Page
10
+30°
MAGNETIC
NORTH
= 360M
TRUE
NORTH
=
= 270 M
TRUE
NORTH
= 0-(-30)
= 30 M
-30°
N
S
E
W
N
S
E
W
+
-
PLUS
MINUS
Example : How to find true north given a compass
and the angle of magnetic declination
90
90
180
180
270
270
0-30
Step 18.
Find true north
It is best to set the wind direction sensor to true north, not magnetic north.
Compass Method
: Get a
compass and the local angle of
magnetic declination – either from
a map, local airport - or use an
online calculator e.g.
http://www.ngdc.noaa.gov/
geomag-web/#declination
...and enter your latitude and
longitude.
Magnetic declination (or variation)
is the angle between magnetic north and true north.
It is positive when the angle measured is east of true north and negative
when west. So, if the declination is positive, true north is left of magnetic
north.
Typical map of magnetic declination for year 2000
(reproduced courtesy of NOAA)
From this map you can see the magnetic variation around the world can be
quite large. Magnetic north also drifts over time, so it is best to refer wind
direction to true north.
Star Method
: Find the North Star or the Southern Cross.
Whichever method you use, establish where true north intersects the
horizon.
