Direction

The direction to any point in space can only True North

be given with reference to something in the
external environment [1]. Typically, one of

Reference line
three reference points are used: True north,
magnetic north, and grid north [2]. The ref-
rence point in true north is the geographic
north pole [3]. For magnetic north, it is the
magnetic north pole [2]. Lastly, for grid north
a local grid (which differs between countries)
is used [2].

e
D
An imaginary line that connects you, or the

lin
ire
point from where the direction is to be deter-

n
ct

io
io

ct
mined, is known as the reference line [1]. An

ire
lin
imaginary line drawn to the destination is the

D
e
direction line [1]. The direction of travel is then
the angle between the reference line and the
45°
destination line [1].
We will be using true north as our reference line.

O
A compasses contains a magnetized needle that 315°
points towards the magnetic north [3]. More tech-
nically, the needle aligns itself with the local mag-
netic field lines of its environment. This leads to
three problems.
First, the geographic and magnetic north poles are The bearing is the angle between a line connecting the observer with
not located at the same place. Thus, their reference an object (i.e. a direction line) and a given reference line [3]. In the ex-
lines are not the same. The angle that represents the ample above, the bearing from the observer (O) to the windmill is 315°
difference between these reference lines is the mag- while the bearing from the observer to the house is 45°. This type of
netic declination. bearing is called a forward bearing (i.e. the bearing from the observer
Second, the field lines differ across the surface of to the object) [3].
the globe. This means that magnetic declination
is not constant across the surface of the Earth.
True North True North
And, thirdly, the magnetic north pole changes po-
sition over time. This means that magnetic declina-
tion is not constant over time.

4°W
135°
225°
A 6°W
8°W
MN TN MN TN MN TN
10°W
12°W
14°W
B 16°W
18°W O
A B C The backward bearing is the bearing from the object towards the obser-
ver. For example, the backward bearing from the observer to the wind-
mill is the same as the forward bearing from the windmill to the obser-
C ver. In this case it is 135°. Similarly, the backward bearing from the ob-
server to the house is the same as the forward bearing from the obser-
ver which, in this case is 225°.
The map above shows a simplified version of the magnetic declination To calculate backward bearing remember LAMS (Less than Add, More
over South Africa as it was in 2020. At point A, the magnetic declination than subtract). If the forward bearing to an object is less than 180°,
is between 4°W and 6°W. At point B, the magnetic declination is be- then you add 180° to the forward bearing to obtain the backward
tween 10°W and 12°W. Finally, at point C, the magnetic declination is be- bearing. If the forward bearing to an object is more than 180°, then
tween 18°W and 20°W. you subtract 180° from the forward bearing to obtain the backward
(adapted from https://ngdc.noaa.gov/geomag/WMM) bearing.

TN
TN TN B B
Reference Line

A magnetic declination of A magnetic declination of

MN MN
e

45° west of North. In other 45° east of North. In other

Lin

words, the angle between a words, the angle between a

n
io

line connecting the observer line connecting the observer

ct

with true north (TN) and a line 45° 45° with true north (TN) and a line
re
Di

connecting the observer with connecting the observer with

magnetic north (MN) is 45°. magnetic north (MN) is 45°. 20
10 0 10
20
30
30
40
40
50

50
60

60
70

70
80

80
90

90

O O
80

80
70

70
60

60
50

50

40
40
30
30
20 20
10 0 10

MN west of TN, MN west of TN, MN east of TN, MN east of TN,

change is easterly: change is westerly: change is easterly: change is westerly: To determine the bearing from point O to point B draw the direction
SUBTRACT the to- ADD the total ADD the total SUBTRACT the to- and reference lines. The reference line should always be towards true
tal change from change from the change from the tal change from the north. Place a protractor at point O so that the 0° is aligned with the
the magnetic de- magnetic declina- magnetic declina- magnetic declina- reference line. Now the bearing to point B can be read off of the pro-
clination. tion. tion. tion. tractor. In this example, the bearing from point O to point B is 35°.

[1] Kimerling, A. J., Buckley, A. R., Muehrcke, P. C. and Muehrcke, J. O. (2016). Map use: Reading, analysis, interpretation, eighth edn, Esri Press, Ney York, NY.
[2] Monkhouse, F. and Wilkinson, H. (1974). Maps and diagrams: Their compilation and construction, Methuen & Co. Ltd., London
[3] Singh, G. (2007). Map work and practical geography, fourth enlarged edn, Vikas Publishing House, New Dehli.