Thread: Which is correct..odometer or GPS

The GPS will be correct everytime short of any long gaps in signal reception.

If the odometer is showing less kms travelled than the GPS then how accurate is the speedo?
If the odometer and speedo are connected then your speedo will be under reading. Which means you might get a speeding ticket.

Usually it is the other way around. Your odometer would read more than the GPS as that is how they are set up at the factory.

Do you have oversize tyres on your car?

yep normally its the other way around........well it is with the defender running standard tyres

Originally Posted by stevo68

. At next fill up the gap is around 10-12kms.

Regards

Stevo

<2% error, do you really give a toss?

GPS wins every time

The GPS will be correct everytime short of any long gaps in signal reception.

If the odometer is showing less kms travelled than the GPS then how accurate is the speedo?
If the odometer and speedo are connected then your speedo will be under reading. Which means you might get a speeding ticket.

Usually it is the other way around. Your odometer would read more than the GPS as that is how they are set up at the factory.

Do you have oversize tyres on your car?

Thanks for that, the speedo and km/hr on the GPS seem pretty accurate, have checked that on numerous occasions. Have just filled up today and going to see what the difference is this time. As for tyres, they are stock standard at the moment.

Quote:
Originally Posted by stevo68
. At next fill up the gap is around 10-12kms.

Regards

Stevo

<2% error, do you really give a toss?

GPS wins every time

Its not so much as giving a toss, it is more out of interest. I have little to no knowledge on these things hence posting the question. I would have thought that a difference of 10-12kms would have been unusual?

Regards

Stevo

There are two questions involved - the accuracy of the odometer, and the accuracy of the GPS.

1. Odometer. The odometer should be consistent, that is, the same percentage error unless either the driving style involves a lot of wheelspin, or the drive member is slipping (which is quite possible on most Landrovers!). As stated above, most cars are set so the speedometer, and hence the odometer, read higher than the correct figure, because the legal requirement allows over but not underreading. Reports on this forum suggest a typical Landrover figure is 8%. Non standard tyre size will change this, oversize tyres reducing the over reading and potentially even going to underreading. Tyre pressure will have almost no effect - particularly with a radial tyre, pressure does not change the circumference of the tread.

2. GPS. It needs to be remembered that a GPS measures position, and the distance travelled (as well as the speed) is a derived function. In particular, the distance travelled, if it follows the jitter* in the position, can easily result in a longer GPS distance. Now the software certainly applies some smoothing to eliminate the problem, but if excessive smoothing is applied, it can round off real corners. I have no feel for how effective the actual algorithm used is in practice, but I find it easy to envision it over reading. Some software may have options as to how much smoothing is applied, and if this is available in your case, try experimenting with it.

*jitter is the apparent change in position due to errors in determination of position. How large it is depends mainly on the geometry of the satellites used and any poor reception, for example due to a partly shadowed antenna.

It might also be useful to compare GPS and odometer distances with the map distances (although I can think of a number of places where these are wrong - for example a couple of years ago I checked the map distance from Yass to Cowra - three maps, three different figures by up to over 10km; they can't all be right!)

John

I have to wonder about traversing hilly country.

Let's assume it's a dead straight but hilly road.

An odometer will measure the distance travelled by the wheels and thus the car as it goes up and down hills.

I assume the GPS measure distance from point A to point B but will it measue the distance travelled going up and down.

An extreme example would be driving up and down a road that looks like sine wave in cross-section.


Where's RMP when you need him?

A GPS position is not only latitude and longitude but altitude as well. So it knows if you are going up or down a hill. Some units are better than others for altitude (longs and lats too).

A GPS track is just a series of points. You can change your GPS to create points by time or distance. You can set the time or the distance.
In older units where track points are limited you would often get dodgy tracks as to get a longer track you sacrificed detail.
Newer units have bigger memories so you can use more points for a given track.
Depends on what you are doing as to what is the best choice.

This is interesting
Testing the Accuracy of Autonomous GPS in Ground Speed Measurement

A hand-held Garmin eTrex Venture Global Positioning System (GPS) receiver operated in autonomous mode was used to investigate the accuracy of autonomous GPS in measuring ground speed. The accuracy of GPS-derived ground speed was tested by collecting and storing in a laptop computer, every 10 sec, ground speed data from the receiver at eight different ground speeds of a vehicle. The targeted ground speeds involved were 5, 10, 15, 20, 25, 30, 40 and 50 km h-1. The vehicle's speedometer was used as an initial indicator of the ground speed. A pulse transmitter was utilized to detect the frequency, for every 10 sec, of a magnet fixed on one of the vehicle's wheels and passing by the transmitter every full rotation of the wheel. Data from the pulse transmitter was acquired and stored in a data logger. Given that the wheel's circumference was known, frequencies were used to calculate the ground speeds which were used as a reference. By the use of matching times of GPS and data logger data records, GPS-derived ground speeds were compared against the reference speed measurements. For nearly 600 data points comprising the data set for the eight test ground speeds, it was found that the error in GPS-derived ground speeds was, on the average, 1.27 km h-1 (less than 7%.) However, high fluctuations in vehicle's ground speed when, for example, turning greatly impacted this error. An error of -80.16% was produced due to a drop of vehicle's ground speed from 18.65 to 11.19 km h-1. Results of steady state ground speed analysis revealed that the average error was less than 1 km h-1 (less than 5.3%), except for the 15 km h-1 data set where the average error reached 1.72 km h-1 (9.92%.