[IRCA] Primer on using magnetic declination data to convert a magnetic bearing to a true bearing

[Peter Jernakoff <Peter.Jernakoff@xxxxxxxxxxxxxx>]

Here's a primer on using magnetic declination data to convert a magnetic
bearing to a true bearing. All you need to remember are two things:

1.  True bearing  =  Magnetic bearing  +  Magnetic declination
2.  East = positive (+) degrees, West = negative (-) degrees     ====>
for both bearing and declination!!!!


==============================================================================
The National Geophysical Data Center (NGDC) has a link, viz.

http://www.ngdc.noaa.gov/seg/geomag/jsp/struts/calcDeclination

that takes you to a tool that can calculate your magnetic declination. I
typed in my zip code, got back my latitude and longitude and this yielded a
magnetic declination of 12 deg. 0 min. **west** for my QTH for today (i.e.
-12 degrees).

An associated FAQ link, viz.

http://www.ngdc.noaa.gov/seg/geomag/faqgeom.shtml#q5d

describes how to use the magnetic declination information to calculate your
true bearing. More specifically, you can compute the true bearing from a
magnetic bearing by adding the magnetic declination to the magnetic
bearing:

      True bearing  =  Magnetic bearing  +  Magnetic declination

This works so long as you follow the convention of degrees west are
negative (i.e. a magnetic declination of 10 degrees west is -10 degrees and
bearing of 45 degrees west is -45 degrees). All magnetic declination and
bearing degrees east are therefore positive.

Example:
My 12 deg. 0 min. west magnetic declination is by convention -12 deg. Both
of my long wire antennas point east of north (positive degrees) and have
magnetic bearings of +36 degrees and +106 degrees. Thus, said antennas'
true bearings are as follows:

      True bearing of antenna 1  =  +36 deg.  +  (-12 deg.)  =  24 deg.

      True bearing of antenna 2  =  +106 deg.  +  (-12 deg.)  =  94 deg.

Best regards,

-Peter Jernakoff-
K3KMS
Wilmington, DE
www.21centimeter.com


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