Magnetic Declination tables?

[dtrayers]

I have a Merdian Gold. How does the unit know the declination to convert true north to magnetic north? Is it part of the firmware or is it updated by the satellites?

 

I couldn't find a good answer to this. Maybe someone can post a link on how this works?

 

Thanks!

 

- Dave

[robertlipe]

I can't recall where I read this (I follow a number of Magellan info sources) but AFAIR, it has tables in the firmware. The information doesn't change frequently enough to encode it in the satellite signals. Yes, the declination changes over time, but not a huge amount. So if there was need to update it, it would presumably be part of a firmware upload.

[msprague]

This made me curious so I ran a query for Dakota county here. I am surprised at how much it has varied.

Date D(+ East)

(yr) (deg)

1953.00 6d 18m

1954.00 6d 18m

1955.00 6d 18m

1956.00 6d 17m

1957.00 6d 16m

1958.00 6d 15m

1959.00 6d 13m

1960.00 6d 12m

1961.00 6d 11m

1962.00 6d 10m

1963.00 6d 9m

1964.00 6d 8m

1965.00 6d 6m

1966.00 6d 1m

1967.00 5d 56m

1968.00 5d 50m

1969.00 5d 45m

1970.00 5d 40m

1971.00 5d 32m

1972.00 5d 25m

1973.00 5d 17m

1974.00 5d 10m

1975.00 5d 3m

1976.00 4d 53m

1977.00 4d 42m

1978.00 4d 32m

1979.00 4d 22m

1980.00 4d 12m

1981.00 4d 3m

1982.00 3d 55m

1983.00 3d 46m

1984.00 3d 38m

1985.00 3d 29m

1986.00 3d 23m

1987.00 3d 16m

1988.00 3d 9m

1989.00 3d 2m

1990.00 2d 55m

1991.00 2d 49m

1992.00 2d 42m

1993.00 2d 36m

1994.00 2d 29m

1995.00 2d 23m

1996.00 2d 16m

1997.00 2d 9m

1998.00 2d 3m

1999.00 1d 56m

2000.00 1d 49m

2001.00 1d 43m

2002.00 1d 36m

2003.00 1d 30m

_________________________

http://michael.sprague.name/pocketpc/

Geocaching with a Pocket PC FAQ

[+Don&Betty]

You can simply measure the present declination in your area by finding a N-S (true) road on a topo map (perhaps one in your own neighborhood), then go there and measure it's magnetic heading with your compass. Then see what your GPS says (however you would do that.)

[+parkrrrr]

[oops]

[+apersson850]

As far as I can remember, I've seen an upgrade notice somewhere, with some firmware, that declination tables were updated for that particular unit. So they can indeed be updated via firmware updates.

Also, if you know the declination, and consider the built-in table in your GPS to be wrong (obsolete), you can enter a User North to handle such a situation.

 

Anders

[+happycycler]

Thank you msprague for that link. I ran it for my house in St. Louis and found that what I vaguely remembered was essentially true. A very small declination here -- almost zero this last year.

[+EraSeek]

Several usefull items at this link: http://www.thecompassstore.com/decvar.html

For finding your declination, and how much it changes. Maps and figures.

 

 

"See the wonderous works of Providence! The uncertainty of human things!" Geo.Washington

[dtrayers]

Thanks for all the replies. I found another interesting link:

 

http://www.ngdc.noaa.gov/seg/potfld/faqgeom.shtml

 

It appears that adjusting for declination is by a mathematical model updated by emperical observations every five years. So I would think that since handheld GPS receivers are fairly recent, there hasn't been more than one update to the model, if any, since their introduction. I wouldn't be suprised that if in the future the model corrections may be part of the data stream sent to the GPS unit from the satellites.

 

- Dave