Vaisala WS425 - Manual (Page 27)

Chapter 3 _______________________________________________________________ Installation
VAISALA _______________________________________________________________________ 27
Alignment
Magnetic Declination Correction
One transducer arm is permanently marked with an N for north and
another with an S for south.
NOTE Aligning might be easier if you mark the sensor body, for example, with
paint or colored tape, to indicate north and south so that it can be seen
from the ground.
Wind direction can be referenced to either the true north, which uses the
earth’s geographic meridians, or to the magnetic north, which is read
with a magnetic compass. The magnetic declination is the difference in
degrees between the true north and magnetic north. See Figure 4 below.
0003-011
Figure 4 Sketch of Magnetic Declination
NOTE The source for the magnetic declination must be current as the
declination changes over time.