On Tue, 01 Feb 2011 01:19:30 -0500
"Rroger (K8RI on TowerTalk)" <k8ri-on-towertalk@tm.net> wrote:
> Although the idea is sound, easy to implement in one form or another
> there are many places in the US (and the rest of the world) where
> magnetic and true North differ substantially. The extremes in
> continental US are in the NE and NW. IOW New England states (-20 deg)
> and parts of Washington state(+ 20 degrees) with the East coast
> running -10 and the N Central US down to Mexico and the tip of the
> Baja peninsula running +10. Zero degrees runs from the Sestern edge
> of Hudson Bay to near New Orleans.
> http://en.wikipedia.org/wiki/File:IGRF_2000_magnetic_declination.gif
>
> Western Australia is zero while Eastern is +10, but New Zealand runs
> from + 20 in the North to about + 25 in the South. Tasmania is some
> where between + 10 to 20. The N end of Madagascar is -10 while the
> South end is -20.
> NOTE this map does not take into account local variations which can
> be as much as 30 or 40 degrees, nor does it show anything related to
> the South Atlantic anomaly.
>
> Check the magnetic deviation in your particular area if you decided
> to use this simple and effective means of keeping track of your
> antenna heading as it can easily be 10 to 20 degrees off particularly
> in the heavily populated coastal areas. The map linked above makes a
> good reference and starting point, but get an aviation chart for your
> area if in the US and check the "Isogonic lines" (didn't think the
> spell checker would know that one). for your area.
>
> Mineral deposits as found in Upper Michigan, Wisconsin, and Minnesota
> are particularly annoying. Man made structures including power lines
> *May* affect readings.
>
> However I should add that many wind indicators in wireless weather
> stations "do not" depend on the earth's magnetic field, but rather
> depend on them being oriented to North (true or magnetic is up to the
> installer) They use a burst transmission on the 430 band, but I've
> never heard the one I have up. Batteries are going on 3 years even in
> our weather.
>
> 73
>
> Roger (K8RI)
Here is a link to a page which will figure your magnetic declination
for you.
Plug in your zip or lat/long plus the date.
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
My QTH is 1 degree 27 minutes east so when pointing a beam on a tower
with an magnetic compass I'd subtract 1 degree. Problem is this number
changes every year as the north pole is moving, 7 minutes per year to
the west at my QTH.
--
R. Kevin Stover
AC0H
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