Plane table survey

EQUIPMENTS AND ACCESSORIES FOR PLANE
TABLING
The following instruments are used in plane table surveying.
1. Equipments
2. Plane Table
3. Tripod
4. Alidade
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EQUIPMENTS AND ACCESSORIES FOR PLANE
TABLING
• Accessories
• Trough Compass
• Spirit level
• U-Fork with Plumb bob
• Water proof cover
• Drawing paper
• Pins
• Drawing accessories
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EQUIPMENTS
Plane Table:
• The drawing board for plane tabling is made from well-seasoned wood
with its upper surface exactly plane.
• It is normally rectangular in shape with size 75 cm x 60 cm
• It is mounted on a tripod and clamps are provided to fix it in any
direction. The table can revolved about its vertical axis and can be
clamped in any position, when necessary.
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TRIPOD
• The plane table is mounted on a tripod
• The tripod is generally of open frame type, combined rigidity
with lightness. The tripod may be made to fold for convenience of
transportation.
• Tripod is provided with three foot screws at its top for leveling of
the plane table.
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ALIDADE
• The alidade is useful for establishing a line of sight.
• Two Types of alidade are used.
• Simple alidade
• Telescopic alidade
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SIMPLE ALIDADE
• It is used for ordinary work
• It is generally consists of a metal or wooden rule with two vertical vanes
at the ends.
• The eye-vane is provided with a narrow slit while the object vane is open
and carries a horse hair. Both the slits, thus provide a definite line of sight
which can be made to pass through the object to be sighted
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The telescopic alidade is used when it is required to take inclined sights.
It essentially consists of a small telescope with a level tube and graduated arc
mounted on horizontal axis.
It gives higher accuracy and more range of sights.
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TELESCOPIC ALIDADE

ACCESSORIES
• Trough Compass:
• The trough compass is required for drawing the line showing
magnetic meridian on the paper. It is used to orient the table to
the magnetic meridian.
• When the freely suspended needle shows 00 at each end, a line
is drawn on the drawing paper which represents the magnetic
north.
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SPIRIT LEVEL
• A Spirit Level is used for ascertaining If the table is properly level.
• The Table is leveled by placing the level on the board in two positions at
right angles and getting the bubble central in both positions.
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U-FORK WITH PLUMB BOB
• U-fork with plumb bob is used for centering the table over the point
or station occupied by the plane table when the plotted position of
that point is already on the sheet.
• Also, in the beginning of the work, it is used for transferring the
ground point on the sheet.
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WATER PROOF COVER & DRAWING PAPER
• An umbrella is used to protect the drawing paper from rain.
• Drawing paper is used for plotting the ground details.
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MAJOR ADVANTAGES OF PLANE TABLE
SURVEYING
• The plan is drawn by the surveyor himself while the area to be
surveyed is before his eyes. Therefore, there is no possibility of
omitting the necessary measurements.
• The surveyor Can compare the plotted work with the actual features
of the area.
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ADVANTAGES
• It is simple and cheaper than the theodolite survey.
• It is most suitable for small scale maps.
• No great skill is required to produce a satisfactory map and work may be
entrusted to a subordinate.
• It is useful in magnetic areas where compass may not be used.
• The mistakes in writing field books are eliminated.
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DISADVANTAGES OF PLANE TABLE SURVEYING
• It is not intended for very accurate work.
• It is not suitable in monsoon.
• It is essentially a tropical instrument.
• Due to heaviness, it is inconvenient to transport.
• Since there are so many accessories, there is likelihood of them
being lost.
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PRINCIPLE OF PLANE TABLE SURVEY
• The principle of plane tabling is parallelism means,
• Principle: “All the rays drawn through various details should pass
through the survey station.”
• The Position of plane table at each station must be identical, i.e. at
each survey station the table must be oriented in the direction of
magnetic north.
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METHOD OF SETTING UP THE PLANE TABLE
• Three processes are involved in setting up the plane table over the
station.
• Leveling
• Centering
• Orientation
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LEVELING AND CENTERING
• The Table should be set up at convenient height for working on the
board, say about 1 m. The legs of Tripod should be spread well apart
and firmly into the ground.
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LEVELING AND 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. It is done by U-fork and plumb bob.
• For leveling the table ordinary spirit level may be used. The table is
leveled by placing the level on the board in two positions at right angles
and getting the bubble central in both directions.
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ORIENTATION
• The Process by which the positions 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 lines on ground which they
represent.
• There are two methods of orientation:
• By magnetic needle
• By back sighting
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• 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 the table is turned in such a way that the ends of
magnetic needle are opposite to zeros of the scale.
• The board is then fixed in position by clamps. This method is inaccurate in
the since that the results are likely to be affected by the local attraction.
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BY MAGNETIC NEEDLE

A= First survey station
B= Second survey station
• Suppose a line is drawn from station A on paper as ab, representing line
AB on ground
• 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.
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BY BACK SIGHTING

METHODS OF PLANE TABLING
There are four distinct methods of plane tabling:
• Method of Radiation
• Method of Intersection
• Method of Traversing
• Method of Resection
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RADIATION METHOD
• In the radiation method of plane table surveying, the direction of the
objects or points to be located are obtained by drawing radial lines
along fiducially edge of alidade after getting the objects or points
bisected along the line of sight of the alidade. The horizontal
distances are then measured and scaled off on the corresponding
radial lines to mark their positions on the drawing.
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RADIATION METHOD
• Suppose P is a station on the ground from where the object A, B, C and D
are visible.
• The plane table is set up over the station P. A drawing is fixed on the table,
which is then leveled and centered. A point p is selected on the sheet to
represent the station P.
• The north line is marked on the right-hand top corner of the sheet with
trough compass or circular box compass.
• With the alidade touching p, the ranging rod at A,B, C and D are bisected
and the rays are drawn.
• The distances PA, PB, PC and PD are measured and plotted to any suitable
scale to obtain the points a, b, c and d representing A,B,C,D on paper.
26

METHOD OF INTERSECTION
• In intersection method of plane table surveying, the objects or points to be
located are obtained at the point of intersection of radial lines drawn from two
different stations.
• In this method, the plotting of plane table stations are to be carried out
accurately. Checking is important and thus done by taking third sight from
another station.
• The intersection method is suitable when distances of objects are large or
cannot be measured properly. Thus, this method is preferred in small scale
survey and for mountainous regions.
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• Suppose A and B are two station and P is the object on the far bank
of a river. Now it is required to fix the position of P on the sheet by
the intersection of rays, drawn from A and B.
• The table is set up at A. It is leveled and centered so that a point a on
the sheet is just over the station A. The north line is marked on the
right-hand top corner, the Table is then clamped.
• With the alidade touching a, the object P and the ranging rod at B
are bisected, and rays are drawn through the fiducial edge on
alidade,
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The distance AB is measured and plotted to any suitable scale to obtain
point b.
The table is shifted and centered over B and leveled properly. Now the
alidade is placed along the line ba and orientation is done by back sighting
With the alidade touching b, the object P is bisected and a ray is drawn,
suppose this ray intersects the previous rays at point p. the point p is the
required plotted position of P
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METHOD OF TRAVERSING
• This method of plane table surveying is used to plot a traverse in cases
stations have not been previously plotted by some other methods. In this
method, traverse stations are first selected. The stations are plotted by
method of radiation by taking back sight on the preceding station and a
fore sight to the following station. Here distances are generally measured
by tachometric method and surveying work has to be performed with
great care.
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• Suppose A,B,C,D are the traverse stations,
• The table is set up at the station A, a suitable point a is selected on the sheet in
such a way that the whole area may be plotted in the sheet. The table is
centered, leveled and clamped. The north line is marked on the right-hand
top corner of the sheet.
• With the alidade touching point a the ranging rod at B is bisected and a ray is
drawn. The distance AB is measured and plotted to any suitable scale.
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• The table is shifted touching point a the ranging rod at B is bisected and a ray is
drawn. The distance is measured and plotted to any suitable scale.
• The table is shifted and centered over B. It is then leveled, oriented by back sighting
and clamped.
• With the alidade touching point b, the ranging rod at C is bisected and ray is drawn.
The distance BC is measured and plotted to the same scale.
• The table is shifted and set up at C and the same procedure is repeated.
• In this manner, all stations of the traverse are connected.
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• Check lines. To check the accuracy of the plane table traverse, a few check
lines are taken by sighting back to some preceding station.
• Error of closure . If the traverse to be plotted is a closed traverse, the
foresight from the terminating station should pass through the first
station. Otherwise the amount by which plotted position of the first station
on the foresight fails to close is designated as the error of closure. It is
adjusted graphically, if the error is within permissible limits, before any
further plotting works are done.
36

METHOD OF 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, locations of which have been plotted.
• There are four methods of resection.
• By Compass
• By back sighting
• By two point problem
• By three point problem
37

METHOD OF RESECTION
• Suppose It is required to establish a station at position P. Let us select two
points A and B on the ground. The distance AB is measured and plotted to any
suitable scale. The line AB is known as the “base line”
• The table is set up at A. It is leveled, centered and oriented by bisecting the
ranging rod at B. The table is then clamped.
• With the alidade touching point a, the ranging rod at P is bisected and a ray is
drawn. Then a point P1 is marked on this way by estimating with the eye.
38

METHOD OF RESECTION
• The table is shifted and centered in such a way that P1 is just over P. It is
then oriented by back-sighting the ranging rod at A.
• With the alidade touching point b, the ranging rod at B is bisected and a
ray is drawn. Suppose this ray intersects the previous ray at a point P.
This point represents the position of the station P on the sheet. Then the
actual position of the station is marked on the ground by U-fork and
plumb-bob
40

BY COMPASS
• This method is used only for small scale or rough mapping.
• Let A and B be two visible stations which have been plotted on the sheet
as a and b. Let C be the instrument station to be located on the plan.
• Set the table at C and orient it with compass. Clamp the table.
• Pivoting the alidade about a, draw a ray towards A, as Similarly,
pivoting the alidade about b, draw a ray towards B, as bb’, The
intersection of aa’ and bb’ will give point c on the paper.
41

THE TWO POINT PROBLEM
• In this problem, two well-defined points whose positions have
already been plotted on the plan are selected. Then, by perfectly
bisecting these points, a new station is established at the required
position.
• Suppose P and Q are two well-defined points whose positions are
plotted on map as p and q. It is required to locate a new station at A
by perfectly bisecting P and Q
• An auxiliary station B is selected at a suitable position. The table is
set up at B, and leveled and oriented by eye estimation. It is then
clamped.
• With the alidade touching p and q, the points P and Q are bisected
and rays are drawn. Suppose these rays intersect at b
43

THE TWO POINT PROBLEM
• With the alidade centre on b, the ranging rod at A is bisected and rays is
drawn. Then, by eye estimation, a point a 1 is marked on this ray.
• The table is shifted and centre on A with a1 just over A. It is leveled and
oriented by back sighting. With the alidade touching p, the point P is bisected
and a ray is drawn. Suppose this ray intersects the line ba1 at point a1, as was
assumed.
• With the alidade centered on a1 the point Q is bisected and a ray is drawn.
Suppose this ray intersects the ray bq at a point q1. The triangle pqq1 is
known as the triangle of error, and is to be eliminated.
• The alidade is placed along the line pq1 and a ranging rod R is fixed at some
distance from the table. Then, the alidade is placed along the line pq and the
table is turned to bisect R. At this position the table is said to be perfectly
oriented.
• Finally, with the alidade centered on p and q, the points P and Q are bisected
and rays are drawn. Suppose these rays intersect at a point a. This would
represent the exact position of the required station A. Then the station A is
marked on the ground.
45

THE THREE POINT PROBLEM
In this problem, three well defined points are selected, whose position have
already been plotted on the map. Then, by perfectly bisecting these three
well-defined points. A new station is established at the required position.
No auxiliary station is required in order to solve this problem. This table is
directly placed at the required position. The problem may be solved by
following methods
• (a) Bessel’s method
• (b) Mechanical Method
• (c) The trial and error method
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The graphical method or Bessel’s method
(i) suppose A,B, and C are three well-defined points which have been plotted
as a, b and c. Now it is required to locate a station at P.
(ii) The table is placed at the required station P and leveled. The alidade is
placed along the line ca and the point A is bisected. The table is clamped.
With the alidade in centre on C, the point B is bisected and rays is drawn
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• Again the alidade is placed along the line ac and the point C is bisected and the
table is clamped. With the alidade touching a, the point B is bisected and a ray
is drawn. Suppose this ray intersects the previous ray at a point d
• The alidade is placed along db and the point B is bisected. At this position the
table is said to be perfectly oriented. Now the rays Aa, Bb and Cc are drawn.
These three rays must meet at a point p which is the required point on the
map. This point is transferred to the ground by U-fork and plumb bob.
49

The Mechanical Method
• Suppose A, B and C are the three well-defined points which have been
plotted on the map as a, b and c. It is required to locate a station at P.
• The table is placed at P and leveled. A tracing paper is fixed on the map
and a point p is marked on it.
• With the alidade centered on P the points A, B and C are bisected and
rays are drawn. These rays may not pass through the points a, b and c
as the orientation is done approximately
50

• Now a tracing paper is unfastened and moved over the map in such a
way that the three rays simultaneously pass through the plotted
positions a, b and c. Then the points p is pricked with a pin to give an
impression p on the map. P is the required points on the map. The
tracing paper is then removed.
• Then the alidade is centered on p and the rays are drawn towards A, B
and C. These rays must pass through the points a, b and c
52

The method of Trial and error
• Suppose a, B and C are the three well-defined points which have been
plotted as a, b and c on the map. Now it is required to establish a station
at P.
• The table is set up at P and leveled. Orientation is done by eye estimation
• With the alidade, rays Aa, Bb and Cc are drawn. As the orientation is
approximately, the rays may not intersect at a point, but may form a
small triangle the triangle of error.
• To get the actual point, this triangle of error is to be eliminated. By
repeatedly turning the table clockwise or anticlockwise. The triangle is
eliminated in such a way that the rays Aa, Bb and Cc finally meet at a
point p. This is the required point on the map. This point is transferred to
the ground by U-fork and plumb bob.
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THE METHOD OF TRIAL AND ERROR
54

POINTS TO BE KEPT IN MIND IN PLANE TABLING
• The following points should be kept in mind while doing plane table
survey.
1. Ground points shall be marked as A, B, C… etc. and plan. Points (on
Paper) shall be marked as a, b, c etc.
2. The rays from survey stations to the objects shall be drawn by dashed
line.
3. The alidade should be properly pivoted while sighting the objects
4. The first survey station and the scale of the map shall be so chosen that
the entire area can be plotted on the paper.
5. While establishing magnetic north on the paper using trough compass,
things causing local attraction shall be kept away of the table.
6. The Plane table should be clamped after centering and leveling. The
table should be rotated at the time of orientation
55

ERROR IN PLANE TABLING
The various sources of error may be classified as :
• Instrumental errors
• Errors in manipulation and sighting
• Errors in plotting
56

INSTRUMENTAL ERRORS
• The surface of drawing board is not plane
• The edge of alidade is not straight.
• The object vane and sight vane are not in a straight line.
• The edge of alidade is not parallel to the line of sight.
• The fixing clamp is not proper.
57

ERRORS IN MANIPULATION AND
SIGHTING
• Defective Leveling
• Defective Sighting
• Defective Orientation
• Defective Centering
• Movement of Board between sights
58

ERRORS IN PLOTTING
• Defective scale of map
• Wrongly intersecting the rays drawn from two different stations.
59

REFERENCES
• Surveying and leveling- S.C.Saxena
• Surveying and leveling- R.P.Rethaliya
• Images: Google Search
TO BE USED FOR EDUCATION/INFORMATION
PURPOSE ONLY
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Plane table survey

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Plane table survey