The direction in which the compass needle

points is known as Magnetic North, and the

angle between Magnetic North and the True
North direction is called magnetic declination
or variation TN MN
magnetic declination
 Magnetic declination at a place is the
horizontal angle between the true meridian
and the magnetic meridian
 If the magnetic meridian is to the right side
of the true meridian, declination is said to MM
the eastern or positive. TM
Declination East (δ E) δ
 If the magnetic meridian is to the left side of
the true meridian, declination is said to the
western or negative TM
 Declination West (δ W) MM

δ
 . Isogonic lines are lines on the Earth's surface
along which the declination has the same
constant value
 Lines along which the declination is zero are
called agonic lines.
The magnetic declination at any place is not
constant

Direction of magnetic meridian at a place changes

with time. Hence the value of Magnetic bearing
of a line is not constant

Magnetic bearing of a line – horizontal angle

between the magnetic meridian(MN) and survey
line.
 True bearing of a line – horizontal angle
between the true meridian(TN) and survey
line. Direction of True meridian is fixed hence
True bearing of a line is constant

 If magnetic declination at a place at the time

of observation is known , True bearing can be
calculated from the observed Magnetic
bearing
 True bearing = Magnetic bearing + or -
declination
 TB = MB + or – δ
Use + sign if δ is to East
Use - sign if δ is to West
Determination of True bearing
 If the magnetic bearing of a line OA is 197° and
the magnetic meridian is deflected 3° towards east of
the true meridian
true bearing of a line=magnetic bearing +or- declination
 TB=MB +δ
 TB = 197° + 3° = 200° (Ans.) TN MN
δ= 3°

A MB = 197°
If the magnetic bearing of a line OB is N 37° W
and the magnetic declination is 2° E find the
true bearing.
TN MN
TB = MB + or – δ
convert QB into WCB B N37 W
N 37° W = 360-37°= 323° o
TB = MB + δ
= 323°+ 2°
= 325°
= N 35° W
The magnetic declination at any place is not
constant as the magnetic needle does not
constantly point in the same direction
Variation in magnetic declination are refers to
changes in the Earth's magnetic field on time
scales. They are
1. Secular variation
2. Annual variation
3. Diurnal variation
4. Irregular variation
 It is a slow and gradual process
 Magnetic meridian swings in one direction
for a long time (100 to 200 years) and
gradually comes to rest and then swings in
the opposite direction. The change
produced annually bysecular variation
amounts in different places from 0 to ± 12
minutes but does not remain constant at any
place.
 Yearly change in the variation of
earth's magnetic field at a specified location.
 It has a period of 1 year.
 Its value may be 1’ or 2’
 Diurnal Variation: It is an oscillation of' the
needle from its mean position during the day.
The amount of this variation varies from 1
minute to about 12 minutes at different
places.
 This is more in magnetic poles and less in
equator, more in day and less in night and
more in summer than in the winter at the
same place.
 Irregular variations are due to magnetic
storm, flood, earthquakes, and other solar
influences
 As the name suggests, these variations are of
irregular and indefinite pattern totally
unpredictable. Its value may be 1° or 2°
 Due to magnetic influence of earth compass
needle is inclined downwards towards the
pole. This inclination of the compass needle
with the horizontal is called dip of the needle

horizontal plane
θ –angle f dip

magnetic needle (North end)

 Angle made by the earth magnetic field lines
with the horizontal plane is called magnetic
dip
 Value of dip varies from place to places
 The angle of dip varies from o ° at equator
and 90 ° at the poles 9o °

o° o°o°

9o °
 The lines of force around the magnetic
equator of the Earth are perfectly horizontal.
So the magnetic needle will become
horizontal there. Thus, the angle of dip at
the magnetic equator of the Earth will be 0
degrees
 At poles angle of dip will be 90∘ because
earth's magnetic field will be almost vertical
there
 Lines joining locus of points having same
value of dip are called isoclinic lines.
 The locus of the points having zero dip is
called aclinic line such as the magnetic
equator
 Advantages
 They are portable and light weight.
 They have fewer settings to fix it on a station
 The error in direction produced in a single
survey line does not affect other lines.
 It is suitable to retrace old surveys.
 Disadvantages
 It is less precise compared to other advanced
methods of surveying.
 It is easily subjected to various errors such as
errors adjoining to magnetic meridian, local
attraction etc.
 Imperfect sighting of the ranging rods and
inaccurate leveling also causes error.
 Errors can be arising due to various reasons
during the process of surveying, they are
classified as:
 Instrumental errors
 Personal errors
 Natural errors
 As the name suggests they are arise due to
the wrong adjustments of the instruments.
Some other reasons are:
 If the plane of sight not being vertical, it
causes error in sighting and reading.
 If the magnetic needle is not perfectly
straight or if it is sluggish, readings may not
be accurate.
 They arise mainly due to the carelessness of
the surveyor. They are:
 Inaccurate leveling
 Inaccurate reading
 Inaccurate centering
 Natural errors are occurring due to the various
natural causes which affect the working of
compass. It has nothing to do with the surveyor
and to minimize them, some corrections in
calculations applied. They are:
 Local attraction
 Proximity to the magnetic storms
 declination