In a message dated 2/9/2006 11:57:28 P.M. Eastern Standard Time,
[log in to unmask] writes:

Is there  any way to know more accurately how much of a correction to make in
any given  time period?


The USGS publishes a model for the declination of magnetic north each five
years. It can be used to compute the declination at any lat, long and year over
 the five year period. They (USGS) also publish the historical values of
magnetic  north declination (measured at county courthouses usually)for the period
1750 to  about 1975 when spherical harmonic models became accepted. Magnetic
north  declination is a function of three variables: Latitude, longitude and
calendar  time. To read more about magnetic north declination and surveyors use
of  compasses go to _www.datacache.com_ (http://www.datacache.com)
Charles.

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