Campbell Scientific CSAT3 3-D Sonic Anemometer - Manual (Page 56)

Appendix A. CSAT3 Orientation
Declination angles are always subtracted from the compass reading to find
True North. A declination angle East of True North is reported as positive a
value and is subtracted from 360 (0) degrees to find True North as shown
FIGURE A-2. A declination angle West of True North is reported as a negative
value and is also subtracted from 0 (360) degrees to find True North as shown
in FIGURE A-3. Note that when a negative number is subtracted from a
positive number, the resulting arithmetic operation is addition.
For example, the declination for Longmont, CO (10 June 2006) is 9.67°, thus
True North is 360° - 9.67°, or 350.33° as read on a compass. Likewise, the
declination for Mc Henry, IL (10 June 2006) is -2.68°, and True North is
0° - (-2.68°), or 2.68° as read on a compass.
FIGURE A-2. A declination angle East of True North (positive) is
subtracted from 360 (0) degrees to find True North
FIGURE A-3. A declination angle West of True North (negative) is
subtracted from 0 (360) degrees to find True North
A-2