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-   -   Is GPS MPH accurate (http://forums.sv650.org/showthread.php?t=137112)

AndyW 11-08-09 11:41 AM
Re: Is GPS MPH accurate
 
Quote:
Originally Posted by flymo (Post 2000647)
A little tongue in cheek perhaps but with an element of serious point there too. Accuracy is a relative term.

Under most conditions I would expect a vehicle GPS unit to be more accurate than a typical speedo in terms of what is displays to you...
The GPS needs to take account of height. If you are going up a 1:4 on a twisty switchback road, distance driven != distance between points.
For an (unfortunatly impossible) example to illustrate, if you drove vertically upwards, the GPS would be showing a speed of 0, whereas you'd be exceeding escape velocity at 11.2 km/s

flymo 11-08-09 12:04 PM
Re: Is GPS MPH accurate
 
Quote:
Originally Posted by AndyW (Post 2000821)
The GPS needs to take account of height. If you are going up a 1:4 on a twisty switchback road, distance driven != distance between points.
For an (unfortunatly impossible) example to illustrate, if you drove vertically upwards, the GPS would be showing a speed of 0, whereas you'd be exceeding escape velocity at 11.2 km/s
That may be true but its not real world most of the time in the UK is it?

AndyW 11-08-09 01:38 PM
Re: Is GPS MPH accurate
 
Quote:
Originally Posted by flymo (Post 2000843)
That may be true but its not real world most of the time in the UK is it?
Lots of hills in Wales and Lake District sufficiently steep to affect reading, though by how much is open to the more mathematically minded
Scratches head and tries to remeber math from 30 years ago:
sketch as right angled triangle, distance travelled = hypot, distance between points ignoring height = adjacent, theta = angle of incline
cos theta=a/h
we want difference between a and h as a fraction to multiply the GPS speed reading by...
erm, I give up I need more coffee to try and work this out without drawing it out.

flymo 11-08-09 01:41 PM
Re: Is GPS MPH accurate
 
ok, so putting aside this as a suitable measure for mountian rescue rovers :-), generally speaking it would be a fairly accurate system in comparison to a typical speedo that hasnt been calibrated.

-Ralph- 11-08-09 03:37 PM
Re: Is GPS MPH accurate
 
Vehicle GPS is only really any good at steady speed in a straight line. Try measuring your speed with one when braking from 60 down to a 30 limit and see if you arrive at the sign knowing for sure what your speed is. They will not show the right speed until you've been at 30 for 50-100yds. Once you are steady and straight though, they are accurate to 1 mph or so, which is good enough for me and better than a speedo.

hindle8907 11-08-09 03:44 PM
Re: Is GPS MPH accurate
 
ok well i was on the motorway in a straight line for around 2 minutes and my speedo showed 102 and my gps showed 104 so this means my speedo is nearly bang on .

MattCollins 11-08-09 05:26 PM
Re: Is GPS MPH accurate
 
Quote:
Originally Posted by AndyW (Post 2000821)
The GPS needs to take account of height. If you are going up a 1:4 on a twisty switchback road, distance driven != distance between points.
Considering GPS is 3 dimensional that'd be an interesting one to put to the test.

AndyW 12-08-09 07:49 AM
Re: Is GPS MPH accurate
 
Quote:
Originally Posted by MattCo (Post 2001258)
Considering GPS is 3 dimensional that'd be an interesting one to put to the test.
Just because it can doesn't mean it does. I'll bet loads of someone elses money everything gets turned into latitude and longitude and that is what is used to calculate positional info, not the raw positional data.

keith_d 12-08-09 08:10 AM
Re: Is GPS MPH accurate
 
IIRC the Americans insisted on adding +/- 10m of positional inaccuracy to the civilian GPS signal, which doesn't bode well for measuring low speeds.

However, my Speedo is legally required to be wrong to allow for variations in tyre size and drive train components.

So I'd be willing to bet my Tomtom is more accurate than my speedo for straight line speed, though at a track a regular GPS wouldn't stand much chance.

Keith.

MattCollins 12-08-09 08:12 AM
Re: Is GPS MPH accurate
 
AndyW, That's why I said that it 'd be an interesting one to put to the test... I don't know... and wouldn't bet against you.

:)


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