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Re: Magnetic Variation
From: Scott Owen
Date: 2010 Mar 01, 19:07 -0600
From: Scott Owen
Date: 2010 Mar 01, 19:07 -0600
Guy Schwartz wrote: > Hello all: > > I just took a test today and one of the questions had to do with magnetic > variation. The answer to the question was that magnetic variation changed > seasonally. I checked Chapman's, Dutton's and Bowditch, no mention of > seasonally adjusting? > > Anyone have any information or was the test wrong? > > Thanks > > Guy I suppose that "seasonally" could be interpreted to mean that magvar does indeed change and that this change is incorporated for navigational purposes on an "annual" basis, though "seasonally" in most people's lexicon means quarterly. Today, in order to get the correct local magnetic variation one consults the compass rose on a modern chart and with some rather simple math the job is done. As Andres points out almost everyone uses the World Magnetic Model for determining magvar. At the bottom of the page from the below link are two pdf files one showing 2010 magvar and another "secular rate of change" whatever that is. http://www.ngdc.noaa.gov/geomag/WMM/DoDWMM.shtml From a historical perspective, I am sure at some point, cartographers and navigators accounted for magvar without the aide of electronic computers, charts with magvar data in the compass rose, and worldwide airborne magnetic surveys. So if there was an "old school" computation for finding local magvar, what is it? Scott