Fundamental lines of theodolite and relations

The fundamental lines of a transit theodolite are the imaginary lines and axes about which the instrument is built. They are:

1. Vertical axis - the axis about which the whole upper part of the instrument rotates horizontally.
2. Horizontal (trunnion) axis - the axis about which the telescope rotates in the vertical plane.
3. Line of collimation (line of sight) - the line joining the intersection of the cross hairs to the optical centre of the objective and its extension.
4. Axis of the plate level (bubble line) - the tangent to the plate bubble at its centre; it is horizontal when the bubble is central.
5. Axis of the telescope level (altitude bubble) - the tangent to the altitude bubble at its centre, used for levelling the line of sight.

Relationship in perfect adjustment

• The axis of the plate level is perpendicular to the vertical axis. (So the vertical axis is vertical when the plate bubble is central.)
• The line of collimation is perpendicular to the horizontal axis. (So the telescope sweeps a plane, not a cone.)
• The horizontal axis is perpendicular to the vertical axis. (So the plane swept is vertical.)
• The axis of the telescope level is parallel to the line of collimation.
• The vertical axis passes through the centre of the graduated horizontal circle, and the horizontal axis through the centre of the vertical circle.

            line of collimation
         ---------------------->
   telescope  \
            ---O--- horizontal (trunnion) axis
               |
               |   vertical axis
               |
           plate level (axis perpendicular
           to vertical axis)