GPS records speed when not moving

Hi,

I bought a dash-cam for my lorry, it incorporates GPS for the recording of my speed and location. Something bugging me, is when I am not moving the speed is still fluctuating anywhere between 0 - 2mph. Shouldn't it just sit at 0.0 MPH?

It worries me a little is if I need to use the video as evidence they may say that it is not good enough as the speed is not accurate.

Thanks.

 

Fix anomalies

If you leave a GPS on with breadcrumb trail enabled it will create a scribble around your location as though it has been moving, sometimes a considerable distance.

Speed is calculated from the time it takes to get from one fix to the next and as the accuracy can vary upwards of 5 metres it is calculating the speed between those fixes.

When your truck is moving this variation is inconsequential as your movement outstrips the "dancing" effect there are so many fixes in the same direction and the distance between them gives you an accurate speed average.