Telescopic Alidade in Plane Table Surveying

In plane tabling, the telescopic alidade is a crucial instrument that is used to measure horizontal and vertical distances directly without needing to use chainage. Here is an explanation of how this instrument works and its features:

Definition of Telescopic Alidade

A telescopic alidade is an optical instrument used in plane table surveying to measure vertical and horizontal distances and angles. It is mounted on a plane table and is used to sight objects, measure vertical and horizontal angles, and measure distances.

Features of Telescopic Alidade

The following are some features of the telescopic alidade:

- It consists of a telescope that is mounted on a rotating axis, and it can rotate in a horizontal plane.
- The telescope has crosshairs or reticles which are used to sight objects and measure angles.
- The alidade is graduated in degrees and minutes, and it can be used to measure horizontal angles.
- The vertical angle can be measured using a clinometer that is attached to the alidade.
- The alidade has a prism that reflects light from a distant object into the telescope, making it easier to sight objects in bright sunlight.

Advantages of Telescopic Alidade

The following are some advantages of using a telescopic alidade in plane table surveying:

- It enables direct measurement of horizontal and vertical distances without resorting to chaining.
- It is easy to use and provides accurate measurements.
- It is lightweight and portable, making it ideal for use in rough terrain.
- It eliminates errors associated with chaining such as sag, slope, and temperature effects.

Conclusion

In conclusion, the telescopic alidade is an indispensable instrument in plane table surveying. It enables direct measurement of horizontal and vertical distances and angles, which eliminates the need for chaining. The alidade is easy to use, accurate, lightweight, and portable, making it ideal for use in rough terrain.