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[sir moanalot]

i went onto the "legals questions answered" guys slot (cheers dudes) and was unsure whether my speedo or my sat nav was more accurate, as my speedo was saying 69 and my sat nav was saying 64/5 so in a 60 zone i was sweating it a bit as i had passed a mobile camera van.
well i have contacted my companys fleet operator and found out nothing yet !!!!!  but 14 days is up tomorrow so a little bit more to sweat, anyone else sweating on a ticket?

[Paulus]

The satnav should be LOADS more accurate that a car speedo.

[Kevin Wood]

The satnav should be LOADS more accurate that a car speedo.

In ideal circumstances (good view of the sky, plenty of satellites locked, straight track and constant speed) GPS speed is spot on. Problem is, it becomes much less accurate as soon as you throw bends in the road, tree cover, etc. at it.

Useful to get an idea of how your speedo reads though.

Kevin

[rickyboy]

I get the odd camera flash when I'm facing the camera on the other side of the road.  You tend to find if it's a 30mph limit the camera will flash at 41mph on the speedo, if it's a 70mph limit then it will flash at 81mph on the speedo and so on.  Most speedo clocks are out by about 4-5mph and the law has to give a certain % of leeway when nailing you using speed cameras.

I wouldn't go testing this theory out though!!!  

[Tony H]

Its pretty common for speedo's to overead by about 10%

[Entwood]

GPS speed is highly accurate under CERTAIN CIRCUMSTANCES. GPS will measure the time between two points in 3 dimensional space to a high degree of accuracy... the problems are :

1. GPS makers do not tell us what the time interval is - so we have to assume it is quite small - The larger it is the bigger the errors that :

2. GPS works in straight line in 3D, so if you are on a bend, or a hill then you will have an error ( draw your "true" path on a piece of paper to see what I mean)

3. GPS cannot take acceleration or deceleration into account .. it works purely on the total time and total distance between sampling points.

4. A minimum of 4 good satellite signals must be present throughout the sampling period.

So .. if you are on a dead level, dead straight road, driving at a constant speed in an exactly straight line, throughout the (unknown) sampling period, with good satellite coverage, ... then your SatNav will give a good indication of your true speed.

Note the "dead level" requirement .... the surface of the earth curves ....  

(the last comment is slightly facetious ... the difference caused by the earth's curvature is negligible ... but hills DO make a difference ..)    

[Tony H]

GPS speed is highly accurate under CERTAIN CIRCUMSTANCES. GPS will measure the time between two points in 3 dimensional space to a high degree of accuracy... the problems are :

1. GPS makers do not tell us what the time interval is - so we have to assume it is quite small - The larger it is the bigger the errors that :

2. GPS works in straight line in 3D, so if you are on a bend, or a hill then you will have an error ( draw your "true" path on a piece of paper to see what I mean)

3. GPS cannot take acceleration or deceleration into account .. it works purely on the total time and total distance between sampling points.

4. A minimum of 4 good satellite signals must be present throughout the sampling period.

So .. if you are on a dead level, dead straight road, driving at a constant speed in an exactly straight line, throughout the (unknown) sampling period, with good satellite coverage, ... then your SatNav will give a good indication of your true speed.

Note the "dead level" requirement .... the surface of the earth curves ....  

(the last comment is slightly facetious ... the difference caused by the earth's curvature is negligible ... but hills DO make a difference ..)    

Not to good on altitude either, the gps on my boat will often give an altitude figure of up to 65 feet

[phil her up]

I get the odd camera flash when I'm facing the camera on the other side of the road.  You tend to find if it's a 30mph limit the camera will flash at 41mph on the speedo, if it's a 70mph limit then it will flash at 81mph on the speedo and so on.  Most speedo clocks are out by about 4-5mph and the law has to give a certain % of leeway when nailing you using speed cameras.

I wouldn't go testing this theory out though!!!    

I think the older gatso cameras with the square windows only work if you are on the same side of the road (flash you from behind) at least my GPS snooper dosent beep when i'm approaching with the camera on the other side of road :-/

[Entwood]

Altitude is interesting when it comes to GPS mapping... "cheap" mapping will make the earth a sphere, when we all know it is an oblate spheroid.

There is also - for boats - the problems of tide... the actual "surface" of the earth is moving vertically in this case !!

What actually happens.. on most systems, is the earth is asumed to be a "ball" of constant proportions with a "mean" fixed surface, all altitudes are given relative to that surface. Now, try getting a GPS company to tell you where/what that surface is set at ...... "Sorry sir, thats confidential" ...

I've never yet had an accurate "height" from a GPS when compared to a calibrated altimeter !!!

[Ian_D]

I get the odd camera flash when I'm facing the camera on the other side of the road.  You tend to find if it's a 30mph limit the camera will flash at 41mph on the speedo, if it's a 70mph limit then it will flash at 81mph on the speedo and so on.  Most speedo clocks are out by about 4-5mph and the law has to give a certain % of leeway when nailing you using speed cameras.

I wouldn't go testing this theory out though!!!  

How did you find out when the camera flashes? Or do you just collect speeding fines?  

[phil her up]

Altitude is interesting when it comes to GPS mapping... "cheap" mapping will make the earth a sphere, when we all know it is an oblate spheroid.

There is also - for boats - the problems of tide... the actual "surface" of the earth is moving vertically in this case !!

What actually happens.. on most systems, is the earth is asumed to be a "ball" of constant proportions with a "mean" fixed surface, all altitudes are given relative to that surface. Now, try getting a GPS company to tell you where/what that surface is set at ...... "Sorry sir, thats confidential" ...

I've never yet had an accurate "height" from a GPS when compared to a calibrated altimeter !!!

Hey entwood, have you swallowed a dictionary rocket scientist

[rickyboy]

Other side of the road buddy.  I drive for a living (around 300+ miles a day).  At the weekend part of my run is from Berwick to Edinburgh and the cameras on the other side of the road flash too late to catch you.  Very few cameras can catch you face on (in fact don't think any do).  The lines on your side of the road aren't for catching you they are for catching cars overtaking on the other side.

[LouisCorney]

I thought GPS could measure altitude - or is that just the military?

[Entwood]

GPS measures nothing, it just gives a 3D position in space, relative to the sattelites received

It is how it is mapped and reported by the inbuilt algorithms that matters. Military GPS can hardly be considered "cheap", especially when overlaid on a military standard terrestial map database.

Put these together and an extremely accurate 3D position is obtained...position and height.... not quite in the same ballpark as Mr TomTom & Mr NavTeq who can't even get roads in the right place ..let alone the hills !!!

[Danny]

i'm sweating on one if unless the 28th reaches me first, i passed a marked traffic car going in the opposite direction, i hit the brakes then released them to look less obvious but was still doing nearly 50 in the 40 stretch, when i looked in my mirror his blues were on but he never caught me up

couple more days...