Map, Compass & GPS

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

Wednesday, April 26, 2017

Plot Your Position With a Compass

 A quality compass is an integral part of the backcountry navigator’s kit.  Sighting with a compass is an important skill that can determine direction to an object or help the hiker locate and identify his position in the backcountry.

This post discusses the steps to be taken to use a compass to plot one’s location on a topographic (topo) map in the back country.  In the vocabulary of navigation this is also known as “fixing” or determining “position.”

The first step is to ensure that the hiker has adequate maps both in quality and quantity.   I recommend carrying a set of maps that include 7.5’ United States Geological Survey (USGS) maps and a second map type such a United States Forest Service map.  The USGS map gives me the detailed information of the immediate area while the other map covers a much broader and larger area. 

Before heading for the trail, take a look at the maps at home.  Scouting from your desk allows you to find significant land features that will surround the direction of travel.  Features such as distinct mountain peaks, a stream, and a ridge line are just of few topographic “hand rails” that can be of value in the field.  By spending some time at home with the map the hiker develops a mental map, a mental picture of the trek in advance of the actual journey. 

Account for declination before leaving the trailhead.  I like to keep my navigation simple and personally use a compass that can be adjusted for declination such as the Brunton 8010G.  Declination information found at the bottom of a topographic map is frequently out of date.  Check the web site  for the current declination.

 “A compass is basically a magnet mounted on a pivot, free to turn in response to the pull of the earth’s magnetic field.  The housing protects the needle and helps you relate the direction in which the needle points to directions on the map and on the land.  A compass by itself can’t tell you where you are or what you are looking at but it can tell you about direction….”

Staying Found, The Complete Map & Compass Handbook, by June Fleming

Sighting with a compass allows the hiker to do several things.

First, sighting on a distant object can provide direction to that object and repeated sightings can provide course corrections along the way.  Secondly, with several sightings on different objects a person’s position can be determined and plotted.

Compass direction to an object is known as the “bearing” or azimuth.   Bearing is the more common term in outdoor recreation and is a term used heavily in GPS navigation.  For example, if a mountain peak is due north of you, the bearing to the peak is 000° (read as zero zero zero degrees.)  A compass can also assist the hiker by orienting a map and following a line of bearing taken from a map.

The picture below offers a quick review of the components of a baseplate compass.

To sight or take a bearing do the following:

  1. Using the owner’s manual, adjust the compass for declination.
  2. While holding the compass at waist level, turn squarely towards a distant object.  Hold the compass so that the direction of travel arrow points directly at the object. (Point the direction of travel arrow away from you, perpendicular to your body.)

  1. While holding the compass, turn the compass housing (the dial) and align the orienting arrow (a red arrow engraved in the rotating housing) underneath the red magnetic needle.

 To determine and plot or “fix” a position, the next step is to plot bearings on the map. In a “nut shell” this means that bearings to three clearly identifiable features are used.  Ideally, objects that have a bearing separation of 30° – 60°.  Good bearing separation provides better fixing information and plots on the map cleanly.  The bearings are then plotted on a map and where the three lines cross is the hiker’s location.  This complete process is called triangulation.

The following are suggestions for triangulating a position in the back country.

  1. Identify three (or more) distinct objects to sight on.  Note that the objects need to be on the topo of the area. 
  2. Orient the topo using the compass.  Orienting the topo means that the map’s left or right border is pointing to true north or 000° degrees true. 
  3. Sight on an object such as a mountain peak or church spire.  (Note that not many objects in the backcountry are so distinct and crisp.  Do the best with what you have.)  Ensure the direction of travel arrow is pointed towards the object.  Be as accurate as you can, point directly at the object.
  4. Turn the compass housing until the orienting arrow is directly under red magnetic needle.  Do not move or rotate the compass housing, keep the new bearing in place.
  5. At this point, and while plotting the bearing on the map, the compass will now be used like a protractor.  Importantly, the movement of the magnetic needle is not important.
  6. Lay the compass on the map with either the top left or right corner of the baseplate on the landmark.  This will be a pivot point while aligning the compass.

  1. With the edge of the baseplate in position, rotate the compass (swing) left or right until the N (north) of the compass housing aligns with map North (the top of the map.) 
  2. Draw a line (along the baseplate) from the object (e.g., the mountain peak) to your approximate area.  Draw a nice long line.
  3. Repeat the process two more times using other distant objects to sight on.
  4. Ideally the three lines will intersect in the immediate area; this is the hikers location.  But because of compass error and human error the point of intersection maybe spread out.  Still, triangulation will put you in the ballpark.  Use terrain association to help narrow down your position.


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