COMPASS
Réf. 52017920
TRUE NORTH (GEOGRAPHIC), MAGNETIC NORTH AND DECLINATION
TO MAINTAIN THE DIRECTION OF AN OBJECT
TO REACH THE DESTINATION SHOWN ON THE MAP
FINDING YOUR POSITION ON THE MAP
We tend to use the words «north», «south», etc. rather informally, without considering
that there are two kinds of north, more specifically true north and magnetic north.
True north is determined geographically and is common throughout the world.
The circles of longitude on a map are drawn in correlation with geographic poles,
and lines indicate the direction of true north. The direction of these lines differs
from the direction in which the magnetic needle points. On a regular map, the top
is true north and the bottom is true south. Accordingly, the right side and the left
side represent east and west respectively.
The magnetic poles are close to the geographic poles but
they do not coincide. The angle formed by the straight lines
pointing from the place of observation to the geographic
and magnetic north poles is called declination, figure 1.
Declination varies from one point of the globe to another.
The lines of identical declination are very irregular; however,
topographic maps of any location provide the local declination.
When talking about a direction of so many degrees, you have
to either subtract or add the local declination, based on your
position relative to the zero declination line, to calculate the
correct geographic angle of the direction. To get the geographic
bearing, take the bearing with a compass and, if the local
declination is to the west, subtract the declination from the
bearing; if, on the contrary, it is in the east, add it to the bearing.
However, it is more convenient for you to use the bearing
obtained, once you have set the magnetic north to 0 °.
To get to the given point on the map, simply determine your position, the direction
of the location and the magnetic north.
1. First lay out the map and draw the directional lines
of magnetic north according to the declination given
on the map. Draw these lines using the sides of the
compass. If, for example, the declination of the location
is 5° to the west, subtract 5° from 360° and bring the
355° graduation to face the index line. Make sure the
arrows of the compass base plate are parallel with
the north-outh line of the map (line of longitude or
vertical lines of the map grid), without moving the
rotating bezel; draw a line as shown in Figure 3. It is
helpful to draw several lines parallel to the magnetic
north that you have just drawn using the coordinate
lines.
2. Determine your position and destination on the map
then connect these two points with a straight line on
the map. Place your compass on the line so that the
index line (with the arrow) points to the destination.
Then turn the bezel so that the arrows on the compass
base plate are parallel with the magnetic north lines
drawn according to the instructions above. (No need
to worry about the direction of the magnetic needle).
Once you’ve done that, hold the compass and slowly
turn it until the north (illuminated) end of the magnetic
needle is parallel with the arrows on the compass base
plate, figure 4. Now proceed in the direction shown by
the arrow, making sure that the magnetic needle remains
parallel with the arrows on the base plate. Continue in
this direction until you reach your destination. Once
you arrive at the first destination, repeat the same
steps to get to your final destination. As you move
towards your goal, you can make sure you are on the
right path by checking your compass and walking in a
straight direction, as far as possible. If the declination
to the right or left of the exact route is large, there is a
higher risk of error at the final destination.
Say, for example, you can see a lake. If you head to this lake and never lose sight
of it, you’ll be fine. However, if you have to cross woods, valleys or the like, from
which you can no longer see the lake, you may lose your way and that’s when
you’ll really appreciate having a compass.
1. Before leaving, keep your compass as
horizontal as possible and point the arrow
on the scale towards the lake.
2. Turn the rotating bezel so that the N (0 °)
points in the direction of the N (illuminated)
end of the magnetic needle.
3. Read the position at the index line. It identifies
the lake; keep it until you reach a place where
you can clearly see the lake again.
In Figure 2, for example, the direction of the object points to 320° (to find the exact
direction, you need to take the declination into consideration, as explained above. As
long as you understand what declination is, however, you can tell the object is 320°
from the magnetic north).
Climb to a height from where you can see two prominent features of the surrounding
landscape which are shown on the map. Let’s imagine you can see a mountain A on
the left and a lake B on the right. Point the arrow of the index line towards mountain
A. Then turn the rotating bezel to align the «N» on the dial with the place where the
north end of the compass needle points. Place the compass on the map and
change the position of the map so that the magnetic north line on the map is
parallel with the magnetic needle and the NS lines on the base plate. Without changing
anything, drag the compass along the map until one side of the scale points towards
mountain A, then draw a line A, figure 5. Then turn to lake B and repeat the previous
step; draw the line. The point where these two straight lines intersect indicates your
position on the map, figure 6.
There are many books on how to use a compass to find your way around. We recommend that you read them or sign up for a course to improve your understanding of how to use these instruments and maps.
N
N
355
。
N
N
N
Figure 1
Figure 3
Figure 4
Figure 5
Figure 6
Magnetic
north
Magnetic
north
Object
Magnetic
north
Magnetic
north
Circles of longitude
Current
position
Walking
direction
Rotating
bezel
Mountain A
Mountain A
Lake B
Lake B
Magnetic
north
Magnetic
north
Current position
Destination
Geographic
north
Geographic
north
Circles of
longitude
Declination
Figure 2
Nature & Découvertes
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