A theodolite is an instrument which is used primarily to measure angles, both horizontal and vertical. It is also used for many other subsidiary work during surveying such as setting up of intermediate points between intervisible points, establishment of intervisible points,prolonging a line, laying out traverse etc.
Types of theodolites:
Classification with respect to a construction:
- Open-faced,vernier-equipped engineer’s transit
- Optical theodolites with direct digital read-out sormicrometer-equipped read-outs(formoreprecisereadings)
- Electronic theodolites
Classification with respect to accuracy:
- One-minute theodolites:the least division of the scale is 1 or 2 minutes
- One-second theodolites: the least division of the scale is 1 or 2 seconds
Components of theodolite:
The vertical axis of these instruments goes up through the center of the spindles and is oriented over a specific point on the earth surface. The circle assembly and alidade rotate about this axis.
Horizontal axis of the telescope is perpendicular to the vertical axis, and telescope and vertical circle tiltonit.
The line of sight (line of collimation) is a line joining the intersection of the reticle crosshairs and the center of the objective lens.The line of sight is perpendicular to the horizontal axis and should be exactly horizontal when the telescope level bubble is centered and when the vertical circle is set at 100g–300g or 0g for vernier transits.
Plate bubble axis is assumed to be tangent to the plate bubble.
Axes of theodolite:
SS:Vertical(standing)axis
TT: Horizontal (Trunnion)axis
PP: Plate bubble axis
CC:Collimation axis (line of sight)
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