# Compass



## rowley (May 14, 2005)

Has anyone found a compass that will work in the front of the Fiat Ducato X250? I have a digital compass but it will only give a valid reading if I hold it by the rear view mirror, not very satisfactory.


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## spykal (May 9, 2005)

Hi

My normal magnetic needle compass, which I use for setting up the TV aerial or the Satellite dish will not work correctly anywhere near the front end of my Peugeot but when driving* I often check the compass display on my Tomtom which bases its data on gps signals not on magnetic north.

* I should add that my Tomtom compass does not work reliably when stationary.

Mike


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## JohnsCrossMotorHomes (Jul 21, 2007)

spykal said:


> Hi
> 
> My normal magnetic needle compass, which I use for setting up the TV aerial or the Satellite dish will not work correctly anywhere near the front end of my Peugeot but when driving* I often check the compass display on my Tomtom which bases its data on gps signals not on magnetic north.
> 
> ...


Magnetic compasses will not work acurately when positioned near other metallic objects. On boats and planes, the compass needs to be 'swung' to allow for variations and a variation card is then fixed to the compass.

There used to be a small one available that stuck to the windscreen with a rubber suction cup which kept it away from the dashboard electrics and metal. It does need to be checked to make sure it is reasonably accurate.

A satellite system may be unreliable when stationary due to it not being updated on a regular basis by the vehicle moving.

Hope this helps.


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## spykal (May 9, 2005)

Hi JCM & Rowley

The compass rose displayed by a GPS can easily fool you into thinking that a compass is a built-in feature and that was why I added my rider to the previous post :wink: . As you pointed out the “compass” displayed by a GPS is based on calculations done by the unit. 

When moving the GPS receiver “knows” where it was a moment ago and it also “knows” where it is right now, it can then calculate the direction between the two and show it in the form of a compass heading. 

When the unit is stationary the innacuracies of the GPS system can fool the reciever into thinking it is moving so it generates a heading on the compass...which of course is not going to be correct and will swing around as the unit makes different calculations based on the innacurate info from the satelites*

So "compass" display on my Tomtom it is not really a compass at all. The display just shows in which direction the unit is moving. Without movement it cannot work. The settings in the configuration of the unit allow you to set it to show the "compass" in several different ways ... heading, north up ect.

I should have added that I have tried one of the car compasses with built in compensation ...it was a failure too... mind you it was not one of the most expensive ones.

Mike


*These innacuracies are caused by what is known as Selective Availability 
The GPS satellites send different signals and depending on who you are they vary in accuracy... the US military get the accurate signals we private users only get a degraded accuracy.... there was talk that they would allow the accurate signal to be used by private users ..I don't know when it will happen ...but as my hand held unit can get me to within a few feet of a Geocache and as I do not have any need to target Osama bin Laden in his hideout cave , that will have to do me.... for now :lol:


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