All maps are based on true North, with the North Pole being the reference point.  Compasses point to the earth’s magnetic North.  Variation is the correction which must be made so a compass reflects true North values.  This difference between magnetic North and true North is expressed in positive or negative degrees.  Depending upon your position on the earth’s surface, the correction can be minimal or quite significant.

World Declination PDF (825KB)

SureShot automatically computes magnetic variation anytime it gets an active GPS fix.  In instances when the operator is calling up stored GPS location information, SureShot consults its non-volatile memory for a stored magnetic variation value.  This correction value is input manually during unit set up.  Consult the SureShot Installation Manual for specific entry instructions.

But first, you must determine the correct value for your general operating area.

Exact local variation values can be found on aviation section charts. 

(Maps Courtesy of The Compass Store)