Map, Compass & GPS

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

Monday, February 12, 2018

Magnetic Compass Accuracy


Navigating with a magnetic compass is a skill that takes study and practice.
When plotting the hiker’s position on a map the objective is to have three lines of bearing intersect just like in the image below; this is a position fix.  That is “pin point” accuracy. This is hard to do with a magnetic compass and may not be achievable.

Blake Miller/Outdoor Quest Image
Many factors impact accuracy.  Some the hiker will have no control over.  
These include:

  1. Visual acuity (e.g., how well the hiker can see.)
  2. Polarity of the compass’ magnetic needle – does it point in the right direction? Polarity may change over time such that the magnetic needle may no longer work accurately.
  3. Smooth movement of the magnetic needle.
  4. Alignment of the compass dial to the compass housing.
  5. Local attraction – Similar to declination, local attraction is magnetic interference unique to a specific location.  It may be caused by buried metal objects or an unusually high concentration of iron or nickel in the ground.
  6. Lack of distant objects to sight on.
  7. Weather (e.g., Fog, clouds, and smoke.)
  8. Terrain may hide the objects that the hiker wants to sight with the compass.
The hiker does have control over the following.

  1. Purchasing a quality compass such as the Silva Ranger.
  2. Correctly adjusting for declination.
  3. Staying away from iron and steel objects such as a car, high tension power lines and a hunting rifle.
  4. Practiced sighting techniques.
  5. Practiced with the procedures of plotting the various lines of bearing.

Blake Miller/Outdoor Quest Image
 The image above closely represents what the hiker will have to deal with and accept.  The crossed lines of bearing provide a rough approximation of a position plotted on the map.

Terrain Association will further "dial in" the hiker's backcountry position.


Blake Miller/Outdoor Quest Image

The image above represents the error of the plotted lines of bearing.  Notice that the lines of bearing have poor angular separation. But by using terrain association the hiker might be able to refine the position fix.  If the hiker is near the river and on the on the river's east side then the position close the road  will better define location.


Navigation is not hard but it does take practice; it is a perishable skill.

When in the wilderness compare both map and compass with a GPS when possible.  Hiking companions should compare their work too.

Read other compass related posts:

     Buying a Magnetic Compass

     Declination

A solid reference is June Fleming's Staying Found and Bjorn Hjellstrom's Be Expert With Map and Compass.

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