Sat Nav Speed V Speedometer

scout353

Has anyone noticed that there is a big difference between the speed registering on your Sat Nav as against the speed registered on the speedometer in the car.

I often find that the car can be registering 135/140 km/hr while the Sat Nav (Tom Tom 920) says the speed is 120 km/hr.

Does anyone know which is the more accurate?

Given the nature of GPS tracking and the calculations required for time and distance I would have thought the Sat Nav would be more accurate - though I wouldn't like to try and argue this in court! :rolleyes:

robinph

The gps is far more accurate and the car speedos are deliberately set to read higher than you are actually travelling. Yes, there is +-X meters that you have to allow for in any GPS reading, but averaged out over the distance that your covering in the car that error can be ignored.

Someone in California did manage to overturn a speed camera fine that their son got whilst driving his car along the Freeway last year. He had fitted a GPS tracker to the kids car without him knowing, and then went and checked the tracklogs when the speeding fine came through and got it cancelled after they then found some problems with the calibration of the cops radar guns.

garydubh

scout353 wrote: »

Has anyone noticed that there is a big difference between the speed registering on your Sat Nav as against the speed registered on the speedometer in the car.

I often find that the car can be registering 135/140 km/hr while the Sat Nav (Tom Tom 920) says the speed is 120 km/hr.

Does anyone know which is the more accurate?

Given the nature of GPS tracking and the calculations required for time and distance I would have thought the Sat Nav would be more accurate - though I wouldn't like to try and argue this in court! :rolleyes:

The two measurement methods are different - one the rotation of a wheel and the other by the average change in position second by second.

The speedo accuracy will vary depending on tyre pressure etc and the GPS accuracy will depend on position accuracy, number of satellites and will have serious errors if position is lost. As the GPS draws straight lines between positions calculated every second, at 120km/hr such a line would be over 30 meters long and any bends/turns curves in that time are not considered. Also GPS calculates distance based on Lat/long and a given earth shape - not exatly the same as measuring distance along a road as is the case with the Speedo.

In other words - really there is no direct comparison! However, you have raised a point - how do you know your speedo is correct to comply with speed limits - where can to go to have it tested and calibrated and if such does not exist what percentage error can you be allowed in speeding cases as a result of the absence of testing and calibration???

scout353

Was in USA last week and had my Tom Tom and the speed on the unit corresponded almost exactly with speedo on car!

Given the above explanation and the fact that most roads in USA are reasonably straight this would seem accurate!!

Maybe manufacturers standards vary enormously too!!!!

Alun

garydubh wrote: »

The two measurement methods are different - one the rotation of a wheel and the other by the average change in position second by second.

Actually GPS receivers (and I mean the actual GPS receiver modules here, not complete GPS units like a nuvi or etrex) calculate velocity based on the observed changes (Doppler shift) in the frequency of the received satellite signals as the receiver moves relative to the satellites. They have to do this calculation anyway as part of their positional calculations, so velocity comes out as a sort of freebie ... there's no need to waste CPU power in the unit itself recalculating something that's already been done for you. If you do the maths, then the error in velocity works out much less than positional errors and is affected very little by bad satellite geometries or other errors that affect positional accuracy.

The "second by second" thing comes from the fact that the GPS modules themselves usually provide positional and other information to the part of the unit that updates the display, draws the map etc. at one second intervals.

Note: It's entirely possible that a consumer GPS unit could completely ignore the velocity information being provided to it by the receiver and work it out for itself, but there'd be little point really.