Re: Finding True North
From: Roger Halstead (Delete-Invallid.groups_at_tm.net)
Date: 09/26/04
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Date: Sun, 26 Sep 2004 17:03:15 GMT
On Fri, 24 Sep 2004 00:33:17 GMT, Chris L Peterson
<clp@alumni.caltech.edu> wrote:
>On Thu, 23 Sep 2004 21:54:58 GMT, Roger Halstead <Delete-Invallid.groups@tm.net>
>wrote:
>
>>It's *probably* a big look up* table just like the imput to a compiler
>>except they use current state and next state tables.
>
>Actually, it is a mathematical model. I read a paper off the USGS site a couple
>of years ago about it, and I've seen a code sample for getting magnetic
>declination from latitude and longitude. About a dozen lines of C and a constant
>array with less than 100 floating point values.
>
>I believe the USGS and NOAA models are supplemented by a GIS based lookup system
>for surveyed anomalies.
And the movement of the magnetic pole itself. As I recall that can be
as much as a mile of more in a year.
As an aside: As I recall, when I started flying the zero degree
isogonic line passed right through Michigan. Now we are close to 6
degrees West in the Midland MI area. 5 degrees through the center of
the state. I believe I read the rate of change is on the order of 1
degree every 10 years, but it does vary from year to year and with
location..
The lines tend to slope from the NNW to the SSE as they go across the
state. For instance the 6 degree line at the South end of Lake
Superior is 85.15W 46.45N and at the north edge of Ohio. 83.10 W
41.30N. Going North the line shifts westward over 2 degrees in a bit
over 6 degrees change in latitude.
Being the variation is 6 degrees W (the compass points 6 degrees West
of true North), I would pick north with the compass and then move the
scope alignment (pointed North) to the right, or East 6 degrees.
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
>
>_________________________________________________
>
>Chris L Peterson
>Cloudbait Observatory
>http://www.cloudbait.com
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