Some of you may find this a interesting read.
A van driver clocked by a speed gun travelling at 61mph in a 50mph zone has used GPS data to successfully challenge the prosecution.
Gareth Powell, was driving along the A4174 near Bristol in November 2008 when he was recorded allegedly speeding. Powell was so certain that he was travelling within the limit that he contacted Navman Wireless who operated the vehicle tracking equipment installed in his van.
Tracking records showed that the van had in fact been travelling at 48mph.
Navman Wireless Director Barry Neill attended court as an expert witness and told the court that the GPS fix was excellent at the time of the alleged offence with eight satellites in view.
As a result the case was dismissed by Magistrates on 30th October this year.
Whilst this is excellent news it yet again begs the question, how many other prosecutions are based on false or erroneous evidence as in this case? Is it time that laser speed gun equipment was withdrawn whilst it is re-assessed for accuracy?
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I've often wondered myself which was the more accurate, ie. the speedo reading, or the speed shown on my built-in Satnav. Every GPS satnav I've had and that is now probably several, they all tended to show my speed as being 3 or 4 MPH less than my speedo especially at motorway speeds, with the difference reducing proportionately as my speed dropped.
This appeared to apply on all the cars I've had over the last few years so one may assume that most car speedometer's are calibrated to error on the low side by a small proportion.
For example my current 320i will show say 70mph on the satnav, and the speedo reading will probably be 74/75mph. I noted when I happened to have my external TomTom 720 switched on at the same time that both SatNavs showed exactly the same speed, so it appears the software formula as far as working out the speed is probably accurate, or similar enough to be reasonably accurate between different GPS units.
However, I often wondered whether the accuracy of the GPS might depend to some extent whether the car was travelling in a reasonably straight line in relation to the satellites, or is this thinking cancelled out due to the fact that as many as 10 or more satellites are being used.
IOW in the event say the car was continually speeding round a small island at say 60mph, the actual satellites would only see a small change in location, almost like running on the spot...lol, just a theory.
So in effect the location accuracy which the estimation of speed is based on may not be very accurate at all unless the car is moving in a reasonable straight line.
So if one is travelling say round a lot of twisting bends perhaps even coming back on yourself almost, would the speed accuracy on the GPS be the same as say travelling in a straight line on a motorway, even though your car speed is the same?