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A compass consist of
A magnetic needle
A graduated circle
The line of site
Box house the above
The two forms of compass that all used commonly for
Difference between prismatic compass & surveyor's compass.
Item Prismatic compass Surveyor's compass
Graduated card The graduation circle is fixed to broad needle.
It does not rotate with line of sight.
The graduation circle is fixed to the box
rotates with line of sight
There is a prism at viewing end. No prism. Only slit
The graduations are in WCB system. The graduation are in Q.B system.
The graduations are marked inverted. The graduations are marked directly.
Magnetic Needle Magnetic needle does not act as index. Magnetic needle acts as index.
Tripod Tripod may not be provided, the instrument can
be used even by holding suitably in hand
The instrument can’t be used without
Reading The reading is taken with the help of a prism
provided at the eye slit
Taken by directly seeing through the top of
Compass traversing: Important Definition
True meridian: Line or plane passing through
geographical north pole and geographical south pole
Magnetic meridian: When the magnetic needle is
suspended freely and balanced properly, unaffected by
magnetic substances, it indicates a direction. This
direction is known as magnetic meridian. The angle
between the magnetic meridian and a line is known as
magnetic bearing or simple bearing of the line.
Designation of magnetic bearing
◦ Whole circle bearing (WCB)
◦ Quadrantal bearing (QB)
WCB: The magnetic bearing of a line measured clockwise from the North Pole towards the
line is known as WCB. Varies 0-360°
Quadrantal Bearing: The magnetic bearing of a line measured clockwise or anticlockwise
from NP or SP (whichever is nearer to the line) towards the east or west is known as QB.
This system consists of 4-quadrants NE, SE, NW, SW. The values lie between 0-90°
◦ QB of OA = N a E
Reduced Bearing: When the whole circle bearing of a line is
converted to quadrantal bearing it is termed as reduced bearing.
Fore and Back Bearing:
◦ In WCB the difference between FB and BB should be exactly
◦ Use the +ve sign when FB<180°
◦ Use the –ve sign when FB> 180°
Magnetic declination: The horizontal angle between the magnetic meridian and true meridian is known as
Dip of the magnetic needle: If the needle is perfectly balanced before magnetisation, it does not remain in
the balanced position after it is magnetised. This is due to the magnetic influence of the earth. The needle is
found to be inclined towards the pole. This inclination of the needle with the horizontal is known as dip of
the magnetic needle.
Method of correction for traverse:
◦ First method: Sum of the interior angle should be equal to (2n-4) x 90. if not than distribute the total
error equally to all interior angles of the traverse. Then starting from unaffected line the bearings of all the
lines are corrected using corrected interior angles.
◦ Second method: Unaffected line is first detected. Then, commencing from the unaffected line, the bearing
of other affected lines are corrected by finding the amount of correction at each station.
Plane Table Surveying is a graphical method of survey in which the field
observations and plotting are done simultaneously.
It is simple and cheaper, 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
The following instruments are used in plane table surveying.
U-Fork with Plumb bob
Water proof cover
DISADVANTAGES OF PLANE
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.
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.
Methods Of Plane Tabling
There are four distinct methods of plane tabling:
Method of Radiation
Method of Intersection
Method of Traversing
Method of Resection
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
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
Error In Plane Tabling
The various sources of error may be classified as :
Errors in manipulation and sighting
Errors in plotting
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
Movement of Board between sights
Errors in Plotting
Defective scale of map
Wrongly intersecting the rays drawn from two different