Theodolite is an instrument used for the measurement of vertical and horizontal angles. Theodolite is used to measure the angles up to the accuracy of 10″ or 20″ depending on the least count of the theodolite.
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Types of Theodolite
In the transit theodolite the telescope can be revolved completely about its horizontal axis in the vertical plane.
Non Transit Theodolite
In the non-transit theodolite, the telescope can’t be revolved completely but can be revolved about its horizontal axis in a vertical plane up to a certain extent to measure the angle of elevation or depression.
If a theodolite is fitted with vernier scale, then it is called vernier theodolite.
If it is fitted with micrometer than it is called micrometer theodolite.
Size of a Theodolite
The size of a theodolite is specified by the diameter of the main horizontal graduated circle. For instance, 10 cm theodolite means the diameter of the main graduated circle is 10 cm. For engineering purposes, 8cm to 12 cm diameter theodolites is used.
Transit Theodolite Parts
Trivet or Base Plate
It is a circular plate and has a central threaded hole to fit it on the tripod. Three foot screws are attached to base plate with ball and socket arrangement.
They are provided for the levelling of the theodolite. The lower part of the foot screws are attached to the base plate and upper part to the tribranch plate.
It is a triangular plate carrying three foot screws at its end.
The assembly of base plate, foot screws and tribranch plate is known as levelling head.
Spindles or axes are attached to the tribranch plate. There are two spindles in the theodolite. Inner spindle or axis which is solid and conical. The outer spindle is hollow and co-axial with the inner spindle.
It is attached to the outer axis and contains a horizontal scale graduated from 0 degrees to 360 degrees in a clockwise direction. Each degree is then sub-divided into further divisions. The value of one division maybe 15 or 20 seconds.
It is provided with the clamp screw and tangent screw. When the clamp screw is tightened the plate is fixed with the outer axis. The tangent screw is used for fine-tuning.
It is attached to the inner axes and contains verniers A and B. It is provided with the upper clamp and tangent screw. When tangent screw is tightened it is attached with the inner axis.
Two plate bubbles are attached at a right angle to each other on the upper surface of the vernier plate. These bubbles are meant for leveling of the instrument for horizontal angle measurement.
Standard or A frame
Standard or A frame supports the telescope, the vertical circle and the vernier scales.
It is pivoted between the standards at a right angle to the horizontal axis. It can be moved in a vertical plane and consists of focusing screws, tangent screw, and clamping screw
It is fixed with the telescope and moves with it. It is divided into four quadrants, each quadrant is graduated from 0 degrees to 90 degrees in opposite directions.
It is provided for levelling when measuring vertical angles.
Sometimes compass is provided for measuring the magnetic bearing of a line.
Least count of vernier theodolite
It is the difference between the value of the smallest division on the main scale and that of the smallest division of the vernier scale. It is the smallest value measured by the theodolite.
Let (n-1) small divisions of the main scale is divided into n small divisions in the vernier scale
Then n x v = (n-1)d
v = value of smallest division of Vernier scale.
d = value of smallest division of main scale.
So least count = d-v= d- (n-1)d/n= d/n
for example; d= 20’ and n = 60; least count = 20/60 x 60 = 20”
Fundamental Lines of Theodolite
There are six fundamental lines of a theodolite.
1. Vertical axis
It is the axis of rotation of the telescope in the horizontal plane.
2. Horizontal axis.
It is the axis of rotation of the telescope in the vertical plane. It is also known as trunnion axis.
3. Axis of the telescope
Is is an imaginary line passing through the optical center of the object glass and the optical center of the eyepiece.
4. Axis of the plate bubble tube
It is an imaginary line tangential to the longitudinal curve of the plate bubble tube at the mid point.
5. Line of collimation
It is an imaginary line passing through the intersection of cross hair at the diaphragm and the optical center of object glass and its continuation
6. Axis of altitude bubble
It is an imaginary line tangential to the longitudinal curve of the altitude bubble tube at the midpoint.
- It is used for measuring horizontal angles.
- For measurement of vertical angles.
- Measuring magnetic bearing.
- Deflection angles can be measured.
- Measuring the horizontal distance between two points.
- Estimating the vertical height of an object.
- Finding difference of elevation between two points.
- Ranging a line.