Orienteering Guide 06
Orienting a Map and Understanding Declination
Map Orientation
A map is oriented when the directions on the map match the directions on the ground. When the map is properly oriented, a ridge drawn to your right should actually be to your right, and a trail shown ahead should line up with the real trail ahead.
True north and magnetic north
Most maps are drawn with north at the top, usually referenced to true or grid north. A magnetic compass points toward magnetic north. The angle between true north and magnetic north at a location is called magnetic declination. Declination varies by location and changes slowly over time.
Use the declination printed on a current map when appropriate or obtain a current value from an authoritative source such as NOAA. Old map declination values can become inaccurate.
East and west declination
Declination may be described as east or west depending on which side of true north magnetic north lies. Avoid memorizing a slogan such as “east is least” unless you fully understand what the conversion is doing. The safer approach is to follow the instructions for your specific compass and map, or use a compass with adjustable declination and set the current local value correctly.
Orient a map with an adjustable-declination compass
- Set the compass declination adjustment for the current location according to the compass manufacturer’s instructions.
- Set the bezel to north, usually 0 or 360 degrees.
- Place the compass flat on the map with an edge or the orienting lines parallel to the map’s north-south grid or margin.
- Rotate the map and compass together until the magnetic needle aligns with the orienting arrow.
- Check nearby terrain features to confirm the map now matches the ground.
Orient a map by terrain
When recognizable features are nearby, you can also rotate the map until roads, valleys, ridges, lakes, buildings, or trail junctions line up with their real positions. Terrain orientation is an excellent cross-check even when a compass is available.
Map bearing versus field bearing
A bearing measured on the map is referenced to the map’s north lines. A magnetic compass in the field responds to magnetic north. If declination is not built into the compass, the bearing must be converted correctly. When in doubt, use a current local declination value and follow the exact conversion method taught with your compass system rather than guessing.
