Dumb question about magnetic declination

Heres a dumb question from a long time GPS user:

Years ago, I had a GPS (can't remember which model) which allowed you to
enter the magnetic declination so that you could produce a magnetic north
reading if desired. This value was stored in EEPROM (non volatile memory)
and could be changed as needed.

Now I have a couple of GPSs that seemingly automatically compute magnetic
north without the benefit of my wisdom. My Magellan Explorist 600 also has
an option "electronic compass" built in that can do the same.

Question - Where does the GPS get its magnetic declination info from? How do
they account for changes - there seems to be no manual input? Would it be
in a table that is modified as part of a firmware upgrade? Downloaded vias
satellite? Corrected by WAAS? Is the same true of "electronic compasses"?
How can one verify the magnetic declination amount for your area?

Jeez - the thinks you find to wonder about when some newbie ask a question!

I can answer PART of your question... Each GPS (that can
show magnetic directions) has a lookup table "grid" of
horizontal MAGnetic VARiation (MagVar) for all of the
world... The unit interpolates inside this grid at its
computed location...

Magnetic Declination is the same thing, but with marine
'terminology'... Magnetic Inclination is the vertical
direction of the magnetic field (for what that's worth)...

Whether this table gets updated with firmware updates, at
least (Garmin) doesn't say... But as new units come out,
they incorporate the latest table (I'm told)... MagVar
changes about 0.1° per year in the US...

If you can get a flight sectional chart of your area, there
will isogonic lines on it, which are the MagVar for the date
of the chart... See:
http://www.gly.fsu.edu/~kish/field/projects/p4/D2.htm
Notice that in 1995, the middle of the US and central Europe
had zero MagVar...

Pieter wrote:

--
Jack

Get general GPS information at:
http://www.gpsinformation.net/

Pieter wrote:

The magnetic declination information is in a lookup table in the
firmware of the receiver.

Geomagnetic Data Home
http://www.ngdc.noaa.gov/seg/geomag/

Sam Wormley wrote:

I like that site... It says that in 1970, the MagVar in
Atlanta was practically zero -which I remember it was,
because I still had an airplane (needing sectional charts)
at that time...
--
Jack

Get general GPS information at:
http://www.gpsinformation.net/

Here is a good freeware :

http://vtopo.free.fr/declimag.htm

using the latest table.

BR
http://utilsea.free.fr