Plane Table Surveying For Civil Engineering

Plane Table Surveying

1. Plane Table Surveying
Plane Tabling is a graphical method of survey in which the field observations and
plotting proceed simultaneously.
It is a means of making a manuscript map in the field while the ground can be seen by
the topographer and without intermediate steps of recording and transcribing field
notes.
Instrument used:
1.The plane table with levelling head having arrangements for (a) levelling, (b) rotation
about vertical axis, and (c ) clamping in any required position.
2.Alidade for sighting: a) Plain Alidade b) Telescopic Alidade
3.Plumbing fork and plumb bob.
4.Spirit level.
5.Compass
6.Drawing paper with a rainproof cover.

2. Principle of 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.
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 it occupied at the starting stations.
Plane tabling is a graphical method of surveying.

3. Temporary Adjustment of a Plane Table
or
Working Operations
1.Levelling: For small scale work, levelling is done by estimation. For
work of accuracy, an ordinary spirit level may be used. The table is
levelled by placing the level on the board in two positions at right
angles and getting the bubble central in both directions. For the
more precise work, a Johnson Table or Coast Survey Table may be
used.
2.Centering: The table should be so placed over the station on the
ground that the point plotted on the sheet corresponding to the
station occupied should be exactly over the station on the ground.
The operation is known as centering the plane table. This is done by
using a plumbing fork.

4. 3. Orientation:
Orientation is the process of putting the plane table into some fixed
direction so that line representing a certain direction on the plan is
parallel to that direction on the ground.
This is essential condition to be fulfilled when more than one instrument
station is to be used.
If orientation is not done, the table will not be parallel to itself at
different positions resulting in an overall distortion of the map.
The process of centering and orientation are dependent to each other.
For orientation, the table will have to be rotated about its vertical axis,
thus disturbing the centering.
Precise work requires that the plotted point should be exactly over the
ground point, repeated orientation and shifting of the whole table are
necessary.

5. Methods of orienting the plane table
1.Orientation by means of trough compass.
2.Orientation means of back sighting
Orientation by trough compass: For orientation, the compass is so
placed on the plane table that the needle floats centrally, and a fine
pencil line is ruled against the long side of the box. At any other station,
where the table is to be oriented, the compass is placed against the line
and the table is oriented by turning it until the needle floats centrally.
The table is then clamped in position.

6. Orientation by back sighting:
Orientation can be done precisely by sighting the points already plotted the
sheet.
Two cased may arise:
A.When it is possible to set the plane table on the point already plotted on
the sheet by way of observation from the previous station.
B.When it is not possible to set the plane table on the point.
For the conditions indicated in A the orientation can be done by back sighting.
To orient the table at the next station, say B, represented on the paper by a
point b plotted by means of line ab drawn from a previous station A, the alidade
is kept on the line baand the table is turned about its vertical axis in such a way
that the line of sight passes through the ground station A. when this is achieved,
the plotted line ab will be coinciding with the ground line AB (provided the
centering is perfect) and the table will be oriented. The table is then clamped in
position.

7. 4. Sighting the points:
when once the table has been set, i.e., when levelling, centering and
orientation has been done, the points to be located are sighted through
the alidade.
The alidade is kept pivoted about the plotted location of the instrument
station and is turned so that the line of sight is passes or bisect the signal
at the point to be plotted.
A ray is drawn from the instrument station along the edge of the alidade.
The points are finally plotted on the corresponding rays either by way of
intersection or by radiation.

8. Methods of Plane Tabling
1.Radiation
2.Intersection
3.Traversing
4.Resection
The first two methods are generally employed for locating the details
while the other two methods are used for locating the plane table
stations.

9. Radiation
In this method, a ray is drawn from the instrument station towards
the point, the distance is measured between the instrument station
and that point, and the point is located by plotting to some scale the
distance so measured.
This method is more suitable when the distances are small and one
single instrument can control the points to be detailed.
The method has a wider scope if the distances are obtained
tacheometricallywith the help of telescopic alidade.

10. Procedure to conduct the plane table surveying
by radiation method:
The following steps are necessary to locate the points from an
instrument station T:
1.Set the plane table at T, level it, and transfer the point on the
sheet by means of plumbing fork, thus getting point t
representing T. Clamp the table.
2.Keep the alidade touching t and sight to A. Draw the ray along
the fiducial edge of the alidade.
3.Similarly, sight different points B, C, D, E etc., and draw the
corresponding rays.
4.A pin may be inserted at t, and the alidade may be kept
touching the pin while sighting the points.
5.Measure TA, TB, TC, TD, TE etc., in the field and plot their
distances to some scale along the corresponding rays, thus
getting a, b, c, d, e, etc. Join these if needed.

11. Intersection ( Graphical Triangulation)
Intersection is resorted to when the distance between the point and the instrument station is
either too large or cannot be measured accurately due to some field conditions.
The location of an object is determined by sighting at the object from two plane table stations
(previously plotted) and drawing the rays.
The intersection of these rays will give the position of the object.
It is therefore very essential to have at least two instrument stations to locate any point.
The distance between the two instrument stations is measured and plotted on the sheet to some
scale.
The line joining the two instrument station is known as base line.
No linear measurement other than that of base line is made.
The point of intersection of the two rays forms the vertex of a triangle having the two rays as two
sides and the base line as the third line of the triangle.
Due to this reason, intersection is also sometime known as graphic triangulation.

12. Procedure to conduct the plane table survey by intersection method
The following is the procedure to locate the points by
method of intersection:
1.Set the plane table at A, level it and transfer the
point A on the sheet by way of plumbing fork. Clamp
the table.
2.With the help of trough compass, mark the north
direction on the sheet.
3.Pivoting the alidade about a, sight it to B. Measure
AB and plot it along the ray to get b. The base line ab
is thus drawn.
4.Pivoting the alidade about a, sight the details C, D, E,
etc., and draw corresponding rays.
5.Shift the table at B and set it there. Orient the table
roughly by compass and finally by back sighting A.
6.Pivoting the alidade about b, sight the details C, D, E,
etc. and draw the corresponding rays along the edge
of the alidade to intersect with the previously drawn
rays in c, d, e, etc.
7.The position of the points are thus mapped by way
of intersection.

13. Intersection
The method of intersection is mainly used for mapping details.
If this is to be used for locating a point which will be used as
subsequent plane table station, the point should be got by way of
intersection of at least three or more rays.
Triangle should be well conditioned and the angle of intersection of
the rays should not be less than 45º in such case.
Graphical triangulation can also proceed without preliminary
measurement of the base, as the length of base line influence only the
scale of plotting.

14. Traversing
Plane table traverse involves the same principles as transit traverse.
At each successive station the table is set, a foresight is taken to the
following station and its location is plotted by measuring the distances
between the two stations as in the radiation method described earlier.
Hence, the traversing id not much different from radiation as far as
working principles are concerned.
The only difference is that in the case of radiation the observations are
taken to those points which are to be detailed or mapped while in the case
of traversing the observations are made to those points which will
subsequently be used as instrument stations..
This method is widely used to lay down survey lines between the
instrument stations of a closed or unclosed traverse.

15. Procedure for Traversing Method
1.Set the table at A. Use plumbing fork for
transferring A on to the sheet. Draw the
direction of magnetic meridian with the
help of trough compass.
2.With the alidade pivoted about a, sight to B
and draw the ray. Measure AB and scale off
ab to some scale. Similarly, draw a ray
towards E, measure AE and plot e.
3.Shift the table to B and set it, Orient the
table accurately by back sighting A. Clamp
the table.
4.Pivoting the alidade about b, sight to C.
Measure BC and plot it on the drawn ray to
the same scale.
5.Similarly, the table can be set at other
stations and the traverse is completed.
(At any station a portion of the traverse may be
checked if two or more of the preceding
stations are visible.)

16. Resection
Resection is the process of determining the plotted position of the station occupied by the plane
table, by means of sights taken towards known points, location of which have been plotted.
The method consist in drawing two rays to the two points of known location on the plan after the
table has been oriented.
The rays drawn from the un-plotted location of the station to the points of known location are
called resectors, the intersection of which gives the required location of the instrument station.
If the table is not correctly oriented at the station to be located on the map, the intersection of
the two resectorswill not give the correct location of the station.
The problem, therefore, lies in orienting table at the stations and can be solved by the following
four methods of orientation.
I.Resection after orientation by compass
II.Resection after orientation by back sighting
III.Resection after orientation by three point problem
IV.Resection after orientation by two point problem

17. Resection after orientation by compass
The method is utilized only for small scale or
rough mapping for which the relatively large
errors due to orienting with the compass
needle would not impair the usefulness of the
map.
The method is as follows:
1.Let C be the instrument station to be located
on the plan. Let A and B be two visible
stations which have been plotted on the
sheet as a and b.
2.Set the plane table at C and orient it with
the compass. Clamp the table.
3.Pivoting the alidade about a, draw a resector
towards A; similarly sight B from b and draw
a resector. The intersection of the two
resectorswill give c, the required point.

18. Resection after orientation by back sighting
If the table can be oriented by back sighting along a
previously plotted back sight line, the station can be
located by the intersection of the back sight line and the
resectordrawn through another known point.
The method is as follows:
1.Let C be the station to be located on the plan and A
and B be the two visible points which have been
plotted on the sheet as a and b.
2.Set the table at A and orient it by back sighting B
along ab.
3.Pivoting the alidade at a, sight C and draw a ray.
Estimate roughly the position of C on this ray as c
1.
4.Shift the table to C and center it approximately with
respect to c
1. Keep the alidade on the line c
1a and
orient the table by back sight to A. Clamp the table
which has been oriented.
5.Pivoting the alidade about b, sight B and draw the
resectorbBto intersect the ray c
1a in c.
6.Thus, c is the location of the instrument station.

19. Resection by three point problem
Statement: Location of the position, on the plan of
the station occupied by the plan table by means of
observations to three well defined points whose
positions have been previously plotted on the plan.
In other words, it is required to orient the table at
the station with respect to three visible points
already located on the plan. The table is said to be
correctly oriented at P when the three resectors
through a, b and c, meet at a point.
The intersection of the three resectorsin a point
gives the location of the instrument station.
Thus in three point problem, orientation and
resection are accomplished in the same operation.
The following are some of the important methods
available for the solution of the problem.
a)Mechanical Method ( Tracing Paper Method)
b)Graphical Method
c)Lehmann’s Method (Trial and error Method)

20. Resection by two point problem
Statement: “Location of the position on the plan, of the station occupied by
the plane table by means of observations to two well defined points whose
positions have been previously plotted on the plan”.
Let us take two points A and B, the plotted positions of which are known. Let
C be the point to be plotted. The whole problem is to orient the table at C.
Procedure:
1.Choose an auxiliary point D near C, to assist the orientation at C.
2.Set the table at D in such a way that ab is approximately parallel to AB
(either by compass or by eye adjustment). Clamp the table.
3.Keep the alidade at a and sight A. Draw the resector.
4.Similarly keep the alidade at b and sight B. Draw a resectorto intersect
the previous one in d.
5.The position of d thus plotted, the degree of accuracy depends upon
the approximation that has been made in keeping ab parallel to AB.
Transfer the point d to the ground and drive a peg.
6.Keep the alidade at d and sight C. Draw the ray. Marks point c
1on the
ray by estimation to represent distance DC.
7.Shift the table to C, orient it by taking backsightto D and center it with
reference to c
1. The orientation is thus the same as it was at D.
8.Keep the alidade pivoted at a and sight it to A . Draw the ray to
intersect with previously drawn ray from D in c.

21. 9. Thus, c is the point representing the station C, with reference to approximate
orientation made at D.
10. Pivoting the alidade about c, sight B. Draw the ray to intersect with the ray
drawn from D to B in b’.Thus b’ is the approximate representation of B with
respect to the orientation made at D.
11. The angle between ab and ab’ is the error in orientation and must be
corrected for. In order that ab and ab’ may coincide ( or may become parallel)
keep a pole P in line with ab’ and at a great distance. Keeping the alidade along
ab, rotate the table till P is bisected. Clamp the table. The table is thus correctly
oriented.
12. After having oriented the table as above, draw a resectorfrom a to A and
from b to B, the intersection of which will give the position of C occupied by the
table.
[Note: It is to be noted here that unless the point P is chosen infinitely distant,
ab and ab’ cannot be made parallel. Since the distance of P from C is limited due
to other considerations, two-point problem does not give much accurate
results. Also more labouris involved because the table is also to be set on one
more station to assist the orientation.]

22. Advantages of plane tabling
Advantages
1)The plan is drawn by the out-door surveyor himself while the country is before his eyes, and
therefore, there is no possibility of omitting the necessary measurements.
2)The surveyor can compare plotted work with the actual features of the area.
3)Since the area is in view, contour and irregular objects may be represented accurately.
4)Direct measurements may be almost entirely dispensed with, as the linear and angular
dimensions are both to be obtained by graphical means.
5)Notes of measurements are seldom required and the possibility of mistakes in booking is
eliminated.
6)It is particularly useful in magnetic areas where compass may not be used.
7)It is simple and hence cheaper than the theodolite or any other type of survey.
8)It is most suitable for small scale maps.
9)No great skill is required to produce a satisfactory map and the work may be entrusted to a
subordinate.

23. Disadvantages of Plane Tabling
(1)Since notes of measurements are not recorded, it is a great
inconvenience if the map is required to be reproduced to some
different scale.
(2)The plane tabling is not intended for very accurate work.
(3)It is essentially a tropical instrument.
(4)It is most inconvenient in rainy season and in wet climate.
(5)Due to heaviness, it is inconvenient to transport.
(6)Since there are so many accessories, there is very likelihood of
being lost.