G-1
APPENDIX G. LOCAL MAGNETIC DECLINATION
G.1 DETERMINING TRUE NORTH
Orientation of the antenna is done after the
location of True North has been found.
1.
Establish a reference point on the horizon
for True North (or other direction relative to
True North). True North is usually found by
reading a magnetic compass and applying
the correction for magnetic declination as
discussed below. 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
.
A general map showing magnetic declination for
the contiguous United States is shown in Figure
G-1. Magnetic declination for a specific site can
be obtained from a USGS map, local airport, or
through a computer service offered by the
USGS called GEOMAG (recommended).
Section G.2 has a listing of the prompts and the
declination determined by GEOMAG for a site
near Logan, Utah.
Declination angles east of True North are
considered negative, and are subtracted from 0
degrees to get True North as shown in Figure
G-2. Declination angles west of True North are
considered positive, and are added to 0 degrees
to get True North as shown in Figure G-3.
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 G-1. Magnetic Declination for the Contiguous United States
G.2 PROMPTS FROM GEOMAG
GEOMAG is accessed by calling 1-800-358-
2663 with a computer and telephone modem,
and communications program such as TERM or
GraphTerm (PC208 Software). GEOMAG
prompts the caller for site latitude, longitude,
and elevation, which it uses to determine the
magnetic declination and annual change. The
following Menu and prompts are from
GEOMAG:
MAIN MENU
Type
Q for Quick Epicenter Determinations (QED)
L for Earthquake Lists (EQLIST)
M for Geomagnetic Field Values (GEOMAG)
X to log out
Enter program option: M
Would you like information on how to run
GEOMAG (Y/N)? N
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