“Declination: A Noun. The horizontal angle between
the true geographic North Pole and the magnetic North Pole, as
figured from a specific point on the Earth.”
Declination is a term that causes
“brain cramps” for many of my students in my map and compass classes.
When I mention Magnetic Declination eyes roll. That said, declination is a physical limitation that the hiker must understand.
The web site
www.magnetic-declination.com has an excellent discussion of what
declination is and what causes it:
“Magnetic declination
varies both from place to place, and with the passage of time. As a
traveler cruises the east coast of the United States, for example, the
declination varies from 20 degrees west (in Maine) to zero (in Florida),
to 10 degrees east (in Texas), meaning a compass adjusted at the
beginning of the journey would have a true north error of over 30
degrees if not adjusted for the changing declination. The magnetic
declination in a given area will change slowly over time, possibly as
much as 2-25 degrees every hundred years or so, depending upon how far
from the magnetic poles it is. Complex fluid motion in the outer core of
the Earth (the molten metallic region that lies from 2800 to 5000 km
below the Earth's surface) causes the magnetic field to change slowly
with time. This change is known as secular variation. Because of secular
variation, declination values shown on old topographic, marine and
aeronautical charts need to be updated if they are to be used without
large errors. Unfortunately, the annual change corrections given on most
of these maps cannot be applied reliably if the maps are more than a few
years old since the secular variation also changes with time in an
unpredictable manner.”
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