SECTION 1. PREPARATION AND SITING
1-5
1.4 DETERMINING TRUE NORTH FOR
WIND VANE ORIENTATION
Magnetic declination, or other methods to find True
North, should be determined prior to installing the
weather station. True North is usually found by
reading a magnetic compass and applying the
correction for magnetic declination*; where
magnetic declination is the number of degrees
between True North and Magnetic North. Magnetic
declination for a specific site can be obtained from a
USFA map, local airport, or through an internet
service called NSSDC CGM (Section 1.4.1). A
general map showing magnetic declination for the
contiguous United States is shown in Figure 1.4-1.
Declination angles east of True North are
considered negative, and are subtracted from 0
degrees to get True North as shown Figure 1.4-2.
Declination angles west of True North are con-
sidered positive, and are added to 0 degrees to get
True North as shown in Figure 1.4-3. For
example, the declination for Logan, Utah is 14.5
°
East. True North is 360
°
- 14.5
°
, or 345.5
°
as read
on a compass.
*
Other methods employ observations using
the North Star or the sun, and are
discussed in the Quality Assurance
Handbook for Air Pollution Measurement
Systems, Volume IV - Meteorological
Measurements
4
.
Subtract declination from 360
°
Add declination to 0
°
22 E
18 E
16 E
14 E
12 E
10 E
8 E
6 E
4 E
2 E
0
2 W
4 W
6 W
8 W
10 W
14 W
12 W
16 W
18 W
20 W
20 E
FIGURE 1.4-1. Magnetic Declination for the Contiguous United States
