Magnetic Declination - Stating The Declination

Stating The Declination

There are three main ways of stating the declination for a given location:

• In a diagram
  • On some maps intended for wilderness or navigational use, including the topographic maps of the U.S. Geological Survey (USGS), a diagram shows the relationship between magnetic north in the area concerned (with an arrow marked "MN") and true north (a vertical line with a five-pointed star at its top), with a label near the angle between the MN arrow and the vertical line, stating the size of the declination and of that angle, in degrees, mils, or both. (On USGS maps, the diagram is near the lower left hand corner, and the information labelled "GN" (grid north) in the same diagram is irrelevant to this discussion.)
• As the numeric size of the angle between magnetic and true north, and the direction from true north to magnetic north.
  • For instance, "10° W" would indicate that magnetic north lies 10 degrees counter-clockwise from true north.
  • Lines of equal declination (isogonic lines) are shown on aeronautical and nautical charts.
• As the signed number of degrees, where a positive angle indicates clockwise from true north and a negative counter-clockwise.
  • For instance, "−10°" would indicate the same as the "10° W" just discussed.

Declination converts between true and magnetic bearings: True Bearing equals Magnetic Bearing plus Magnetic Declination. (See http://www.ngdc.noaa.gov/geomag/faqgeom.shtml question 5d