
Theodolite - common questions
What is the difference between a theodolite and a total station?
A theodolite measures angles only. A total station adds an electronic distance meter and an onboard computer, so it produces coordinates directly instead of requiring separate distance measurement and manual calculation. Everything a theodolite does, a total station also does - which is why theodolites are now mostly teaching and alignment instruments.
Is a theodolite still used today?
Yes, in specific roles. Engineering colleges use them to teach angle observation, and industry uses them for verticality checks, structural alignment and machine setting where distance is irrelevant. For general survey work producing coordinates, total stations and GNSS have replaced them almost entirely.
Why are angles observed on both faces?
Observing on face left and face right and meaning the two readings cancels collimation error, horizontal axis error and vertical index error. It roughly halves the effect of instrument maladjustment without any adjustment being made, and it is the single most effective habit in angle measurement.
What accuracy can a theodolite achieve?
Grades typically range from about 20" for teaching instruments down to 2" for precise work. Achieved accuracy depends heavily on careful centring over the station, correct levelling and observing on both faces - a well-used 5" instrument routinely outperforms a carelessly set up 2" one.
Related instruments
Need a theodolite - or the survey it produces?
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