Plane table survey, accessories and Set-up

The Plane Table surveying is one of the fastest and easiest methods of surveying. This method of surveying in which the fieldwork and plotting are done simultaneously called plane tabling. The principle of plane tabling is parallelism, meaning that the rays drawn from stations to objects on the paper are parallel to the lines from the stations to the objects on the ground.

It is commonly employed for small and medium scale mapping, but this survey is recommended for work where great accuracy is not required. It is less costly than a theodolite survey. It is very advantageous in magnetic areas, but it is not suitable for work in a wet climate. As the fitting and fixing arrangement of this instrument is not perfect, most accurate work cannot be expected.

ACCESSORIES OF PLANE TABLE SURVEY:-

Some basic surveying instruments are required for plane table surveying.

The Plane Table – The plane table is drawing board of size 750×600mm. The plane table made of well-seasoned wood like pine, teak etc. The top survey of the table is extremely smooth and well levelled. The bottom surface consists of a threaded circular plate for fixing the table.

The Alidade – 

              There are two types of alidade

a)                             Plain and

b)                             Telescopic

Plain Alidade – The plain alidade consists of a metal or wooden and it has two edges. One straight and other is bevelled and is known as the fiducial edge. It consists of two vanes at both ends which are hinged with the ruler. One known as the ‘object vane’ is open and fine thread between the top and bottom silt; the other is called the ‘sight vane’ and provide with the narrow silt.

Telescopic Alidade – Where the plain alidade took only horizontal sights, this telescopic alidade consists of a telescope meant for inclined sight or sighting distant objects clearly. It is used for increasing the accuracy of the sight taken. The alidade has no vanes at the ends but is provided with the fiducial edge.

Spirit Level:- In-plane table survey, sprit level is very essential instrument. It contains a small bubble of sprit. A bubble (mercury is usually used) is put inside the tube. The bubble is visible on the top.

The Compass:- 

                           There are two compass -

a)   The Trough Compass – The trough compass is rectangular box made of non- magnetic metal containing a magnetic needle pivoted at the centre.

b)  The Circular Box Compass – It containing a pivoted magnetic needle at the centre. The compass is meant for marking the north direction of the map.

U-fork or Plum Bob – Plum bob is a U-shaped metal strip that has two different types of arm. The top arm is pointed and the bottom arm carries a hook for suspending a plumb bob. The used of plumb bob for centring.

Ranging Rod – It’s a common instrument for all surveying, used for marking the position of different surveying stations and ranging for straight lines.

 Also Read:- Geotextiles-defination-advantages-funtion.

PROCEDURE OF SETTING UP:-

In order to set up a plane table at a station, the following four operations are performed –

Levelling – The table is fixed on over a station point and at a convenient height. The tripod stands are well over the station. The table is levelled by chacking the sprit level at different corners and various points on the table. When the table is levelled, then the levelling screws or by simply adjusting the legs.

Centring – U fork and plumb bob is used for centring the table over the station. Let a point P is selected on the sheet to represent station P on the ground. The pointed end (at the upper hand) of the plumbing fork is kept on the paper and at the other end, plumb bob is fixed. The station P by turning the table clockwise or anticlockwise or adjusting the legs. This processed is known by Centring.

Making the north-line – The compass is adjusted exactly the right-hand top corner with its north end approximately towards the north. The compass needle exactly coincides with the 0-0 mark, now the line representing the north line. It should ensure that the table is not turned.

Orientation – The process by which the position occupied by the board at various survey stations are kept parallel is known orientation. In-plane table survey orientation is the most important parts of set up. Orientation must be done when the plane table is set at more than one station. Orientation may be done by two following processed-:

Magnetic needle – This method is suitable when local attraction is not suspended in the area and when it is not possible to bisect the previous station from the new station.

Backsight – This method is more accurate and is always preferred. In this method, a particular line drawn from the previous station is drawn again from the new station. This process is called a backsight method.

Video Link:- Plane table survey