[time-nuts] OT- RMC sentence and magnetic variation/compensation in GPS module
Hal Murray
hmurray at megapathdsl.net
Sat Feb 5 21:26:21 UTC 2011
> but obviously I can't get it from the internet
Are you sure?
> still wondering how the GPS could provide a useful value if the modeling is
> difficult, varying widely in time and space.
If you ignore local quirks like big chunks of iron ore, the magnetic
declination is a set of smoothly curved lines on a map. I expect it
compresses well so it won't take much space in a ROM.
US Map here:
http://www.ngdc.noaa.gov/geomag/declination.shtml
(lots of other info too)
World map here:
http://en.wikipedia.org/wiki/Magnetic_declination
Also, there is a fancy changes-over-time map down near the bottom.
On the other hand, there are places with really strong local magnetic
screwups. Navigation charts often have notes on them. I have a map with a
note saying the error is 173 degrees.
--
These are my opinions, not necessarily my employer's. I hate spam.
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