plane table survey

PLANE TABLE SURVEY
Introduction:-
In case of plane table survey, the measurements of survey lines of the traverse and
their plotting to a suitable scale are done simultaneously on the field. Following are the
cases in which the plane table survey is found to be useful:

(a) Compass survey cannot be carried out with success in industrial areas of the town.
Plane table survey will be the best alternative in such cases.

(b) For preparing plans on a small scale, plane table survey proves to be speedy, easy
and accurate.

(c) The city or town has expanded within two or three decades and it is required to
plot the developed area on the previously plotted plan of the existing area.

Instruments required:

Alidade, Drawing board, Plumbing fork, Spirit level and Trough compass
Temporary adjustments of plane table:
Following three distinct operations at each survey station are carried out for the
temporary adjustments of a plane table.
(1) Centering
(2) Leveling
(3) Orientation
(1) Centering
The legs of tripod are well spread out to get the convenient height for working on
the board. Then, the operation of centering is carried out by means of plumbing fork or
U-frame and plumb bob. This process ascertains the fact that the point on paper
represents the station point on ground. The pointed end of the plumbing fork is kept on
point on paper and at the other end, a plumb bob is fixed. The table or board is shifted
bodily till the plumb bob hangs exactly over the peg of the station.
(2) Leveling
The process of leveling is carried out with the help of spirit level and it consists of
making the table level either by ordinary tilting the board or by ball and socket
arrangement or by adjusting the legs of tripod.
(3) Orientation
The process by which the position occupied by the board at various survey
stations are kept parallel is known as the orientation. Thus, when a plane table is
properly oriented, the lines on the board are parallel to the lined on ground which they
represent.
The methods of orientation are:
 
(i) Orientation by magnetic needle: In this method, the magnetic north is drawn
on paper at a particular station. At the next station, the trough compass is
placed along the line of magnetic north and then the table is turned in such a
way that the ends of magnetic needle are opposite to the zeros of the scale.
The board is then fixed in position by clamps. This method is inaccurate in
the sense that the results are likely to be affected by the local attraction.
(ii) Orientation by back sighting: In this method, the orientation is carried out by
the back sighting of a particular line. Suppose a line is drawn from station A
on paper representing line AB on ground. The table is centered and leveled at
station B and then the alidade is placed along the line ba. The table is turned
till the line of sight bisects the ranging rod at A. The board is then clamped in
this position. This method is better than the previous one and it gives perfect
orientation.
Methods of plane table survey:
Following are the four methods by which an object might be located on paper by
plane table:
(1) Radiation
(2) Intersection
(3) Traversing
(4) Resection