WHAT IS DECLINATION by wuu19113

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									USING THE GLOBAL POSITIONING SYSTEM                                       APPENDIX C—WHAT IS DECLINATION?




WHAT IS DECLINATION?
The earth’s magnetic field is neither uniform, stationary, nor perfectly aligned with the
planet’s poles. True north in California can be determined with a compass reading minus the
location’s magnetic declination. Magnetic declination is the angle between the (magnetic)
north compass heading and the heading to true (geographic) north.
When we talk about the location of the magnetic pole, we are referring to an average
position. Long-term changes in position (called secular variation) mean that the declination
for any given site changes over time.

WAYS TO FIND THE DECLINATION AT A SITE
                                                          variation can be significantly larger (up to 8
The easiest way to determine the magnetic
                                                          minutes per year). For this reason, if you are
declination of a given location is to look at a
                                                          using a map older than 20 years, check a
topographic map of the area. All large scale
                                                          more recent source for the current magnetic
(1:24000) USGS topographic maps, publish
                                                          declination at your site.
a declination correction index somewhere on
the margin of the map. For USGS                           A relatively recent isogonic map or web sites
quadrangles, look at the lower left-hand                  that have current magnetic declination
portion of the map.                                       information are good sources. The 1995
                                                          isogonic map for North America is shown to
                                                          the right.
                                                          If you know the latitude and longitude of
                                                          your site, you can find the declination at
                                                          http://www.geolab.nrcan.gc.ca/geomag/e_cg
                                                          rf.html.
                                                          You can also call the local airport control
                                                          tower.
                                                          More about magnetic declination can be
                                                          found at
                                                          http://www.cam.org/~gouletc/decl_faq.html.



The figure above is the declination
correction index taken from the USGS 7.5-
degree Blue Hills, Mass. quadrangle. When
this map was published (1971), the magnetic
declination was 15 degrees west of true
north. The present magnetic declination for
this location is 16 degrees west of true north.
The change is due to secular variation of the
magnetic field. The annual secular variation
can be very small (less than 2 minutes per
year) in the north; elsewhere the secular

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