Compass surveying

INTRODUCTION
•Chain surveying can be used when the area to be
surveyed is comparatively is small and is fairly flat.
•But when the area is large , undulating and crowded
with many details , triangulation(which is the
principle of chain survey) is not possible.
•In such an area , the method of surveying is used.

Principle of compass surveying
•The principle of compass surveying is traversing;
which involves a series of connected lines.
•The magnetic bearing of the lines are measured by
prismatic compass.
•Compass surveying is recommended when the area
is large, undulating and crowded with many details.
•Compass surveying is not recommended for areas
where local attraction is suspected due to the
presence of magnetic substances like steel
structures, iron ore deposits, electric cables , and so
on.

Traversing
•In traversing , the frame work consist of connected
lines.
•The length are measured by a chain or a tape and the
direction measured by angle measuring instruments.
•Hence in compass surveying direction of survey
lines are determined with a compass and the length
of the lines are measured with a tape or a chain. This
process is known as compass traversing.

Compass
•A compass is a small instrument essentially
consisting of a graduated circle, and a line of sight.
•The compass can not measures angle between two
lines directly but can measure angle of a line with
reference to magnetic meridian at the instrument
station point is called magnetic bearing of a line.

Types of compass
• There are two types of compass they are as follows:-
• The prismatic compass

Elements of prismatic compass
• Cylindrical metal box: Cylindrical metal box is having diameter of 8to 12 cm. It
protects the compass and forms entire casing or body of the compass. It protect
compass from dust, rain etc.
• Pivot: pivot is provided at the center of the compass and supports freely suspended
magnetic needle over it.
• lifting pin and lifting lever: a lifting pin is provided just below the sight vane. When
the sight vane is folded, it presses the lifting pin. The lifting pin with the help of lifting
lever then lifts the magnetic needle out of pivot point to prevent damage to the pivot
head.
• Magnetic needle: Magnetic needle is the heart of the instrument. This needle measures
angle of a line from magnetic meridian as the needle always remains pointed towards
north south pole at two ends of the needle when freely suspended on any support.
• Graduated circle or ring: This is an aluminum graduated ring marked with 0ᴼ to
360ᴼ to measures all possible bearings of lines, and attached with the magnetic needle.
The ring is graduated to half a degree.
• Prism : prism is used to read graduations on ring and to take exact reading by
compass. It is placed exactly opposite to object vane. The prism hole is protected by
prism cap to protect it from dust and moisture.

• Object vane: object vane is diametrically opposite to the prism
and eye vane. The object vane is carrying a horse hair or black
thin wire to sight object in line with eye sight.
• Eye vane: Eye vane is a fine slit provided with the eye hole at
bottom to bisect the object from slit.
• Glass cover: its covers the instrument box from the top such
that needle and graduated ring is seen from the top.
• Sun glasses: These are used when some luminous objects are to
be bisected.
• Reflecting mirror: It is used to get image of an object located
above or below the instrument level while bisection. It is placed
on the object vane.
• Spring brake or brake pin: to damp the oscillation of the
needle before taking a reading and to bring it to rest quickly, the
light spring brake attached to the box is brought in contact with
the edge of the ring by gently pressing inward the brake pin

Temporary adjustment of prismatic compass
• The following procedure should be adopted after fixing the prismatic compass
on the tripod for measuring the bearing of a line.
• Centering : Centering is the operation in which compass is kept exactly over
the station from where the bearing is to be determined. The centering is
checked by dropping a small pebble from the underside of the compass. If the
pebble falls on the top of the peg then the centering is correct, if not then the
centering is corrected by adjusting the legs of the tripod.
• Leveling : Leveling of the compass is done with the aim to freely swing the
graduated circular ring of the prismatic compass. The ball and socket
arrangement on the tripod will help to achieve a proper level of the compass.
This can be checked by rolling round pencil on glass cover.
• Focusing : the prism is moved up or down in its slide till the graduations on
the aluminum ring are seen clear, sharp and perfect focus. The position of the
prism will depend upon the vision of the observer.

Observing Bearing of Line
• Consider a line AB of which
the magnetic bearing is to be
taken.
• By fixing the ranging rod at
station B we get the
magnetic bearing of needle
wrt north pole.
• The enlarged portion gives
actual pattern of graduations
marked on ring.
NORTH
OBJECT B
A
SOUTH
LINE OF
SIGHT
90
180
270
0

The Surveyor`s Compass
•It is similar to a prismatic compass except that it has
a only plain eye slit instead of eye slit with prism
and eye hole.
•This compass is having pointed needle in place of
broad form needle as in case of prismatic compass.

BEARINGS
• The bearing of a line is the horizontal angle which it makes with
a reference line(meridian).
• Depending upon the meridian , there are four type of bearings
they are as follows:
• 1) True Bearing: The true bearing of a line is the horizontal
angle between the true meridian and the survey line. The true
bearing is measured from the true north in the clockwise
direction.
• 2) Magnetic Bearing: the magnetic bearing of a line is the
horizontal angle which the line makes with the magnetic north.
• 3) Grid Bearing: The grid bearing of a line is the horizontal
angle which the line makes with the grid meridian.
• 4) Arbitrary Bearing: The arbitrary baring of a line is the
horizontal angle which the line makes with the arbitrary
meridian.

Working of Surveyor`s Compass
• 1) Centering
• 2) LEVELING
• 3) OBSERVING THE BEARING OF A LINE
• First two observation are same as prismatic compass but third
observation differs from that.
• 3) OBSERVING THE BEARING OF A LINE : in this
compass ,the reading is taken from the top of glass and under
the tip of north end of the magnetic needle directly. No prism
is provided here.

BEARINGS
TRUE
MERIDIAN
MAGNETIC
MERIDIAN
TRUE BEARING
MAGNETIC
BEARING
A
B
MN
TN

Designation of bearing
• The bearing are designated in the following two system:-
• 1) Whole Circle Bearing System.(W.C.B)
• 2) Quadrantal Bearing System.(Q.B)

Whole circle bearing system(W.C.B.)
• The bearing of a line measured with respect to magnetic
meridian in clockwise direction is called magnetic bearing
and its value varies between 0ᴼ to 360ᴼ.
• The quadrant start from north an progress in a clockwise
direction as the first quadrant is 0ᴼ to 90ᴼ in clockwise
direction , 2nd 90ᴼ to 180ᴼ , 3rd 180ᴼ to 270ᴼ, and up to 360ᴼ
is 4th one.

Quadrantal bearing system(Q.B.)
• In this system, the bearing of survey lines are measured wrt
to north line or south line which ever is the nearest to the
given survey line and either in clockwise direction or in anti
clockwise direction.

Reduced bearing (R.B)
• When the whole circle bearing is converted into Quadrantal
bearing , it is termed as “REDUCED BEARING”.
• Thus , the reduced bearing is similar to the Quadrantal
bearing.
• Its values lies between 0ᴼ to 90ᴼ, but the quadrant should be
mentioned for proper designation.

• The following table should be remembered for conversion of WCB to RB.
W.C.B OF ANY
LINE
QUADRANT IN
WHICH IT LIES
RULES FOR
CONVERSION
QUADRANT
0 TO 90 I RB=WCB N-E
90 TO 180 II RB=180-WCB S-E
180 TO 270 III RB =WCB-180ᴼ S-W
270 TO 360 IV RB=360ᴼ - WCB N-W

Fore bearing and Back bearing
• The bearing of a line measured in the forward direction of the
survey lines is called the ‘fore bearing’(F.B.) of that line.
• The bearing of a line measured in direction backward to the
direction of the progress of survey is called the ‘back
bearing’(B.B.) of the line.

FB of line AB
BB of line AB
A
NORTH
NORTH
Θ1 Θ2
B
FB of AB = Θ1(from A to B)
BB of AB= Θ2(from B to A)
Remembering following points:
1) In the WCB system ,the differences
b/n the FB and BB should be exactly
180ᴼ. Remember the following
relation :
BB=FB+/-180ᴼ
+ is applied when FB is <180ᴼ
- is applied when BB is >180ᴼ
2) In the reduced bearing system the FB
and BB are numerically equal but the
quadrants are just opposite.

Computation of Angles
• Observing the bearing of the lines of a closed
traverse, it is possible to calculate the included
angles, which can be used for plotting the traverse.
• At the station where two survey lines meet, two
angles are formed-an exterior angles and an
interior angles. The interior angles or included
angle is generally the smaller angles(<180ᴼ).
A
B
C
D

EXAMPLES
A
E
B
F
B
BB
/_A
INCLUDED
ANGLE
AT STATION A,FB of AB and BB of EB were
measured. Difference of these two bearing
give interior angle A.
Angle A =FB OF AB – BB OF EA
EXTERIOR ANGLE
B
B
B
B FB
A
C
M
AT STATION B ,FB of BB OF AB were
measured . Difference of these two bearing
will give you exterior angle B.
EXTERIOR ANGLE B=BB OF AB-FB OF
BC
INTERIOR ANGLE B=360ᴼ-EXTERIOR
ANGLE

Meridian
• Bearing of a line is always measured clockwise wrt some reference line
or direction. This fixed line is known as meridian.
• There three types of meridian:
• 1) Magnetic meridian: The direction shown by a freely suspended
needle which is magnetized and balanced properly without influenced by
any other factors is known as magnetic meridian.
• 2) True meridian : True meridian is the line which passes through the
true north and south. The direction of true meridian at any point can be
determined by either observing the bearing of the sun at 12 noon or by
sun’s shadow.
• 3) Arbitrary meridian: In case of small works or in places where true
meridian or magnetic meridian cannot be determined, then ,any direction
of a prominent object is taken as a reference direction called as arbitrary
meridian.

Compass surveying

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