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# Speed watch

1. (Original post by dennishodgson)
Thanks Roger,
Sorry I have not got the distance in miles, in km the distance between the speed limit signs is 1038km I can not see how I was doing 60mph for a full 2 mins maybe I peaked at 60mph for a matter of seconds, they say my average speed was 47mph could you work out the speed it woul take me to cover this distance i.e. slowest and quickest time??
The recording was at 0543 to 0545 I will have to go offline now but will lookout for your reply and thanks for all your help.
The time between 0543 to 0545, at first glance, is not accurate enough. As Roger calculated, your average speed covering 1.038km in 2 minutes works out at 19.5 mph. It is very likely that the time stated is the time of day only and not the actual elapsed time between the markers.

i.e. To contest this, you would need to request the actual elapsed time recorded by the cameras which would no doubt be far more accurate to within say a few milliseconds. You could dispute this if you knew for certain that their equipment did not record the distance covered to an accuracy of better than 1 minute, but I highly, highly, doubt that is the case.

At a constant average speed of 60 mph, the distance of 1.038 km would take 38.699 seconds.

At an average constant speed of 47 mph, the distance of 1.038 km would take 49.403 seconds.

So if the elapsed time recorded to the nearest 100 milliseconds was less that 49.4 seconds for 1.038km, then you would most definitely have exceeded 47 mph for the whole of that time.
2. (Original post by uberteknik)
As stated, the time between 0543 to 0545 at first glance, is not accurate enough. As Roger calculated, your average speed covering 1.038km in 2 minutes works out at 19.5 mph. It is very likely that the time stated is the time of day only and not the actual elapsed time between the markers.

i.e. To contest this, you would need to request the actual elapsed time recorded by the cameras which would no doubt be far more accurate to within say a few milliseconds. You could dispute this if you knew for certain that their equipment did not record the distance covered to an accuracy of better than 1 minute, but I highly, highly, doubt that is the case.

At a constant average speed of 60 mph, the distance of 1.038 km would take 38.699 seconds.

At an average constant speed of 47 mph, the distance of 1.038 km would take 49.403 seconds.

So if the elapsed time recorded to the nearest 100 milliseconds was less that 49.4 seconds for 1.038km, then you would most definitely have exceeded 47 mph for the whole of that time.
It's a SatNav tracking him for his employer, not the police.

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3. (Original post by dennishodgson)
They do work in seconds every 10 secs, but the print out given to me is a full 2 minutes surely would you not dispute the figures given has it does say 60mph for 2 minutes I don't think I was only doing 20mph?? what would be the figures for 46mph as they give me a tolerance of 10% plus 2 miles an hour i.e. 46mph.
Not disputing I was over the 40mph limit but I need to get rid of the 60mph sorry to be a pain !!
You could post the print-out. I doubt that there will be inconsistencies on it though. Your employer may be misinterpreting it, but I still wouldn't be happy with my employees doing 60mph in a 40mph zone.
4. (Original post by dennishodgson)
They do work in seconds every 10 secs, but the print out given to me is a full 2 minutes surely would you not dispute the figures given has it does say 60mph for 2 minutes I don't think I was only doing 20mph?? what would be the figures for 46mph as they give me a tolerance of 10% plus 2 miles an hour i.e. 46mph.
Not disputing I was over the 40mph limit but I need to get rid of the 60mph sorry to be a pain !!
Were you going at 60 in a 40 at all?

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5. (Original post by Doonesbury)
It's a SatNav tracking him for his employer, not the police.

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The SatNav timing accuracy would be beyond doubt though. I suspect the recording software calculates the average speed based on distance sampling between 10 second rolling markers over the two minute period and plotting the gradient between markers to get the speed.
6. This thread should be in Cars and Motoring - tagging AngryJellyfish to move it for you.

GPS is on the whole accurate, so the speed will be more or less accurate.
7. (Original post by Doonesbury)
Were you going at 60 in a 40 at all?

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It was in June so can not remember soz
8. (Original post by WoodyMKC)
Sat Navs read your speed directly, so it's sort of futile trying to refute it.
Actually, they don't. Speedometers directly read your speed, GPS units differentiate to calculate it. Which is more accurate depends on a whole host of things, although in Europe (but not everywhere) speedos are typically calibrated to over-read.

(Original post by IWMTom)
GPS is on the whole accurate, so the speed will be more or less accurate.
That's a bit of a myth. GPS can be very accurate, but it's also subject to errors and outages. Many millions of pounds are spent in the aviation industry to improve the dependable accuracy. I've frequently seen speeds (and locations) on GPS tracks which simply cannot be correct.
9. (Original post by CurlyBen)
Actually, they don't. Speedometers directly read your speed, GPS units differentiate to calculate it. Which is more accurate depends on a whole host of things, although in Europe (but not everywhere) speedos are typically calibrated to over-read.
Yeah I misunderstood what he was saying first time around until he clarified afterwards. It of course works on a "you got from this point to this point in this time, so you were going this fast" fashion.
10. (Original post by CurlyBen)
Actually, they don't. Speedometers directly read your speed, GPS units differentiate to calculate it. Which is more accurate depends on a whole host of things, although in Europe (but not everywhere) speedos are typically calibrated to over-read.

That's a bit of a myth. GPS can be very accurate, but it's also subject to errors and outages. Many millions of pounds are spent in the aviation industry to improve the dependable accuracy. I've frequently seen speeds (and locations) on GPS tracks which simply cannot be correct.
Thanks at last someone on my side ?? I have visited location measured the road andistance etc there is no way alleged speed is as recorded so above reply is correct.
May I thank everyone for your replys and help, closing this post now.

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