Plane Table Survey

Plane Table Surveying

• Plane Table Surveying is a graphical method of
survey in which the field observations and
plotting are done simultaneously.
• It is simple and cheaper than theodolite survey.
It is most suitable for small scale maps.
• The plan is drawn by the surveyor in the field,
while the area to be surveyed is before his eyes.
Therefore, there is no possibility of omitting the
necessary measurements.
 Equipments and Accessories for
Plane Tabling
The following instruments are used in plane
table surveying.
Equipments
• Plane Table
• Tripod
• Alidade
 Equipments and Accessories for
Plane Tabling
Accessories
• Trough Compass
• Spirit level
• U-Fork with Plumb bob
• Water proof cover
• Drawing paper
• Pins
• Drawing accessories
 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.
Plane Table
Plane Table
 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.
Tripod
Scanned by CamScanner
 Alidade
• The alidade is useful for establishing a line of
sight.
• Two Types of alidade are used.
• Simple alidade
• Telescopic alidade
 Simple Alidade
• It is used for ordinary work
• It is generally consists of a gun 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
Simple Alidade
 Simple Alidade
• To draw the rays, one of the edge of alidade is
beveled and this perfectly smooth working
edge is known as the fiducially edge.
• The fiducially edge is graduated to facilitate
the plotting of distances to a scale.
Telescopic Alidade
• 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.
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.
Trough Compass
 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.
Spirit Level
 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.
U-Fork With Plumb Bob
 Water Proof Cover
• An umbrella is used to protect the drawing
paper from rain.
 Drawing Paper
• Drawing paper is used for plotting the ground
details.
 Advantages and Disadvantages of
Plane Table Surveying
Advantages
• 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.
 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.
 Advantages and Disadvantages of
Plane Table Surveying
Disadvantages (Limitations)
• 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.
 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.
Principle Of Plane Table Survey
 Method Of Setting Up The Plane
Table
• Three processes are involved in setting up the
plane table over the station.
• Leveling
• Centering
• Orientation
Leveling
 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.
 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.
Centering
 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
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 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.
Orientation
By Back Sighting
• 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.
 Methods Of Plane Tabling
• There are four distinct methods of plane
tabling:
• Method of Radiation
• Method of Intersection
• Method of Traversing
• Method of Resection
 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.
Radiation Method
 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.
Radiation Method
 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.
 Method Of Intersection
• 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,
Method Of Intersection
 Method Of Intersection
• 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
Method Of Intersection
 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.
 Method Of Traversing
• 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.
Method Of Traversing
 Method Of Traversing
• 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.
 Method Of Traversing
• 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.
Method Of Traversing
 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
 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.
Method of Resection
 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
 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.
By Compass
 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.
 The Two Point Problem
• 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
The Two Point Problem
 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.
 The Two Point Problem
• 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.
 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
 The Three Point Problem
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
Bessel’s Method
 The Three Point Problem
• 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.
 The Three Point Problem
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
The Mechanical Method
 The Three Point Problem
• 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
 The Three Point Problem
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.
The method of Trial and Error
 PLANETABLESURVEYING
283
The orientation will be correct
only when the- triangre of error is reduced to one point.
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ancl rotate the table to sight
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orientation (but more u.,
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the size
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u oornll' error can be reduced to
The final and correct position
meer in one single point,
of the tabre wilr be such that the
gi"i"g it.^pointp. raysAa, Bb andcc
The whore problem, thus, involves
approximate fixation of p' a fair knowledge of Lehmann,s Rures
so ,il"i ,tr"'oi."gr;'"i;;#";., for the
The lines joining A' B' c (or be reduced to a minimum.
.t.trilil;:T";,i-,*,r,"e ,r,.Jei a, b,.B,,ri"r;,
c) form a triangle. known as the
2, Great Triangre.
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the great triangre ABC, the
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 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
Points to be kept in mind in plane
tabling
 Points to be kept in mind in plane
tabling
• 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.
 Error In Plane Tabling
• The various sources of error may be classified
as :
• Instrumental errors
• Errors in manipulation and sighting
• Errors in plotting
 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
perpendicular to the alidade.
• The edge of alidade is not is not parallel to the
line of sight.
• The fixing clamp is not proper.
 Errors In Manipulation And
Sighting
• Defective Leveling
• Defective Sighting
• Defective Orientation
• Defective Centering
• Movement of Board between sights
 Errors in Plotting
• Defective scale of map
• Wrongly intersecting the rays drawn from two
different stations.