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Appendix 1
Using the magnetic compass of the PI 4100
Proper operation of the PI 4100 dictates that, in order to make a measurement, the receiving loop
antenna’s vertical axis is to remain upright while the longitudinal axis is pointed at the center of the
transmitting antenna. Since certain measurements will be made when the transmitting antenna is not
visible to the operator, a magnetic compass is supplied as an integral component of the PI 4100 and it can
be used for approximating the direction of proper antenna alignment.
At first thought it may seem that a magnetic compass is superfluous when the instrument also
includes a Global Positioning Satellite (GPS) receiver. However, the user should quickly realize that the
GPS receiver “knows” only its physical location on the earth. The PI 4100 program is, therefore, capable
of calculating the True bearing to the transmitter antenna if transmitter coordinates have been entered into
the PI 4100 transmitter data base. The GPS cannot “know” the direction that the PI 4100 loop antenna is
pointed, for a given measurement, but the magnetic compass that is mounted on the antenna does enable
the operator to track that information. However, if the operator is to gain the optimal precision from the
magnetic compass, it is appropriate that he or she understand and compensate for differences between
Compass Bearing (as indicated by a magnetic compass) and True Bearing which is displayed on the PI
4100 data screen.
A magnetic compass functions by allowing a free swinging magnet to align itself with the earth’s
magnetic flux lines that result from the flow of the earth’s inner core. These flux lines extend between a
Magnetic North Pole and a Magnetic South Pole. Magnetic poles differ from the geographic poles by
several degrees. And, they move over time which means that the difference between Magnetic North and
True North also changes. This difference is known as magnetic declination (or variation). Magnetic
declination also varies from place to place and, like the position of the magnetic poles, it changes from
time to time. It is carefully tracked by geologists around the world and the value of declination is
published on some topographical maps and virtually all air and sea navigational charts. [If you wish to
calculate magnetic declination at any location on earth, at any date from 1900 to 2010, for which you
know the geographical coordinates (Latitude and Longitude) that information may be obtained from the
U.S. National Oceanic and Atmospheric Administration’s (NOAA) web site at the following internet
address:
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp]
If the PI 4100 operator enters the appropriate magnetic declination information when the entering the
transmitter coordinates (via the Tx add menu or the Tx edit menu) the magnetic bearing to the station is
displayed in the BRG ### º Mag block in the center left of the display.
There is one additional factor which must be considered with regard to “compass bearing” and its
relationship to “magnetic bearing.” Magnetic deviation may, or may not, exert an influence on magnetic
heading to yield the final compass bearing. Magnetic deviation encompasses the combined magnetic
field distortion effects caused by ferromagnetic material in close proximity to the compass. The effect of
magnetic deviation is site specific and cannot be predicted accurately. Accordingly, it is important for the
PI 4100 operator to understand that the magnetic bearing that is displayed on the display screen may
differ, slightly, from the compass heading as indicated by the instrument’s magnetic compass and that this
difference can vary from one measurement location to the next.
The foregoing discussion is summarized by the two following, age old, formulas: Compass Bearing
± Deviation = Magnetic Bearing and Magnetic Bearing ± Declination = True Bearing.
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