Map, Compass & GPS

Map, Compass & GPS
Wild flowers along Fall Creek on the way to the Green Lakes - Oregon

Monday, September 16, 2013

Direction - Bearngs and Azimuths

In land navigation, bearings and azimuths are used to describe the angular measurement to a distant object.  This is an essential component of compass and GPS usage.

In the field, the hiker needs a way to express the direction to an object or the direction of intended travel.  The two terms most commonly used for such expression are bearings and azimuth.  Both are described by units of angular measurement.

Azimuth is the common term used by the military in land navigation.

Bearing is the term most frequently used by those recreating in the backcountry.  Bearing will be used during the rest of this discussion.  Bearing is the horizontal angle measured clockwise from North.

The degree is the common unit of measurement of a bearing.  A degree can be broken down into the following terms:

                        A circle = 360°                     

1 degree (1°) = 60 minutes

                        1 minute = 60 seconds

Thankfully, most direction information is expressed simply in degrees.  For example, the direction to a destination calculated by a Global Positioning System (GPS) receiver will be stated in degrees.

Degrees will always be described in relationship to a circle and a starting point.  A circle is comprised of 360 degrees; check your compass dial to confirm.  In land navigation, the starting point is from north.  Commonly, the angular measurement is calculated from one of three options.   These are True North, Magnetic North and Grid North.

True North is the direction from any point on the globe to the North Pole.  All lines of longitude are lines that run true north and south.  Angular measurement is calculated from the starting value (0°) clockwise to the desired direction.  For example, east has a direction of 90°.  (The best practice is to state the bearing in three digits, thus, 90° becomes 090°.)  Bearings measured from the North Pole are referred to as degrees true.

Magnetic North is the direction to the Magnetic North Pole.  Note that Magnetic North and True North are not the same.  The angular difference between True North and Magnetic North is known as declination.  (For more information read my post Declination). The angular measurement is measured clockwise from the magnetic north arrow to the desired direction.

Grid North for the recreating hiker is in relationship to Universal Transvers Mercator (UTM) grid.  The globe is bounded by 60 UTM zones that stretch north to south.  The central meridian of the UTM zone is aligned to true north.

For the backcountry navigator, the simplest method is to use bearings that are in relation to true north.  In my navigation classes I suggest that students “match the map” that will be used in the field.  In most cases a map is oriented to the North Pole.  A GPS can be setup such that the receiver provides direction information in degrees true.  I also recommend consider the purchase of a declination adjustable compass such as the Suunto M3 or the Silva Ranger.  Though the magnetic needle always point to magnetic north, the dial provides direction data in degrees true.  Thus, both instruments (GPS and Compass) will match the map.


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