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BigPerk
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 Posted: Sat Jan 09, 2010 7:03 pm Post subject: |
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I hope it wasn't me that upset anyone  . Apologies if I have - not intended 
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David
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GerryC
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| Posted: Sun Jan 10, 2010 2:39 pm Post subject: |
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It wasn't aimed against anyone at all BigPerk. More about not putting off a new poster from coming back on behalf of all of us.
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Gerry
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djc1610
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Location: St Neots, UK
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| Posted: Tue Aug 10, 2010 11:30 am Post subject: |
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The simple honest answer to the question is "none of them" as they all involve difficulties in getting exact measurements.
The relevant question is which is the most accurate? Certainly the speedo is the least accurate with, I imagine, the camera most accurate.
However if you are caught speeding then the speedo and sat nav readings will carry little weight. In the court it's the camera that is all important.
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David
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peterc10
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| Posted: Tue Aug 10, 2010 11:36 am Post subject: |
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Resurecting and old post (and a hoary old chestnut)
If you are travelling at a steady speed on a straight level road the sat nav is supposed to be accurate to within 0.1mph. Big ifs" of course. But more accurate than the camera and much more accurate than the speedo
Now if you are talking about evidence in legal proceedings thats another matter entirely .........
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Peter
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M8TJT
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Joined: Apr 04, 2006
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Location: Bexhill, South Sussex, UK
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| Posted: Tue Aug 10, 2010 12:02 pm Post subject: |
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| peterc10 wrote: | | But more accurate than the camera |
What makes you think that a satnav is more accurate than a laser or 10[cm]Ghs (CPS) (3cm or 1 1/4") doppler radar speed measuring device. Neither are much affected by atmospherics (at short ranges), both use the speed of light (or more strictly speaking, the speed of propogation through the medium) as their reference not some old atomic clock  in a bunch of satellites whirling round the earth twice a day.
Edited to correct 10cm to 3 cm and to add the Imperial equivalent in response to DennisN's complaint about using metric.
Last edited by M8TJT on Tue Aug 10, 2010 2:04 pm; edited 1 time in total |
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DennisN
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Location: Keynsham
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| Posted: Tue Aug 10, 2010 12:38 pm Post subject: |
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| M8TJT wrote: | | peterc10 wrote: | | But more accurate than the camera |
What makes you think that a satnav is more accurate than a laser or 10cm doppler radar speed measuring device. Neither are much affected by atmospherics (at short ranges), both use the speed of light (or more strictly speaking, the speed of propogation through the medium) as their reference not some old atomic clock in a bunch of satellites whirling round the earth twice a day. |
Damn, you beat me to reply yet again! I was just going to say that's exactly what my dear old granny used to tell me - you can't beat a doppler propagator astrologist thing, we do all our seeds with one (although, to be honest with you, we generally use a three inch one - can't be messing with this new fangled centimetres stuff). 
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Dennis
If it tastes good - it's fattening.
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peterc10
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| Posted: Tue Aug 10, 2010 1:02 pm Post subject: |
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Yes in theory they are more accurate. But there are plenty of problems such as slippage and defected beams that can make them fallible. And they have to be callibrated properly, and sometimes aren't. Isn't that part of the reason why ACPO have an unofficial "allowance" over the speed limit before there's a NIP in the post?
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Peter
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DennisN
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| Posted: Tue Aug 10, 2010 6:11 pm Post subject: |
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| peterc10 wrote: | | ACPO have an unofficial "allowance" over the speed limit before there's a NIP in the post? |
Not Mr Brunstrom!
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Dennis
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Skippy
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Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Tue Aug 10, 2010 8:06 pm Post subject: |
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| peterc10 wrote: | | Resurecting and old post (and a hoary old chestnut) |
Yep. 
| peterc10 wrote: | | If you are travelling at a steady speed on a straight level road the sat nav is supposed to be accurate to within 0.1mph. |
You forgot to add "With a clear view of the sky and a good GPS fix". 
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Guivre46
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| Posted: Tue Aug 10, 2010 8:22 pm Post subject: |
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Yes it is straight line speed it measures. So if you are speeding along a twisty turny road, you are likely to be going faster than your nav says.
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Mike R [aka Wyvern46]
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DennisN
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| Posted: Tue Aug 10, 2010 8:36 pm Post subject: |
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| Guivre46 wrote: | | Yes it is straight line speed it measures. So if you are speeding along a twisty turny road, you are likely to be going faster than your nav says. |
Indeed - probably by at least 0.1, or even 0.5mph I dunno how frequently the satnav position is refreshed, nor how far you go in that space of time (loads of others here have that detail right at their fingertips, so they'll give an example of 50mph at 1/2 second refresh, assuming twisty turny negotiable at 50mph without bending hedges).
For the technically minded, I'm thinking a road from A to B which covers say 387 yards on the road, but only 363 yards as the crow flies and as the satnav measures. Theoretically 50mph as a crow/satnav, how fast over the tarmac. Note, this test assumes the satnav only refreshes at A and B.
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Dennis
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Guivre46
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| Posted: Tue Aug 10, 2010 10:19 pm Post subject: |
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OK, the maths defeats me. I was just repeating something I'd read, which had seemed reputable, but now I'm not sure. Maybe they tested it on one of those zig-zag Pyrenean descents?
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Mike R [aka Wyvern46]
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mikealder
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Joined: Jan 14, 2005
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Location: Blackpool , Lancs
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| Posted: Tue Aug 10, 2010 10:52 pm Post subject: |
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As long as you have a decent signal from a number of different sats the accuracy will be 0,1MPH or better. Bear in mind the GPS signal updates position once per second for most commercial kit, and this will cause a slight delay in the indicated speed during heavy braking/ acceleration, but put this in to reality, we drive road legal cars not something like the F1 boys are used to, the only time you will pull 4G in a road car is during a crash.
The rate of change when cornering/ braking/ accelerating even in a high performance road legal car is actually quite small, try the maths (basic trig) and you will soon work out that even on a twisty road with steep incline/ decline the inaccuracy isn't worth bothering with - Mike |
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Guivre46
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| Posted: Tue Aug 10, 2010 11:09 pm Post subject: |
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Oh dear, I never liked homework - extra maths!
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Mike R [aka Wyvern46]
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mikealder
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| Posted: Tue Aug 10, 2010 11:14 pm Post subject: |
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| When I get home to the main PC I will have access to some data I wrote covering the maths involved from a few years ago, I will post the applicable parts at weekend - I stupidly switched the PC off at home this morning so I cannot access the files on it from where I am located, extra maths maybe but it does give you a better understanding of how small the speed error actually is in reality - Mike |
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