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How does a GPS compass work?

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All recreational GPS units can tell you your current bearing, e.g. North, South, East, West and all the points in between. So how come some GPS units advertise an electronic compass and other units don't? It's confusing and it can mean a big price difference so here are the facts.

There are two common ways to work out your bearing.

The Magnetic Compass

A traditional compass uses a suspended pointer which swings around to line up with the earth's magnetic field. The pointer indicates where North is. You can then rotate an outer dial to align with North and read off your current bearing.

The Differential Compass

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This is an alternative way to take a bearing. You must be moving and it's essential to know your precise position so it's perfect for GPS. As you move through the countryside the GPS periodically records your position. By comparing where you were to where you are now the GPS can work out which direction you are heading and uses this to indicate the current bearing.

Limitations of a Differential Compass

You have to be moving. The faster and further you move the more accurate this technique can be. If you stop moving then the GPS can still work out your precise position but cannot give you a bearing. You also need to be within sight of the satellites so, for example, a GPS based differential compass would not work in a cave.

And so back to the Magnetic Compass.

GPS manufacturers recognise that whilst a differential compass is fine most of the time, users still like the security of a magnetic compass. The traditional magnetic compass has been fitted with electronic sensors and bundled into some of the GPS devices. This is what is meant when a GPS is advertised with an electronic compass. It costs a little more but allows you to take bearings when stationary or out of sight of the satellites.

Electronic Compass Hints and Tips

Whether your GPS compass is using differential GPS readings or you have a built in electronic option, check through these hints to make sure you are getting the most from it.

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Hold the GPS level to get an accurate reading
The electronic compass will only work when the GPS is level. This means it will not work when the GPS is mounted at an angle on a vehicle or on a bike. You should hold the GPS level when taking a bearing.

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Always point the GPS forwards in the direction of travel
You need to point the GPS forwards when taking differential bearings. Whilst it knows where it is, it does not know which way round you are holding it so assumes you don't have it upside down or twisted around.

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Try walking a few steps to confirm the reading
To get a correct bearing from a GPS differential compass when you stop, point the GPS ahead and walk forward a few steps. It only takes a few yards before the correct reading is recovered.

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At speed the differential compass is used
The electronic compass can be configured to switch to differential mode above a certain speed. On the move the accuracy of the differential compass is just as good and it no longer needs to be level.

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Switch the compass off when not in use
The electronic compass can eat batteries very quickly. Most devices enable you to switch it off and on as required.

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Don't forget to calibrate your electronic compass
Make sure you calibrate your electronic compass before use. This is usually done by rotating the GPS in slow circles as instructed by the unit.

Written by TrackLogs Digital Mapping. Copyright TrackLogs Digital Mapping 2002-2014. All rights reserved. Please send any comments, questions or feedback to support@tracklogs.co.uk


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