PLANE TABLE SURVEYING

PLANE TABLE SURVEYING

PRINCIPLE AND ACCESSORISES OF PLANE TABLE.

PRINCIPLE

  The principle of plane tabling is parallelism, meaning that the rays down from stations to subjects on the paper are parallel to the lines from the stations to the objects on the ground. The relative positions of the objects on the ground are represented by their plotted positions on the paper and lie on the respective rays.

  The table is always placed at each of the successive stations parallel to the position is occupied at the starting station. Plane tabling is graphical method of surveying. Here, the field work and plotting are done simultaneously, and such survey does not involve the used oh field book.

  Plane table survey is suitable for filling interior details when traversing is done by theodolite. Sometimes traversing by plane table may also be done. But this survey is recommended for the work where grate accuracy is not required. As the fitting and fixing arrangement of this instrument is not perfect, most accurate work can not be expected.  

ACCESSORISES OF PLANE TABLE.

1). Plane table: The plane table is a drawing board of size by 750 mm x 600 mm made of well-seasoned wood like teak, pine, etc. the top surface of the table is well leveled. The bottom surface consists of a threaded circular plate for fixing the table on the tripod stand by a wing nut.

  The plane table is meant for fixing drawing sheet over it. The positions of the objects are located on this sheet by drawing rays and plotting to any suitable scale (Fig. 1).

  

Fig. 1 

2). The alidade: There are two types of alidade – plain and telescopic.

a). Plain alidade. The plain alidade consists of a metal or wooden ruler of length about 50 cm. one of its edges is beveled and is known as the fiducial edge. It consists of two vanes at both ends which is hinged with the ruler. One is the known as the ‘object vane’ and carries a horse hair; the other is called ‘sight vane’ and is provided with a narrow slit (Fig. 2).  

  

Fig. 2 

b). Telescope alidade. The telescope alidade consists of a telescope meant for inclined sight or sighting distant objects clearly. This alidade has no vanes at the ends but is provided fiducial edge.

  The function of the alidade is to sight objects. The rays should be drawn along the fiducial edge (Fig. 3).

Fig. 3 

3). The sprit level: The sprit level is a small metal tube containing a small bubble of sprit. The bubble is visible on the top along a graduated glass tube.

  The sprit level is meant for leveling the plane table (Fig. 4).

Fig. 4 

4). The compass: There are two types of alidade – (a) the tough compass and (b) the circular box compass.

a). The Touch Compass. The tough compass is a rectangular box made of non-magnetic metal containing a magnetic needle pivoted at the centre. This compass consists a ‘0’ mark at both ends to locate the N-S direction (Fig. 5).

Fig. 5

b). The Circular Box Compass. It carries a pivoted magnetic needle at the centre. The circular box fitted on a square base plate (Fig. 6).

  Sometimes two bubble tubes are fixed at right angles to each other on the base plate.

  The compass is meant for making the north direction of the map.

Fig. 6

5). U-fork or plumbing fork with plumb bob: The U-fork is a metal strip bent in the shape of ‘U’ having equal arm lengths. The top arm is pointed, and bottom arm carries a hook for suspending a plumb bob (Fig. 7).

  This is meant for centering the table over a station.

Fig. 7 

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