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Magnetic Compass Accuracy, Sighting, and Triangulation by Blake Edwards

The quality of a compass wil make a difference in its accuracy
The quality of a compass will make a difference in its accuracy

Navigation is a practiced skill, and when not practiced on a regular basis it becomes easily forgotten. Sighting and triangulating with a magnetic compass is a skill that the responsible outdoorsman should never allow to lapse.

The accuracy of a magnetic compass and the information presented depends on land characteristics, the quality of the compass and the ability and knowledge of the hiker.

The hiker will experience natural factors and conditions in the backcountry that impact accuracy. Some of these factors the hiker will have no control over but may be able to mitigate. One of the most important factors is declination. Declination is the angular difference between true north and magnetic north.

Why is this important? It is because declination’s value varies over time and place. Many outdoorsmen fail to realize how declination impacts the accuracy of arriving at a desired destination. The outdoorsman must understand how to apply the difference of East and West declination –specifically- whether to subtract or add the difference. It can result in either arriving at the desired destination or being way off course.

The Declination value can be found at the map key of most maps. It is important to note that many maps are out of date and the declination value presented has changed. The most current information can be found at the website www.magnetic-declination.com.

Further compounding the declination issue is what is known as local attraction. Local attraction is a result of materials such as iron or nickel deposits located near the earth’s surface at a specific spot. Like declination, local attraction will impact the compass. High voltage power lines, car bodies and flashlight batteries will impact the compass too.

Oregon's Mt. Washington’s tall spire is ideal for accurate sighting
Oregon’s Mt. Washington’s tall spire is ideal for accurate sighting

To lessen the impact on a magnetic compass and improve its accuracy, do these 3 steps:

  1. use the most current declination data for an area
  2. use a declination adjustable compass (e.g., the Suunto M3 or Silva Ranger)
  3. stay clear of iron/steel and electrical sources

When triangulating the hiker’s position with several lines of bearing, the size and shape of an object will make a difference. Ideally all objects will be distinct and clear like Oregon’s Mt. Washington. Mt. Washington’s tall spire is ideal for accurate sighting. In practice the hiker will have to accept what is in view and what is pictured on the map.

Another accuracy impact factor is distance. Distance will magnify the errors. Objects that are five or more miles away may have a significantly large cross range error when lines of bearing are plotted.

The quality of the compass itself will make a difference in accuracy.

The two smaller black compasses (above) have small bearing tick marks that are spaced every 15°. Increments of 15° are of little value for accurate navigation – they provide a rough trend of direction.

The three bottom compasses are good baseplate compasses. Depending on the technical reference, these have an estimated accuracy of roughly +/- 2°; perhaps even more. The three compasses have degree increments of 2° labeled on the rotating dial. While looking at the dial it is difficult to interpolate a bearing (azimuth) more refined that +/-1°. Baseplate compasses have no sighting system and are held a waist level when sighting on an object; not a very precise method to achieve an accurate bearing measurement.

A baseplate compass is adequate for general hiking and orientation
A baseplate compass is adequate for general hiking and orientation

While, a baseplate compass, such as Suunto’s M3, is adequate for general hiking and orientation, I prefer using the Silva Ranger mirrored sighting compass (below).

Silva Ranger mirrored sighting compass
Silva Ranger mirrored sighting compass

Because of its sighting system, it is more accurate than the traditional baseplate compass. A quality baseplate compass such as the Suunto M3 will cost about $30. The Silva Ranger will run about $55 or more.

A significant feature to improve sighting accuracy is with the mirrored black box component of the compass.
A significant feature to improve sighting accuracy is with the mirrored black box component of the compass.

How accurate is this compass?

The degree increments on the rotating dial are spaced at 2° like the other baseplate compasses.

The Silva Ranger offers the hiker the ability to mechanically adjust the compass for declination. A small set screw on the back of the rotating dial can be turned to match easterly or westerly declination.

A significant feature to improve sighting accuracy is with the mirrored black box component of the compass. When the box is raised a notch on the top edge becomes a convenient sight (below).

This notch allows the compass to sight on a distant object more precisely than just being held at waist level. Other manufacturers making similar models also have notch cut into the box at the hinge; this is very helpful.

The sight line is scribed into the mirror. When the compass is sighted, the black box is raised and the sight line comes into view.

Ideally, the sight line will intersect the magnetic needle pivot point.

I recommend that the compass be held like this for improved accuracy when sighting on a landmark.
I recommend that the compass be held like this for improved accuracy when sighting on a landmark.

In my navigation classes I recommend that map and compass go with the hiker on every outing. Further, two weeks before a backcountry journey the compass goes everywhere; even to the mall. Practice sighting, determine a back bearing, and triangulate one’s position at every opportunity. Review the fundamentals of the grid system (e.g., Universal Transverse Mercator (UTM) that will support the compass work. This advance effort pays off in the field by the development of a greater sense of confidence and understanding.

About Blake Miller

Blake Miller received significant hands-on navigational training during his 20 years of service in the US Navy.  He has taught map and compass, GPS and wilderness survival classes in Central Oregon since 1998.  As a part-time faculty member at Central Oregon Community College, he currently teaches land navigation classes to Natural Resource students.  He has been an active member of the Deschutes County’s Search and Rescue (SAR) team since 2009.   Blake is also a featured speaker at regional SAR conferences, Sportsman Shows, and Cabelas. His articles have been published in several national magazines.  Blake can be contacted through www.outdoorquest.biz.

A frequent contributor to SectionHiker, Blake has also written the following navigational articles:

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7 comments

  1. Interesting how few know the box type Compasses like Recta make. Suunto has one model too. I think these offer far and away the best sighting.

  2. Thanks Chris…Never heard of them and I’ll have to check them out. Website? Blake

    • Hi Blake – check out Suunto MB-6, or recta.ch, e.g. the DP-6

      • I just checked the Recta website and they say their compasses are manufactured by Suunto in Finland these days. The DP-6 sure looks like the MB-6. I’m going to check them out.

      • Chris, Turns out that the Recta was my first orienteering compass back in the mid ’70s……lost in safe keeping somewhere. Today I really like that Silva Ranger style of compass.

  3. What are the situations where a degree of accuracy better than 2 degrees are required? Unless you are in a landscape devoid of topographic features shown on a map, and you need to know your position precisely, it seems a bit much to go with a sighted compass. Otherwise the contours of the land or other features should fill any small precision gap.

    • Good question. 2 degrees is probably the best many will get with a standard baseplate compass. It’s a matter of personal choice and what is important. For me, a Silva Ranger fits my needs nicely.

      I also carry a Garmin 62S too.

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