1. COMPASS SURVEYING
Arun Ravi. K. R
Instructor, Draughtsman (Civil)
PART - I

3. LEARNING
OBJECTIVES
Compass surveying
The prismatic compass
Surveyors Compass
Setting of the Prismatic
Compass

4. Compass surveying
Compass surveying is a type
of surveying in which the directions
of survey lines are determined with a
magnetic compass,
The length of the surveying lines are
measured with a tape or chain.
The compass is generally used to
run a traverse line.

5. Principal of Compass surveying
A freely suspended magnetic needle always lies in North- South
direction

6. Uses of Compass surveying
It is used where speed is more important than
accuracy, hence it is used for preliminary survey
works such as road, railway, military etc.
Accuracy of readings depends on Local attraction
of that site

7. There are two forms of compass in common use
1. The prismatic compass
2. The surveying or surveyor’s compass.
Types of Compass

8. The prismatic compass
The prismatic compass consists of cylindrical metal box of
8cm to 12 cm diameter.
METAL BOX

9. The prismatic compass
In the centre of the compass consists of a pivot
which carrying a magnetic needle. Magnetic needle
attached to the graduated aluminium ring with the
help of an agate cap
PIVOT
AGATE CAP
MAGNATIC NEEDLE
GRADUATED RING

10. The ring is graduated to half a degree and is ready
by a reflecting prism. prism cap protect the prism from
dust, moisture etc. The opposite side to the prism is the
object vane and it carrying a horse hair, with which an
object is bisected. The eye is applied through eye vane.
PRISM CAP
PRISM
OBJECT VANE
EYE VANE
PRISM

11. The angular readings can be observed directly
by the eye through the prism. The readings can be
made clearly by adjusting the prism to the eye sight
by the focusing screw
PRISM
OBJECT VANE
EYE VANE
FOCUSING SCREW

12. The prismatic compass is protected by a glass
cover. The needle is lifting buy using lifting lever by
pressing lifting pin. The oscillations of the needle
stop by using brake pin
GLASS COVER
LIFTING LEVER
LIFTING PIN
BREAK PIN

13. Reflecting mirror helps to take the
bearings of very high (or) very low objects. A pair
of sun glasses helps to take readings during
heavy sun glare.
SUN GLASS
REFLECTING MIRROR
A metal cover fits over the glass cover as well as the object
vane when the compass is not in use.

14. The compass is placed over a tripoded

15. In the Prismatic compass, graduations are
marked on the ring in a clockwise direction with 0°or
360° at south end of the needle. 90° is marked at the
West 180° at the North and 270° at the East. The
figures are written upside down

16. The greatest advantages of prismatic compass
is, both sighting the object as well as reading
circle can be done simultaneously without
changing the position of the eyes.

17. The bearing shows 330° at the observer’s end
under the prism (ie at the south end)

18. Surveyors Compass
 The graduated ring is directly
attached to the circular box and not
with the magnetic needle.
 The magnetic needle floats freely
over the pivot.
 No prism is attached to the eye
vane and it is having a narrow
vertical slit.
It is similar to prismatic compass except with a following
modifications.

19.  Readings are taken directly
with naked eye against the
north end of the needle.
 The angle is graduated in
quadranted system of having
• 0° at north and south ends
• 90° at East and west ends.
Surveyors Compass

20. The prismatic compass is required to
be temporarily set over the station at
which the bearing of survey line required
to be measured. It is basically a two-step
procedure.
1. Centering
2. Levelling
This is also called the temporary
adjustments of compass.
Setting of the Prismatic Compass

21. The compass is set so that its centre lies exactly above
the station by suspending a plumb bob from. If the conical
end of plumb bob lie exactly over the station, the compass
is considered to be exactly centered. If not, the legs of the
tripod are adjusted in position by moving one leg first and
then simultaneously moving other two legs in
perpendicular direction to first movement.
If the plumb bob is not available, a small piece of stone
or pebble can be used for centering.
1. Centering

22. 2. Levelling
The compass is required to be
levelled so that the aluminium ring
is in horizontal plane. The levelling
can be checked by a spirit level or by
rolling a pin on compass box. If not,
the level can be adjusted by moving
the legs of tripod.

23. ARUN RAVI.K.R
Instructor, Draughtsman (Civil)