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.
This downloadable file contains a map showing variation values around the world:PDF Download (825kb)
At each start-up, SureShot automatically computes magnetic variation once it receives an active GPS fix. SureShot calculates the variation for the current location using a complicated algorithm. The calculated variation (correction) value actually changes daily. This correction value is used to adjust SureShot’s azimuth calculations in order to precisely position an antenna toward the target location.
Approximate local variation values can be found on aviation section charts.
(Maps Courtesy of The Compass Store)