Humminbird AS GR16 Operation Manual page 6

Due to wind and waves, the boat is often traveling in a slightly different direction than its heading. The direction of travel, or Course
Over Ground, is provided by the GPS receiver. You can use the compass Heading with the GPS Course Over Ground and Bearing to
navigate a route.
A compass' Magnetic North is affected by the local variations in the earth's magnetic field around the globe. Nautical charts will
often provide the magnetic declination, or magnetic variation, for a local area so that you can confirm that Magnetic North matches
True North. If you have trailered the boat to a new location, the compass' operation may also be affected by a different magnetic
zone. The Humminbird control head compensates for magnetic declination and also allows you to make additional adjustments
from the menu system. The details of these features are described in this manual.
Overview 6