Temporary adjustments; fundamental lines

Temporary adjustments

These are done at every set-up before observation:

1. Setting up (centering): fix the theodolite on the tripod and bring the plumb bob (or optical plummet) exactly over the station mark.
2. Levelling up: with the foot screws and the plate levels, make the vertical axis truly vertical. Set the plate level parallel to two foot screws, centre the bubble; turn 90 degrees and centre it with the third screw; repeat until the bubble stays central in all positions.
3. Elimination of parallax: focus the eyepiece until the cross hairs are sharp, then focus the objective on the object so that there is no apparent movement between the image and cross hairs when the eye moves.

Fundamental lines of a theodolite

• Vertical axis - about which the instrument rotates horizontally.
• Horizontal (trunnion) axis - about which the telescope rotates vertically.
• Line of collimation - the line of sight through the intersection of the cross hairs and the optical centre of the objective.
• Axis of the plate level and axis of the altitude (telescope) level.

            line of collimation
         ----------------------->
   telescope \
           ---O--- horizontal axis
              |
              |  vertical axis

In adjustment: plate level axis $\perp$ vertical axis; line of collimation $\perp$ horizontal axis; horizontal axis $\perp$ vertical axis; altitude level axis $\parallel$ line of collimation.