Introduction:
Plane tabling is a surveying method used to obtain accurate and detailed information about a small area of land. It involves setting up an instrument called a plane table at a known point and then visually observing and recording the features and measurements of the surrounding area. One of the essential instruments used in plane tabling is the telescopic alidade.

Telescopic Alidade:
The telescopic alidade is a surveying instrument that is mounted on the plane table. It consists of a telescope with crosshairs or a reticle for sighting and a graduated scale for measuring angles. The alidade is used to determine the horizontal and vertical distances between points on the ground.

Obtaining Horizontal Distance:
To obtain horizontal distances using the telescopic alidade, the surveyor first sights a target point with the telescope. The target point can be a prominent feature such as a tree or a building. The surveyor then rotates the telescope until the target point aligns with a reference point on the plane table. The angle between the reference point and the target point is read from the graduated scale on the alidade. By knowing the angle and the distance between the reference point and the telescope, the horizontal distance to the target point can be calculated using trigonometry.

Obtaining Vertical Distance:
The telescopic alidade can also be used to measure vertical distances directly without the need for additional instruments or calculations. This is achieved by using a leveling bubble attached to the alidade. The surveyor adjusts the leveling screws on the alidade until the bubble is centered. The vertical distance between the plane table and the target point can then be read directly from the graduated scale on the alidade.

Advantages of Telescopic Alidade:
The telescopic alidade offers several advantages in plane tabling:

1. Direct Measurement: Unlike other instruments such as planimeters or clinometers, the telescopic alidade allows for direct measurement of both horizontal and vertical distances without the need for additional calculations or conversions.

2. Accuracy: The telescopic alidade provides accurate measurements due to its precise alignment and graduated scale.

3. Efficiency: By eliminating the need for additional instruments or calculations, the telescopic alidade simplifies the surveying process and saves time.

4. Versatility: The telescopic alidade can be used in various surveying applications, making it a versatile tool for plane tabling.

Conclusion:
The telescopic alidade is an essential instrument used in plane tabling for obtaining accurate horizontal and vertical distances directly. Its direct measurement capabilities, accuracy, efficiency, and versatility make it a valuable tool for surveyors in gathering detailed information about a small area of land.