Permanent adjustments of theodolites
The permanent adjustments of theodolites are made to establish fixed relationship between the instrument's fundamental lines. The fundamental lines or axis of a transit theodolite include the following:-
- Vertical axis
- Axis of plate levels
- Axis of telescope
- Line of collimation
- Horizontal axis
- Axis of altitude bubble and the vernier should read zero.
These adjustments once made last for a long time. These are important for accuracy of observations taken from the instrument. The permanent adjustments in case of transit theodolite are:-
- Horizontal axis adjustment.
The horizontal axis must be perpendicular to the vertical axis.
- Vertical circle index adjustment.
The vertical circle must read zero when the line of collimation is horizontal.
- Adjustment of altitude level.
The axis of altitude level must be parallel to the line of collimation.
- Collimation adjustment.
The line of collimation or line of sight should coincide with axis of the telescope.[1] The line of sight should also be perpendicular to the horizontal axis at its intersection with the vertical axis . Also, the optical axis, the axis of the objective slide, and the line of sight should coincide.
- Adjustment of horizontal plate levels.
The axis of plate levels must be perpendicular to the vertical axis.[2]