3CorsaMeal
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Registered: 11th Apr 02
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i assume my speedo under reads, as when i reach a 40mph zone, the car in front disappears about 5-10mph faster than me, and the car behind sits up my arse like i'm driving too slow.
i make sure i do bang on 40mph
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A2H GO
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Registered: 14th Sep 04
Location: Stoke
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quote:
Originally posted by Steve
yeah but if you think about it, it doesnt matter if its only accurate to a few meters as long as the other readings are equally as innaccurate it wouldnt matter 
I thought this, as long as the innaccuracy is the same at both ends it should be accurate. 
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Steve
Premium Member
Registered: 30th Mar 02
Location: Worcestershire Drives: Defender
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my opinion of nath has gone up in this thread
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Nath
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Registered: 3rd Apr 02
Location: MK
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quote:
Originally posted by Steve
my opinion of nath has gone up in this thread
Means alot to me Steve.
Forgot to add car GPS acquires about 3 satelites. Top end GPS aquires up to 30 from up to 3 different constellations. More satelites = more accuracy.
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mwg
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Registered: 19th Feb 04
Location: South Lakes
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quote:
Originally posted by 3CorsaMeal
i assume my speedo under reads, as when i reach a 40mph zone, the car in front disappears about 5-10mph faster than me, and the car behind sits up my arse like i'm driving too slow.
i make sure i do bang on 40mph
thats just because your car is slow mate
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John
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Registered: 30th Jun 03
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The innaccuracy speed readings of a normal GPS at car speeds is tiny even though it's not as accurate as some other methods.
The timing GPS systems are even more accurate.
Accurate to about 1/2mph as far as I can see.
[Edited on 09-09-2010 by John]
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whitter45
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Registered: 15th Nov 02
Location: Norton
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quote:
Originally posted by mwg
quote:
Originally posted by 3CorsaMeal
i assume my speedo under reads, as when i reach a 40mph zone, the car in front disappears about 5-10mph faster than me, and the car behind sits up my arse like i'm driving too slow.
i make sure i do bang on 40mph
thats just because your car is slow mate
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Nath
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Registered: 3rd Apr 02
Location: MK
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quote:
Originally posted by John
The innaccuracy speed readings of a normal GPS at car speeds is tiny even though it's not as accurate as some other methods.
The timing GPS systems are even more accurate.
Accurate to about 1/2mph as far as I can see.
[Edited on 09-09-2010 by John]
Yeah I was thinking about that, in relation to timing. I'm not too clued up on that aspect though.
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Steve
Premium Member
Registered: 30th Mar 02
Location: Worcestershire Drives: Defender
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id like you to teach me about gps nath, and then when iv learnt im going to use it to pin point your house to beat you up
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John
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Registered: 30th Jun 03
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I think it's because as Steve said, if it's roughly the same amount out on every reading you still end up with the same speed.
With location if it's out, it's out.
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antnee
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Registered: 30th Dec 07
Location: Cov Drives: Clio 197
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Mines +/-1 out  Up to 80 anyway, last time I checked higher, it was consistently out (not a %)
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pow
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Registered: 11th Sep 06
Location: Hazlemere, Buckinghamshire
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My Ibiza was HIDIOUSLY out... it never came to rest at 0mph though
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Mike
Organiser: North West and North Wales
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Registered: 20th May 06
Location: nr. Skipton, North Yorkshire
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quote:
Originally posted by sand-eel
By law they have to over read!
Our old Sprinter under read 
I found out one day driving down a dual carriageway at 40 and one of those speed signs flashed up saying 45 so I went and picked the satnav up and it was a fairly consistent 10% under. Now our Transit's roughly 10% over
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Ian
Site Administrator
Registered: 28th Aug 99
Location: Liverpool
User status: Online
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quote:
Originally posted by Nath
Top end GPS aquires up to 30 from up to 3 different constellations. More satelites = more accuracy.
Not really possible as they're not all visible to you at any one time.
Even a cheap 12 channel receiver (which most laptop dongle / phone / TomTom are) will pick up all 12 if they're visible. The difference in accuracy is down to its ability to triangulate at the rate at which it updates the data. At 1Hz, you can't be accurate down to much less than a few metres because everything is moving and you physically don't get the data quick enough.
The solution is to poll the data more often, with better receivers running at 5 or 10Hz. Problem there is the data bandwidth, hence why you don't see Bluetooth receivers running at those sorts of speeds because you have to tokenise the data or lose some of the sentences. But if you can get the data in, you can increase your positional accuracy.
Although speed is a different matter - they are certainly significantly more accurate than a gearbox driven speedo - which is also quite deliberately calibrated to over-read in order that you never accidently speed. And I'm referring there to the ability of the equipment to consistently display accurate readings, not whether the equipment is configured to do so, which is a related but fundementally different question. The gearbox speed is essentially the more accurate apperatus - particularly if you can overlook issues such as tyre flex - as you can't really negotiate the facts. Engine speed x gears x tyre size = road speed. Whereas GPS speed is a calculation based on latent data.
Depends what you need really.
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Ian
Site Administrator
Registered: 28th Aug 99
Location: Liverpool
User status: Online
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quote:
Originally posted by John
if it's roughly the same amount out on every reading you still end up with the same speed.
Not identically, the data in has different margins of error based on your position so you re-calculate at every sample. Although yes, it's not large enough to worry.
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Chris
Premium Member
Registered: 21st Sep 99
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GPS speed is knots, with a convert to MPH
Also the hills will effect the speed figure from gps as it asumes a flat surface.
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John
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Registered: 30th Jun 03
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It's whatever speed the application calculates surely, where does knots come from?
Don't see how a hill would affect it either, you've still travelled the same distance forward, no matter how much you've ascended or descended in that time
Not sure how 'GPS' assumes a flat surface either actually, altitude can be calculated, although not normally very accurately with your bog standard receiver in my experience.
[Edited on 09-09-2010 by John]
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Ian
Site Administrator
Registered: 28th Aug 99
Location: Liverpool
User status: Online
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Speed is the difference in your position between the two points at which you sampled your position. So yes, your altitude does matter, but it is calculated in three dimensions and is factored in.
If you drove 100m forward on flat surface in 10 seconds you would have travelled 10m/s
If you drove 100m according to an aerial view of the car but down a 1 in 4 hill you've in fact described the hypotenuse of a right-angled triangle with 100m as one side and 25m as the other, which is just over 103m, ie. in the same 10 seconds you've travelled further ie. gone faster.
Obviously you also have a change in lateral coordinates unless you're either heading dead north or dead west (an even then earth curvature usually means you still do) and these are always in ther calculation even if they don't contribute much to the triangle.
Knots come from the fact that the NMEA sentences contain them and you convert to the user preference in software.
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ed
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Registered: 10th Sep 03
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You need a pretty decent GPS to get altitude properly:
On phone GPS's you get crazy readings on hills. I tried out Trails on my iPhone when skiing, I gave up when it said I was going at 600mph..
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Steve
Premium Member
Registered: 30th Mar 02
Location: Worcestershire Drives: Defender
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prove that you werent going 600mph
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ed
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Registered: 10th Sep 03
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[Edited on 09-09-2010 by ed]
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Ian
Site Administrator
Registered: 28th Aug 99
Location: Liverpool
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You need that in three dimensions ed 
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AlexW
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Registered: 25th Oct 08
Location: Essex
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quote:
Originally posted by 3CorsaMeal
i wonder if tyre wear accounts for anything, i don't know how much tread depth a tyre comes with, but your losing the few mm's twice on the overall diameter of wheel
From what ive seen yes, our van speedo is fairly accurate on new tyres, and gradually starts overreading as the tyres wear. Mind you, the tread is fairly deep on those tyres.
Also, its not about the size of the wheels, its the rolling radius, so if you had some 13inch wheels with massive tyres and then fitted 15inch wheels with tiny sidewall tyres, (Just an example, ive no idea if they are the same) then the rolling radius would stay similar and cause the speedo to read the same as it did on the 13s.
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ed
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Registered: 10th Sep 03
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C is the distance you've travelled, A is the descent and B would be the distance your GPS would see and work out your velocity from. That's the way I see it?
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Ian
Site Administrator
Registered: 28th Aug 99
Location: Liverpool
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That assumes the absence of lateral adjustment.
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