Theodolite
Theodolite
This domain covers the theodolite's geometry and field use: temporary adjustments, face-left and face-right readings, horizontal and vertical angles, repetition and reiteration, and the errors that averaging removes.
Core rules
- Temporary adjustments: setting over the station, levelling with foot screws, focusing the eyepiece and objective, and eliminating parallax.
- Fundamental lines include the vertical axis, horizontal axis (trunnion), line of collimation, and bubble tube axis. In adjustment they should be perpendicular or parallel as required.
- Face-left and face-right readings differ by about 180° in the horizontal circle. The mean of the two faces eliminates collimation error and horizontal-axis error.
- Repetition measures an angle several times on the same arc and divides by the number of repetitions. Reiteration measures several angles from one zero reference.
- Vertical angle is measured from the horizontal or from the zenith, depending on the instrument. A zenith angle of 90° means the line is horizontal.
Worked example
A horizontal angle is read face left as 30°20'40" and face right as 210°20'20". First reduce the face-right reading by 180°: 30°20'20". Mean = (30°20'40" + 30°20'20") / 2 = 30°20'30". The 20" discrepancy is split, not added as a correction. If the face-right value were used without reduction, averaging would produce a meaningless 120°.
Traps
Do not average raw face-left and face-right readings without applying the 180° difference. Do not confuse the horizontal axis with the vertical axis. Do not treat a 20-second least-count theodolite as a 20-minute instrument. In traversing, always record the angle actually turned and the face used.