New Hampshire Declination

Magnetic declination is the direction and amount of variation between the Magnetic Pole and True North. The amount and direction of declination depends upon how those two poles align relative to a given point on Earth.

Magnetic declination is the difference between true north (the axis around which the earth rotates) and magnetic north (the direction the needle of a compass will point). It is usually printed on the map to the left of the scale bar at the bottom of a USGS 7.5' quadrangle.

Turn the compass over. Insert the metal key (provided with your compass) into the adjustment screw. Turn the key until the declination indicator is the correct number of degrees east or west of 0° (15.6 degrees West in this example).

An easy way to remember whether to add or subtract is "West is best and East is least." So for West declination, add to the true reading (West is best, and therefore a larger number) and for East declination subtract from the true reading (East is least, and therefore a smaller number).

If using a compass with adjustable declination, set your GPS to true north. Your GPS will indicate a bearing of zero degrees must be followed to reach the waypoint. Adjust the declination on your compass so the orienting arrow points to 10 degrees east. Dial zero degrees on your compass.

The angle between grid north and true north is called the convergence angle. To obtain the true declination it is necessary to add or subtract the convergence angle to the Grid Declination.

An easy way to remember whether to add or subtract is "West is best and East is least." So for West declination, add to the true reading (West is best, and therefore a larger number) and for East declination subtract from the true reading (East is least, and therefore a smaller number).

The following equation can be used to calculate the declination angle: 03b4=221223.45°A—cos(360/365A—(d+10)) where the d is the number of days since the start of the year The declination angle equals zero at the equinoxes (March 22 and September 22), positive during the summer in northern hemisphere and negative during winter

The variation for a charted area is shown inside the chart's compass rose. It's usually written along the arrow pointing to magnetic north. It's expressed as degrees east or west. This illustration represents the difference in direction between true north and magnetic north.