Van driver uses GPS data to beat speeding fine.

[67688c]

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 Is it time that laser speed gun equipment was withdrawn whilst it is re-assessed for accuracy?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?

[Birchy63]

GPS works by recording exact co-ordinates, the speed is measured by the time taken to go from one co-ordinate to the next and then averaged over several readings. It should be pretty damn accurate.

[parsco]

GPS works by recording exact co-ordinates, the speed is measured by the time taken to go from one co-ordinate to the next and then averaged over several readings. It should be pretty damn accurate.

Took the words right out of my mouth mate

As above they work in a similar way that SPEX do (average speed cameras), the camera will clock you at a certain position and time you to see how long it takes to get from point A to point B, you get the idea . This is why average speed cameras are so accurate compared to the hand held lasers.

[67688c]

GPS works by recording exact co-ordinates, the speed is measured by the time taken to go from one co-ordinate to the next and then averaged over several readings. It should be pretty damn accurate.

Yes.. but I suppose there has to be a certain small tolerance, IOW if your car only moved say 3 feet from its location would it pick that up, I doubt it would log the small change, may need to be a larger distance perhaps. I feel a bit of a experiment coming on...lol

However, I agree one can assume they are pretty accurate.

Just as well they are, maybe not my lifetime, but certainly in the future every car will have some form of gps tracking system where your location and speed etc and likely toll charges will be logged by government departments.

[Birchy63]

Just as well they are, maybe not my lifetime, but certainly in the future every car will have some form of gps tracking system where your location and speed etc and likely toll charges will be logged by government departments.

Depends on what the government want:

Proactive countermeasure - they fit GPS based restrictors so that its impossible to speed on any given road = Instantly better road safety

Reactive countermeasure - they do as you suggested = More money in their pockets

With speed limits mostly being imposed to generate revenue, I think we all know which they'd choose
Road safety V's Money...... Hmmmmmmm

[si_hoss]

I was led to believe that GPS devices are generally accurate to 0.1mph.

I ride a bike too and sometimes read MCN, ok they are seriously anti-police but they tested a few years back a laser speed gun and proved that unless it bounces back off a flat surface by essentially a trained marksman they can read back a wrong speed. Finding a flat panel on a bike is pretty difficult, and when they tested one on a STATIONARY Ducati sports bike (can't remember which one) it clocked it doing "40mph"

Now like i said MCN are anti-police at the best of times, and there's plenty of police on this forum that have probably be trained to use them, but it does make you wonder...

[Big Knox]

the last time I was done it was spot on. my speedo was reading 90mph and the officer clocked me at 86mph which I would say it about accurate.

[67688c]

I agree I would think the speed calibation is quite accurate as I assume it's a case of mathematic formula, however, I also would say it's only as accurate as the co-ordinate data which it gets from tracking the satellites. IOW the distance you can walk holding the gps before it updates the change in position as you move, shall have to try it, probably may be just a couple of metres perhaps.

Re. the Speed Gun accuracy, I think they may have improved over time, but I do remember Clarkson and his gang doing some tests 2 or 3 years ago when they managed to clock a stationary wall doing 55mph + some other silly readings lol. Though knowing JC that was to be expected.

Police are supposed to be specifically trained in their use, however even in the best hands I'm sure mistakes are sometimes made, such as in the van drivers case above.

Approx 4 years ago my daughter received a NIP from Plymouth police who had clocked her doing 58mph in a 50mph stretch on the A38, she swore she was doing well under 50, she knows the road well and knows there were camera's there quite often. Anyway, she said she was going to plead not guilty, and I advised her to request the photographic evidence which she duly received about 3 weeks after the NIP.

It was a A4 plain paper photocopy of 2 pictures of her car, one photo showed the car quite a long distance away, you couldnt see her or the registration plate, and in a section along the bottom of the photo it showed the time/date and her speed as being 45mph.

The lower second much nearer shot photo printout however looked like someone had gone over where the speed was written with a felt tip pen and erased the speed, the colour was a similar grey to the colour on the printout, so it had been done on the photo and then copied. IOW there was no speed showing on the second photo at all. When I saw it I instantly thought this cannot be right, and despite phone calls to point this out to the police, she ended up going to court in Plymouth and pleaded not guilty, regretfully she defended herself. Unfortunately I was not present myself, but the result was she was fined £500 and received 3 points, I know she was fuming at the time and I dont know how she conducted herself in court, not very good by all accounts, but I always thought she was treated very unfairly. Certainly at least.... just based on the photo printout I saw.

Had she pleaded guilty, it would probably have cost her £60 or whatever the standard fine was. I can only assume she said something in court which upset the magistrates big time!.

As I also suggested to her she pleaded not guilty if she was that certain she was under the speed limit, I felt rather bad about her fine and I ended up paying it for her. She was still at Plymouth UNI doing her degree course and so was not exactly flushed with money at the time.

I dont know what hokey pokey went on with those photo's, but something was definately not right, however, sometimes it's much cheaper to take the fine and points, or at least get a good solicitor if pleading not guilty.

[mark_647]

Interesting read, i have often wondered this and always assumed the GPS was moe accurate.

[Danialj32]

Most people (like me) would just assume that the speed camera's are the most accurate, though i've done it a few times with my sat nav on, and drove through one of those "your speed" display things, and seen a big difference, from all 3 sources!