Instruments used in Plane Table Surveying

Like types of chains used in chain surveying, there are equipment and accessories that are used in plane table surveying. Each instrument contributes to the performance of the plane table surveying. 

As parallelism is the principle of the plane table survey, the following items help to attain that perfectly.

• Plane table survey tripod
• Alidade
  • • • • Plain Alidade
        • Telescopic Alidade
• Plumbing Fork and Plump Bob
• Trough Compass
• Spirit Level
• Ranging Rods
• Drawing Sheets and Tools

Plane Table Survey Tripod

As the name suggests, a plane table pad is like an exam pad which helps to hold the drawing paper in place. The pad is attached to a tripod which allows adjusting the pad to the comfortable height position. It can be rotated horizontally to place it according to the stations. It is made of seasoned wood material.

Alidade

Alidade is a straight ruler kind of object. Based on the line of sight, there are two types of alidade.

• Plain Alidade
• Telescopic Alidade

The difference between the alidades is more like a difference between the tacheometer and theodolite. One helps to view horizontal line sight, and the other helps to measure both horizontal and vertical lines of sight.

Plain Alidade

It consists of two foldable vanes which are bevelled and graduated. Two sight vanes are mounted at two edges. One with narrow slit sight serves as eye vane, and the other wide slit attached with thin wire serves as object vane. Using both vanes, we plot the line of sight on board.

Telescopic Alidade

On hill stations, we have to adjust the vertical eyesight where telescopic alidade comes in hand. In simple alidade, we can’t move it vertically to observe it. Here in telescopic alidade, we can do it.

As you can see from the picture, a telescopic alidade will be mounted on a straight edge ruler to plot the line of sight. It is movable in a vertical position mounted with a vertical scale. With the help of the object sight, we can able to point the ranging rods.

Plumbing Fork and Bob

The whole process of plane table surveying depends on the setup of the plane table in the right position. Plumbing Fork and Bob help to attain the levelling, centering and orientation of the plane table. 

Plumbing fork and plumb bob consist of a upper and lower arm. The upper arm has a pointer fixed at the end. The lower arm has a plumb fixed under. The plumb bob and pointer will be on the same line. Using the help of plumb bob, the ground station point will be marked on the paper (with the help of pointer).

Trough Compass

Trough compass is used to mark the north pole on the drawing sheet.

The trough compass will be 80 to 150 mm long and 30mm wide. A double-edged needle is pivoted in the compass which will freely move inside the compass. Both ends have angle scales of 0° to 5° that helps to point the north and south pole. 

The compass will be protected by the glass to prevent the wind blow.

Spirit Level

Spirit Level used to level the plane table tripod each time. The mercury bubble on the spirit level helps to ensure that the plane table is mounted at the correct level.

Ranging Rod – Ranging rods are used to mark and establish the sight point to the plane table. 

Drawing Sheets & Other Tools 

In a plane table surveying, the plotting (on drawing paper) and field observations are done simultaneously. The map of the land prepared while doing the fieldwork. 

Other tools such as Clip camps, adhesive tapes are used to hold the drawing sheet in the position.

Happy Learning 🙂