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[TowerTalk] Magnetic North Info & Map

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Subject: [TowerTalk] Magnetic North Info & Map
From: (Jim Lux)
Date: Tue Feb 11 12:42:12 2003
The Canadian site is actually fascinating... Lots of stuff there about how 
the magnetic field is NOT adequately represented by a dipole (What, no bar 
magnet in the middle of the earth?)...

At 09:58 AM 2/11/2003 -0500, you wrote:
>Take a look at this interesting site. Knowing the location and
>characteristics of the Magnetic North Pole will help anyone remember how to
>correct a compass reading to True North.
>Another tip for those of us in the northern hemisphere. Find the time for
>your local sunrise and sunset. Divide that time period by two to determine
>your local noon. At that moment, the shadow of your tower points to true

This is only accurate to about a 1/2 degree (good enough for run of the 
mill HF antenna pointing.. not particularly good enough with a 40 dBi dish).

1) The sun is 1/2 degree wide, so the shadow is somewhat indistinct.  If 
you're sighting with a sextant or transit, you can sight on the limb (edge) 
of the solar image.

2) Local solar transit (noon) is exactly halfway between sunrise and sunset 
times only when it happens to occur exactly at the solstice. Otherwise it's 
off by a little bit. The saving grace is that you're essentially making a 
linear approximation to a function that is sinusoidal underneath, and the 
error over an interval of  1/(2*365), is going to be pretty small.

Example: Cleveland OH
March 21, 2003: rise, 629am, transit 1234pm, sunset 640pm       - (29+40)/2 
= 34.5
March 22, 2003: rise, 627am, transit 1234pm  sunset 641pm       - (27+41)/2 
= 34

Thousand Oaks, CA
1 Jan     702am 1159am 457pm    (62+57)/2 = 59.5
2 Jan     702am 1200n  458pm    (62+58)/2 = 60 (00)
22 March, 557am 1203pm 609pm
21 March, 558am 1203pm 608pm

Splitting the rise/set times probably gives you local transit time to an 
accuracy of 30 seconds, which corresponds to about 1/4 to 1/8 degree of 
heading accuracy.

Given that the handy UNSO site actually gives transit time, you can use 
that directly, but you're still limited by the optical accuracy problem 
(1/2 degree wide source) and the fact that you need to know your lat/lon 
accurately. A longitude error of 1/4 degree (around 10-15 miles) 
corresponds to a 1 minute change in time.

UNSO Site for sunrise/set:

There are some other sites that will give you the azimuth of the sun, from 
your location, at an arbitrary time, so you don't need to be constrained to 
making your measurement at local noon. (if a cloud happens to come over at 
just the wrong time!)

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