[time-nuts] OT- RMC sentence and magnetic variation/compensation in GPS module

[Hal Murray]

> 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.