Compass Surveying Notes

Compass surveying
Compass is an instrument which can be used to

measure the direction of a survey line with respect to
magnetic north-south. The magnetic north-south
direction which is the reference direction is called
meridian (reference direction) and the angle between
the line and the meridian is called bearing.Use of
compass for measuring direction of a line simplifies the
surveying to a great extent.
Types of Compass
The types of compass that are used commonly are:
(i) Prismatic compass
(ii) Surveyors compass.
The essential parts of both types are:
(i) a magnetic needle
(ii) a graduated circle
(iii) a line of sight
(iv) a box to house them.
Surveyors Compass
In this type of compass graduation disc is fixed to the
box and magnetic needle is free to rotate above it.
There is no prism provided at viewing end, but has a
narrow slit. After fixing the line of sight, the reading is
directly taken from the top of the glass cover. Hence,
graduations are written directly (not inverted). In this
compass graduations are from zero to 90°, zero being
to north or south and 90° being to east and west. An
angle of 20° to north direction to the east is written as
N 20° E, and an angle of 40° to east from south is
written as S 40° E. Always first direction indicated is
north or south and the last letter indicates east or west
direction. In this system graduated circle rotates with
line of sight and magnetic needle is always towards
north. The reading is taken at the tip of needle. Hence,
on the compass east and west are marked
interchanged and marked‘
Photograph of a surveyors compass.
Method of using a compass

To take a reading from a compass, the following
temporary adjustments are required:
(i) Centring: The compass should be fixed to the stand
and set over the station or hand held. To centre the
compass legs of the tripod stand should be moved
inward-outward or in a circumferential direction. To
check centring plumb may be used or a pebble/stone
dropped from the centre of the compass.
(ii) Levelling: In compass survey perfect levelling is not
necessary, but it should be sufficient to permit free
suspension of magnetic needle. For checking levelling a
bubble level is provided in many compasses. After
centring bubble should be ensured in the middle of the
circle provided for it in the level. If it is not within that
circle, circumferential movements may be provided to
the legs of tripod so that without disturbing centring
the levelling is achieved.
(iii) Focussing the prism: In prismatic compass, to focus
the prism on graduated circle, its attachment is slide up
or down till the readings are clearly visible. There is no
such requirement in surveyors compass.
The following steps are required for observing bearing
of a line, say, AB:
(i) Centre the compass over A.
(ii) Level the compass.
(iii) Focus the prism, if prismatic compass is used.
(iv) Rotate the box till ranging rod at B is sighted
through the line of sight.
(v) Bring/Allow the needle to rest using knob.
(vi) Take the reading and note it in the field book.
Care should be taken to see that the line of sight is not
disturbed between the time of sighting the object and
the time of reading the bearing.
Bearing
A bearing of a line is the angle made by the line with
respect to a reference direction, the reference
direction being known as meridian. The direction
shown by a freely suspended and properly balanced
magnetic needle is called magnetic meridian and the
horizontal angle made by a line with this meridian is
known as magnetic bearing.
The points of intersection of earth’s axis with surface
of the earth are known as geographic north and South
Pole. The line passing through geographic north, south
and the point on earth is called true meridian at that
point and the angle made by a line passing through
that point is called true bearing.
While traversing along lineA B, the bearing of line AB is
called forward bearing of ABand the bearing of BA is
called back bearing. Fore bearing and back bearing
differ by 180°.
Whole circle Bearing
In whole circle bearing (WCB) the bearing of a line at
any point is measured with respect to a meridian. Its
value varies from zero to 360°, increasing in clockwise
direction. Zero is north direction, 90° is east, 180° is
south and 270° is west. This type of bearing is used in
prismatic compass.
The magnetic meridian and the true meridian may not
coincide with each other in a place. The horizontal
angle between these two meridians is known as
magnetic declination. The magnetic north at a place
may be towards east or west of true north.If it is
towards east, it is known as eastern or +ve declination.
Western declination is known as –ve declination as
indicated on the diagrams below. Eastern declination is
to be added to observed magnetic bearings to get true
meridian. To find magnetic declination at a point true
meridian should be established from astronomical
observations and magnetic meridian by a compass.
Maps are made with respect to true meridian.
Magnetic declination varies from time to time and also
from place to place. In the noon sun is exactly on the
geographical meridian. In India, ‘Survey of India’
department conducts astronomical survey and
publishes Isogonic Charts from which magnetic
declinations at any point can be found.
The lines joining the points at which declination is the
same at the given time are called ‘Isogonic Lines’. Lines
joining points of zero declinations are called ‘Agonic
Lines’. The isogonic lines are quite irregular near
geographic poles. The isogonic charts show lines of
equal annual change in declination.
The following types of variations are observed in
declination:
(i) Secular variation,
(ii) Annual variation,
(iii) Daily variations, and
(iv) Irregular variations.
Secular Variation
The magnetic meridian swings like a pendulum to the
left and to the right of true meridian. Its period of
variation is approximately 250 years.
Annual Variation
It is observed that in a year declination varies from 1 0
to 20 .
Daily Variation
The daily variation of magnetic declination is as much
as 100. This variation is also known as ‘Diurnal
Variation’. The following factors influence its
magnitude:
(a) It is more in day and less in night.
(b) It is more in summer and less in winter.
(c) The amount of variation changes from year to year.
(d) It is more near magnetic poles and less near
equator.
Irregular Variation
Due to earthquakes and volcanic eruptions, magnetic
storms occur, resulting into changes in magnetic
meridian. Such changes are from 1° to 2°.
Local Attraction
A freely suspended and properly balanced magnetic
needle is expected to show magnetic meridian.
However, local objects like electric wires and objects of
steel attract magnetic needle towards themselves.
Thus, needle is forced to show slightly different
direction. This disturbance is called local attraction.
The list of materials which cause local attraction are:
(i) Magnetic rock or iron ore
(ii) Steel structures, iron poles, rails, electric poles and
wires
(iii) Iron buttons, steel rimmed spectacles,chain,arrows,
hammer, clearing axe etc.
Surveyor is expected to take care to avoid local
attractions listed in (iii).
Detecting Local Attraction
For detecting local attraction it is necessary to take
both fore bearing and back bearing for each line. If the
difference is exactly 180°, there is no effect of local
attraction on both points. If difference is not 180°,
better to go back to the previous station and check the
forward bearing. If that reading is same as earlier, it
may be concluded that there is local attraction at one
or both stations.
Correcting Observed Bearings
If local attraction is detected in a compass survey
observed bearings may be corrected/adjusted as
follows.
Method:
In this method, errors due to local attraction at each of
the affected station are found starting from the
bearing of station unaffected by local attraction. The
bearing of the successive lines are adjusted
accordingly.
Exercise
Give a class example.
A practical on compass traversing to be given.

Compass Surveying Notes

Uploaded by

Copyright:

Available Formats

Compass Surveying Notes

Uploaded by

Document Information

Original Description:

Original Title

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Compass Surveying Notes

Uploaded by

Copyright:

Available Formats

Compass surveying

Compass is an instrument which can be used to

Method of using a compass

You might also like