From: Ronald Wong (ronwong@inreach.com)
Date: Tue Mar 26 2002 - 02:07:23 PST
Message-Id: <l03102802b8c5e1388add@[209.209.19.220]> Date: Tue, 26 Mar 2002 02:07:23 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: Re: Magnetic declination
Recently, Tom Woosnam said:
>A student of mine says that when he sails off the Washington coast the
>maps he uses show a 5 degree offset from true north.
>...
It may seem to be a minor point to many, but to those who sail as opposed
to those who don't - like surveyors/backpackers/tourists/etc. - there are a
few things that have to be kept in mind:
A. When you are sailing or flying, the navigational aid used to find your
position on the water or in the air is called a "chart" - not a "map".
A map is what everyone else uses.
B. When you use a chart, the difference between the direction the compass is
pointing and the direction of true north is called "variation" and is
frequently indicated by a pair of compass roses in which one, the magnetic
compass rose, is nested inside the other,the true compass rose. Printed
in the center of the compass rose is the variation and the average
change/year at the time of publication. Aeronautical charts don't bother
with variation.
C. When you use a map, the difference between the direction the compass is
pointing and the direction of true north is called "declination" and is
frequently indicated on a topographical map in the form of a declination
diagram found in the margin of the map. The direction of True north is
indicated by a line with a star at the top. Two other lines usually extend
from the base of this line and indicate the direction of grid north and
magnetic north. The declination is found next to the line indicating the
direction of magnetic north.
D. It IS possible to find a map that has compass roses like those found on
a chart and uses the term variation instead of declination, but that
is very much the exception to the rule (National Geographic's topo map
of the Grand Canyon for instance).
E. When you give either the variation or declination, you must also give
the direction of the error in terms of East or West variation/declination.
Since Mr. Woosnam's student didn't, it isn't clear whether the difference
between the students observation and the value that Tom found in
various textbooks - 20 degrees East - was 15 degrees or 25 degrees.
The simplest way of resolving the problem is to get a chart of the
Washington coast. An alternative approach is to direct your browser to the
National Oceanic and Atmospheric Administration's website for magnetic
field information:
http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl
The values are typically good to within 30 minutes of the actual value. An
intense solar storm or local anomaly may cause the observed value to differ
from this but, since these are temporal or local in nature, they wouldn't
appear on a chart/map.
For Seattle, the variation/declination is currently 18° 30' East and it is
decreasing 7'/year.
I wonder what led Tom's student to a figure of 5 degrees. I've come across
charts of the area west of Washington that go back to the beginning of the
last century and they are nowhere near 5 degrees - East or West.
ron
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