What is Plane Table Surveying?
Plane table surveying is a graphical method of surveying in which both the observations and plotting of stations are done simultaneously on a sheet placed on a plane table fitted with a tripod in the field.
It is a simple and fast survey and is mostly suitable for preparing small-scale maps.
Instruments and accessories used in Plane Table Surveying
- Plane Table with Tripod
- Alidade
- Spirit level
- Trough compass
- Plumbing fork with plumb bob
Let’s discuss each of them in detail.
1) Plane Table with Tripod
The plane Table is made of well-seasoned good quality teak or pine wood and is available in different sizes,
- Small size plane table – 50cm x 40cm x 1.5cm
- Medium size plane table – 50cm x 50cm x 1.5cm
- Large size plane table – 75cm x 60cm x 1.5cm
The Upper surface of the board must be smooth and perfectly plane. The plane table is mounted on a tripod in such a way that it can be rotated freely in the horizontal plane and can be clamped in any position.
The tripod is generally of open frame type. The tripod can be folded for the convenience of transportation.
2) The Alidade
The Alidade is a sighting device fitted with a straight ruled edge. Generally used to establish a line of sight. There are two types of alidades namely,
- Plain Alidade and
- Telescopic Alidade
a) Plain Alidade
It is a wooden or a metal rectangular block fitted with two sight vanes at its short end and a ruler at one of the longest edges. The vanes are hinged so that they can be folded when not in use.
One of the vanes known as the eye vane is provided with narrow silt with three holes spaced equally.
The other vane known as the object vane is open and carries a thin hair stretched between the top and bottom. The thin hair is used to bisect the station rod.
The longest edge of the alidade is fitted with a ruler. This edge is known as the working edge or fiducial edge.
The two vanes should be perpendicular to the ruling edge as well as the surface of the plane table.
b) Telescopic alidade
The telescopic alidade is used when it is required to take inclined sights.
It consists of a small telescope fitted with an A-frame which is placed on a metal or wooden block with a fiducial edge.
The telescope increases the range and accuracy of the sights.
3) Spirit level
The spirit level is used to ensure that the table is perfectly leveled.
The spirit level consists of a small cylindrical tube filled with alcohol or any other fluid and has a small bubble at the center. If the bubble is at the center, then it is said to be perfectly leveled.
The spirit level should be placed on the plane table at the right angles in two positions. If the bubble is at the center in both spirit levels, then the table is said to be leveled properly in all directions.
4) Trough compass
A trough compass is a small magnetic compass having a freely suspended magnetic needle showing the direction of the magnetic north.
It is required for drawing a line showing a magnetic meridian on the paper.
It is also used for orienting the table to the magnetic north.
5) Plumbing fork with plumb bob
The plumbing fork is of U shape. One end of the U fork is fitted with a pin and the other end is fitted with the plumb bob.
It is used for centering the plane table exactly over the station and sometimes can be used to transfer a point in the ground to the sheet.
Method of setting up the plane table
There are three processes involved in the setting up of a plane table in plane table surveying. They are,
- Leveling the plane table
- Centering the plane table
- Orienting the plane table
1) Levelling the plane table
First, the table should be set up at a convenient height for the user above the station approximately, and make sure the tripod legs are spread well and firmly pressed into the ground.
The table is leveled by placing the spirit level on the board in two positions at the right angles and getting the bubble at the center in both levels.
For rough and small-scale works, leveling can be done by eye estimation, and for large-scale and important works, leveling should be done by a spirit level.
2) Centering the plane table
In this operation, the location of the ground station position is brought exactly over the drawing sheet by using a plumbing fork with the plumb bob.
3) Orienting the plane table
It is the process of putting the plane table in some fixed direction so that the line representing the particular direction on the plane is parallel to the direction on the ground.
For the orientation process, the table will have to be rotated about its vertical axis on a horizontal plane, thus disturbing the centering. Therefore, after the orientation process, it is advised to check the centering again.
The relative positions of the objects on the sheet will be accurate only if the orientation is proper.
Orientation of the plane table shall be done in two ways,
- Orientation by back sighting
- Orientation by a magnetic needle
a) Orientation by backsight
Let’s understand this through an example. Let’s take two stations on the ground namely, A and B. Consider A as the first station and B as the second station.
Now, the plane table is leveled and centered at station A. The ground station A is brought over the drawing sheet using a U fork and it is marked as “a”.
Now mark a point on the sheet using a small pin. Now place the working edge of the alidade in touch with the pin and sight station B. When station B is sighted, clamp the plane table in position.
Now measure the distance between A and B on the ground and plot it on the sheet with a respective scale and mark the station B on paper as “b”.
Now bring the plane table to B and sight A and do the same process to check the line ab on the sheet.
This method gives a better orientation of the plane table and is always preferred.
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b) Orientation by the magnetic needle
Let’s take two stations A and B on the ground.
The plane table is set up at the station A and leveled using the spirit level. The centering is done by U fork with plumb bob so that the point a on the sheet is exactly over station A.
Then the trough compass is placed on the right-hand top corner of the sheet. Adjust the table in such a way that the compass shows a “0-0” direction (North). Then clamp the table in position and draw a line indicating north on the sheet.
Now place the alidade by touching point ‘a’ on the sheet and try to sight the station B. When station B is bisected using a sight vane, measure the actual distance on the ground between A and B and plot it in the sheet using a suitable scale (like 1cm =10m) and mark the point b on the sheet.
Now the table is shifted to the station B and centered over B so that the point b on the sheet is just over station B. After checking leveling and centering, the trough compass is placed in the sheet in the exact same position along the north line precisely.
Adjust the table clockwise or anticlockwise until the needle coincides exactly with the “0-0” mark of the compass.
While turning the table, care enough care should be taken not to disturb leveling. If disturbed, leveling needs to be adjusted immediately.
When centering, leveling and the needle is exactly at 0-0 at station B, the orientation is said to be perfect.
This method is suitable when the local attraction is not suspected in the surveying area.
Why orientation of the plane table is important?
When there are more than one stations, the plane table has to be shifted to each station to prepare a map on the sheet. While transferring to each station, if there is any small error occurs, then it totally distorts the diagram on the sheet and the relative positions of objects on the sheet are not as same as that on the field. So the total plane table surveying becomes useless.
So enough importance should be given to the orientation of the plane table in Plane table surveying.
Advantages of Plane table surveying
- It is simple and cheaper than the theodolite survey.
- It is most suitable for small-scale maps.
- It is useful in magnetic areas, where a compass survey is not possible.
- The mistakes in writing readings in the field books are eliminated.
- No great skill is required to operate.
Disadvantages of plane table surveying
- It is not intended for very accurate work.
- It is not suitable in monsoon.
- Due to its heaviness, it is inconvenient to transport.
- This involves so many accessories, so there are high chances for them to be lost.
Hope you understand everything you need to know about plane table surveying. If you have any queries (or) if you find this article helpful, let us know in the comment section.
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