Definition :

Plane table surveying is a Graphical method of surveying in which

plotting and the observation is done at a time. In this method of
surveying a table top, similar to drawing board fitted on to a tripod is
the main instrument. A drawing sheet is fixed on to the table top, the
observations are made to the objects, distances are scaled down and
the objects are plotted in the field itself. Since the plotting is made in
the field itself, there is no chance of omitting any necessary
measurement in this surveying. However the accuracy achieved in
this type of surveying is less.

ACCESSORIES USED IN PLANE TABLE:

 The following accessories are required to carry out plane table
survey:
1.Plane table
2. Alidade
3. Plumbing fork with plumb bob
. 4. Spirit level
5. Trough compass
6. Drawing sheets and accessories

1.Plane table:
The most commonly used plane table is shown in Fig. It consists of a
well seasoned Wooden table top mounted on a tripod. The table top
can rotate about vertical axis freely. Whenever necessary table can
be clamped in the desired orientation. The table can be levelled by
adjusting tripod legs
2. Alidade:
It is a straight edge ruler having some form of sighting device. One
edge of the ruler is bevelled and is graduated. Always this edge is
used for drawing line of sight. Depending on the type of. line of sight.
there are two types of alidade: (a) Plain alidade. (b) Telescopic
alidade. a. Plain Alidade: Figure shows a typical plain alidade. A sight
vane is provided at each end of the ruler. The vane with narrow slit
serves as eye vane and the other with wide slit and having a thin wire
at its centre serves as object vane. The two vanes are provided with
hinges at the ends of ruler so that when not in use they can be folded
on the ruler. Plain alidade is not suitable in Surveying hilly areas as
the inclination of line of sight in this case is limited.
4. Plumbing Fork and Plumb Bob:
Figure shows a typical plumbing fork with a plum bob. Plumbing fork
is a U-shaped metal frame with a upper horizontal arm and a lower
inclined arm. The upper arm is provided with a pointer at the end
while the lower arm is provided with a hook to suspend plumb bob.
When the plumbing fork is kept on the plane table the vertical line
(line of plumb bob) passes through the pointed edge of the upper
arm. The plumb bob helps in transferring ground point to the
drawing sheet and vice versa
5. Spirit Level:
A flat based spirit level is used to level the plane table during
surveying. To get perfect level, spirit level should show central
position for bubble tube when checked with its positions in any two
mutually Perpendicular direction.
6. Trough Compass:
It consists of a 80 to 150 mm long and 30 mm wide box carrying a
freely suspended needle at its centre. At the ends of the needle
graduations are marked on the box to indicate zero to five degrees on
either side of the centre. The box is provided with glass top to
prevent oscillation of the needle by wind. When needle is centred
(reading 0–0), the line of needle is parallel to the edge of the box.
Hence marking on the edges in this state indicates Magnetic north–
south direction.

7. Drawing Sheet and Accessories for Drawing

A good quality, seasoned drawing sheet should be used for plane
table surveying. The drawing sheet may be rolled when not in use,
but should never is folded. For important works fibre glass sheets or
paper backed with thin aluminium sheets are used. Clips clamps,
adhesive tapes may be used for fixing drawing sheet to the plane
table. Sharp hard pencil, good quality eraser, pencil cutter and sand
paper to keep pencil point sharp are other accessories required for
the drawing work. If necessary, plastic sheet should be carried to
cover the drawing sheet from rain and dust.

 Temporary adjustments of plane table:

 Setting & levelling of plane table.
 Centring of plane table.
 Orientation of plane table.
 Sighting the points.

The following four methods are available for carrying

out plane table survey:
1. Radiation
2. Intersection
3. Traversing
4. Resection.
The first two methods are used for locating details while the other
two methods are used for locating position of plane table station on
drawing sheet.
Radiation method:
After setting the plane table on a station, say O, is required to find
the plotted position of various the objects A, B, C, D ….. . To get these
positions, rays OA, OB, OC ….. are drawn with soft pencil Then the
distances OA, OB, OC ….., are measured scaled down and the
positions of A, B, C ….., are found on the drawing sheets. This method
is suitable for surveying small areas and is convenient if the distances
to be measured are small. For larger areas this method has wider.

Intersection method:
In this method the plotted position of an object is obtained by
plotting rays to the object from two stations. The intersection gives
the plotted position. Thus it needs the linear measurements only
between the station points and do not need the measurements to
the objects. the method for locating objects A and B from plane table
positions O1 and O2 This method is commonly used for locating: (a)
details (b) the distant and inaccessible points
Traversing method:
This is the method used for locating plane table survey stations. In
this method, ray is drawn to next station before shifting the table and
distance between the stations measured. The distance is scaled down
and next station is located. After setting the plane table at new
station orientation is achieved by back sighting. To ensure additional
checks, rays are taken to other stations also, whenever it is table
possible. Figure shows a scheme of plane survey of closed area. This
method can be used for open traverses also.
Resection method:
This method is just opposite to the method of intersection. In the
method of intersection, the plotted position of stations are known
and the plotted position of objects are obtained by of intersection. In
this method the plotted position objects are known and the plotted
position of station is obtained. If a, b and c are the plotted positions
of objects A, B and C respectively, to locate instrument station P on
the paper, the b, orientation of table is achieved with the help of a, c
and then resectors Aa, Bb, Cc are drawn to get the p, the plotted
position of P. Hence in theresection method major work is to ensure
suitable orientation by any one of the methods. The following
methods are used in the method of resection. (a) by compass (b) by
back sighting (c) by solving two point problem (d) by solving three
point problem.
Resection after Orientation by Compass:
Let a and b be the plotted positions of A and B of two well defined
points in the field. Keeping the through compass along north
direction marked on the drawing sheet table is oriented on station P,
the position of which is to be found on paper. The resectors Aa and
Bb are drawn to locate p the plotted position of station point P. This
method gives satisfactory results, if the area is not influenced by local
attractions. It is used for small scale mapping only.

Resection after Orientation by Back Sighting:

shows in Figure the scheme of resection after orientation by back
sighting. From station A, the position of B is plotted as ‘b’ and ray has
been taken to station P as ap. Then plane table is set at P and
oriented by back sighting A, line AP is not measured But the position
of P is obtained on the paper by taking resection Bb.
Resection after Solving Two Point Problem: The problem of
finding plotted position of the station point occupied by the plane
table with the is help of plotted positions of two well defined points
known as solving two point problem. shows in Figure the scheme of
solving.
Procedure:
Let A and B be two well defined points like lightening conductor or
spire, the plotted positions a and b already known. Now the problem
is to orient the table at P so that by resection its plotted position p
can be obtained. The following steps may be followed to solve this
problems:
(i) Select a suitable point Q near P such that the angles PAQ and
PBQ are not accute.
(ii) Roughly orient the table at Q and draw the resectors Aa and
Bb to get the point ‘q’.
(iii) Draw the ray qp and locate p1 with estimated distance QP.
(iv) Shift the plane table to P and orient the table by back
sighting to Q.
(v) Draw the resector Aa to get ‘p’.
(vi) Draw the ray pB. Let it intersect line bq at b1 .
(vii) The points b and b1 are not coinciding due to the angular
error in the orientation of table. The angle bab, is the
angular error in orientation. To correct it, Fix a ranging rod at
R along ab, Unclamp the table and rotate it till line ab sights
ranging rod at R. Then clamp the table. This gives the correct
orientation of the table which was used in plotting the points
A and B.
(viii) The resectors Aa and Bb are drawn to get the correct plotted
position ‘p’ of the station P. (d) Resection after Solving Three
Let A, B, C be three well defined objects on the field whose
plotted positions a, b and c are known. Now the problem is
to locate plotted position of the station point P. Point
Problem: Locating the plotted position of a station point
using observations to three well defined points whose
plotted positions are known, is called solving three point
problem.