The plan is to go from the Soda Creek trail head and hike off- trail to Blue Lake.  Looking at the USGS topographic map shows that it is a 5 mile hike along a bearing of 020°.  So all one needs to do is adjust the compass and strike out.  Not so fast.  The bearing from the map to Blue Lake is in degrees true.  Magnetic declination will need to be compensated for (that is 16° East in Central Oregon) and the Global Positioning System (GPS) receiver should be set to match the compass or map or both.

Failure to compensate for declination with the magnetic compass will result in a navigation error of well over a mile.

Much of land navigation is based on the relationship to the North Pole; also known as “true north.”  The measure of degrees of direction in relation to true north is called “degrees true.”   Maps are laid out in degrees true.  Land features (buttes, mountains, streams) on a topographic map are in reference to degrees true.  By that I mean the bearing from one mountain peak to another will be referenced in degrees true.

The issue is that the magnetic compass’s direction information is in degrees magnetic.  The angular measurement between degrees true and degrees magnetic is called declination.   The hiker will need to compensate for declination.

In the Navy I learned to keep my navigation simple.

In the Navy during a bridge watch, I evaluated the ship’s position on a navigation chart.  The principle navigation compass (a gyro repeater) reported in degrees true; the backup compass reported in degrees magnetic.    Key to this navigation was the both the principle navigation compass and chart used degrees true.

The hiker can select either degrees true or degrees magnetic.  It is a matter of personal preference; a choice.

Today, what works for me is to use a magnetic compass (e.g., the Brunton 8010G or Silva Ranger style 515 CL) that can be adjusted to report in degrees true (some compasses come with declination increments scribed on the housing but this is not the same thing.)

Brunton’s models can be adjusted by simply turning two components (the outer ring and the clear cylinder that houses the magnetic needle) while the Silva models come with a small flat screw driver to make adjustments.  Check the owner’s manual for detail on how to adjust the compass.

Importantly, the adjustable compass eliminates the requirement to calculate for declination. Do remember that the magnetic needle always points to magnetic north and the adjusting accounts for the angular measurement of declination.

GPS receivers provide an amazing amount of information. They come loaded with many navigation features, maps, tide data and much more, but nothing is more important than position data and the direction to a waypoint.

To keep my navigation simple I want my GPS to provide the same directional information as my compass and map; degrees true.  Because a new GPS receiver will be pre-set to report in degrees magnetic at the factory, the unit will need to be re-set to report in degrees true.

For example, to change this default setting on my Garmin GPSmap 60CS I’ll go to the main menu and select “set up’, then “heading” , then “display”, then “degrees” and look for “north preference”. Select “true.”

That’s it.  The GPS receiver should now be set to provide direction information (e.g., bearing and heading) in degrees true.

For those receivers with an electronic compass remember to “calibrate” the compass after changing or removing batteries.

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