Magnetic Interference

Magnetic Interference


Natural anomalies, things you carry and the surroundings can affect the reading of your compass


Magnetic Interference 

Magnetic Interference



Compass interference comes in two flavours: natural interference, also known as magnetic anomalies, and man made interference.


Man Made Interference

The needle of your compass is magnetized, thus pointing to north. You may have items of kit on your person or in your rucksack that could interfere with the accuracy of the compass.

There is a story of a group of mountain leader trainees where one person’s compass in the group always seemed to point in a different direction to the other group members’ compasses, and was thought to be broken. A brief trouble shooting exercise revealed that the operator was in fact wearing under her waterproof, a heavy metal watch, together with a bunch of metal bangles, all on her compass holding wrist. When the ‘broken’ compass was swapped one from another of the team, the replacement compass also pointed in directions other than north. When another team member used the ‘broken’ one, it pointed correctly to north. Thus, proximity of metal objects produces bad readings. Remember this! 

The same issue has been reported with electronic gizmos, such as mobile phones, GPS units, iPods etc, and especially mobile phones, which contain magnets in speakers. Exposure to magnetic forces can cause reverse polarity. Keep metal away from from your compass when using it and electronic devices away always.

It's not just things you carry that can cause interference. Metal objects around you, such as fence posts, bridges, gates etc can all interfere with the compass.


Reverse Polarity

Reverse polarity is the term used to describe the situation whereby a compass's north pointing needle in fact permanently points south instead. It is cause by exposing the compass to magnetic forces, and in particular these days, mobile phones, as they contain magnets (in speakers). The maxim to follow is never to keep your compass and phone together.

Periodically check that your compass still points north. Do this by orientating your map, based on land and map features alone, and check whether the red part of the needle points to the top of the map.


Video

Some good examples in this video of compass interference.




Magnetic Anomalies

There are a few areas of the UK where your compass will, frankly, tell lies. Metallic content of close-to-surface rocks in the locality can interfere with the compass needle, causing it to point in a direction other than north. The Cullin Mountains of Skye are notorious for this, and below are other known areas. 

Know if your route has a magnetic anomaly, be sure to check your position against a GPS grid reference if position uncertain or visibility is poor.


Known Upland Areas of Magnetic Anomalies

  • Cullin of Skye
  • Hecla on South Uist
  • Ben More on Mull
  • Torridon area
  • Ore Gap in the Lake District has a red colour to it, and it is known to interfere with compasses. Various online forums have also reported interference on nearby Crinkle Crags & Bowfell, although according to the British Geological Survey Magnetic Anomaly maps, there is no significantly different magnetic reading than the rest of the locality. One can only say that the anomaly must be extremely localised.

(If you know of more, please get in contact and the list will be updated.)


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