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 but it is mostly
suitable for small scale survey. The plan on drawing sheet drawn by surveyor in the field itself so
there is chance of occurrence of any mistake is very less.
Equipment and accessories
The drawing board: The board is made of well-seasoned wood and
varies in size from 40cm x 30 cm to 75cm x 60cm or 50 – 60 cm
square.
The Alidade: The alidade consists of metal or box wood straight edge or
ruler about 50cm long. The welled edge of the alidade is called the
fiducially edge.
Tripod: It is rigid and light equipment having three legs. The plane
table is mounted on tripod. It has three foot screw to level the plane
table.
Trough compass: The compass is used to mark the direction of the
meridian on the paper.
U- frame or Plumbing fork: U frame with a plumb bob used for
centering the table.
Water Proof Cover: Water Proof cover protects the sheet from rain.
Spirit level or level tube: A level tube is used to level the plane table.
Drawing sheet: The drawing sheet is fixed on the top of the drawing
board.
Pencil and eraser: A pencil is used for constructing lines and eraser
is used for erasing linesafter completion of the plan.
Setting Up the Plane Table
The setting up the plane table includes the following three operations.
Centering the plane table
Leveling the plane table
Orientation of plane table
Centering the plane table: The table should be set up at a convenient height
for working say about 1m. The legs of tripod should be spread well apart and
firmly fixed in to the ground. The table should be approximately leveled by
tripod legs and judging by the eye. Then the operation of centering is carried
out by means of U-frame and plumb bob. The plane table is exactly placed
over the ground station by U-frame and plumb bob.
Leveling the plane table: The process of leveling is carried out with the help
of level tube. The bubble of level tube is brought to center in two directions,
which are right angles to each other. This is achieved by moving legs.
Orienting the table: The process by which the position occupied by the
board at various survey stations are kept parallel is known as the orientation.
In the plane table surveying, the whole table needs to be moved at several
stations to complete a survey. Every time the table is moved one has to make
sure that the new station is parallel to the previous one otherwise the lines
drawn on paper will not represent the same lines on the field.
Methods of orientation are: -
i. Orientation by Magnetic Needle: This method is used when it is not
possible to bisect the previous station from the new station. This
method is not much reliable and prone to errors due to variations of the
magnetic field.
ii. Orientation by Back Sighting: This is a more reliable method. In this
method, a particular line drawn from the previous station is drawn again
from the new station. This process is called back-sighting. One does not
necessarily have to draw the line the second time rather check if the
new line superposes over the previous one or not.
Methods of Plane Table Surveying
Generally, there are four methods are available to perform plane table surveying. They are:
1. Radiation
2. Intersection
3. Traversing
4. Resection
1. Radiation Method:
The plane table is set up over only one station from which the whole traverse can becommanded.
It is suitable for survey of small areas.
Select a point “O” so that all points to be located are visible from it.
Set up the table at “O”, level it, and do centering.
Select a Point “O” on the sheet so that it is exactly over station “O” onthe
ground.
Mark the direction of the magnetic meridian
Centering the alidade on “O”, bisect the objects of traverse A, B, C, D
and E.
Measure the distances OA, OB, OC, OD and OE and plotted to
convenient
scale to locate a, b, c and d respectively
Join the points a, b, c, d and e on the paper.
2. Intersection method
In this method we can locate the point by plotting two rays from two known
stations. As shown in figure, P and Q are the known station. First the
equipment is placed on P and plot the lines by sighting the stations A, B and Q.
then shift the equipment to station Q and plot the lines by sighting stations A, B
and P. Finally, the intersection of A and B rays is the required location of point
of intersection.
Select two points P and Q such that the points (building corners) to be plotted arevisible
from their stations.
Set the table on P and locate on the sheet.
Pivot on P bisect Q draw a ray.
Measure the distance PQ and locate Q on the sheet to a convenient scale.
Now pq is known as the base line.
Pivot ‘p’ bisects the inaccessible objects A and B (building corners) and draw rays.
Shift the table to ‘a’ such that q is over Q and do temporary adjustments.
Place the alidade along qp and the rotate the table till p is bisected clamp table.
Pivot on q bisect the objects A and B and draw rays.
The instruction of rays drawn from P and Q will give the points a and b.
To check the accuracy measured AB and compare with plotted distance ab.
The same procedure is applied for other features of the campus. each point isbisected from
two stations.
3. Traversing method
This is used for running survey lines of a closed or open traverse. Traversing is the connection of
series of straight lines. In the case of traversing, the plane table is located at one point for
supposing A as shown below.
a. Select the traverse stations A,B,C,D,E etc on the ground.
b. Set the table on starting station ‘a’ and perform temporary adjustments.
c. Mark the magnetic meridian.
d. Locate A on the sheet as ‘a’.
e. Pivot on ‘a’ bisect the next station B and draw a ray
f. Measure the distance AB and locate ‘b’ on the sheet with a suitable scale.
g. Shift the table to next station B, set the table over B, and do temporary adjustments.
h. Place the alidade along ‘ba’ and bisect A for doing orientation of plane table.
i. Pivot on b bisect c draw a ray
j. Measure the distance BC and locate ‘c’ on the sheet with the suitable scale.
k. Repeat the same procedure at every successive station until the traverse is completed.
l. Finally, traverse lines are plotted on the drawing sheet
4. Resection
Resection is a method of plane table surveying in which the location of the plane table is unknown
and it is determined by sighting it to known points or plotted points. It is also called the method of
orientation and it can be conducted by two field conditions as follows.
The three-point problem
The two-point problem
Advantages of plane table survey
The sighting and plotting are done simultaneously hence there is no chance of missing
any detail
Irregular objects can also be plotted accurately as the lay of land is in view
It is the most rapid and useful for filling in details
The error and mistakes in plotting can be checked by drawing the check lines
No great skill is required
It is less costly in comparison to theodolite survey
It is advantages in areas, where compass survey is not reliable. Example: area affected
by magnetic field
Disadvantages of plane table survey
it is not suitable for, work in wet climate and in a densely wooded country
plane table is heavy and difficult to carry
it does not give very accurate result
it becomes difficult, if plan or map is to be plotted on some different scale due to the
absence of field notes
