Odo v GPS mileage

[Dave Hall]

I`ve decided to find out exactly how far out my odo is so I`ve taken to checking the trip mileage against the GPS for the last 500 miles or so.

Assuming the Odo is turning over consistently and that the GPS is very accurate I would have assumed the same difference every time but over 100 miles I`ve had odo readings ranging from 105 to 111.

I`m confused....can anyone shed any light.

 

[John Armstrong]

Garmin quote their distance measure's the distance between readings every second. If the DOP is bad this could affect the results of the GPs accuracy. Also if you're spending more time banked over then that would affect the odo's reading (based on what I've read about race bikes and the revs in corners).

HTH.

 

[MikeO]

Dave,

I'm guessing a bit here, but I think this might be it...

The GPS measures speed, and therefore distance, by taking a number of 'fixes' of your position, working out the distance travelled, comparing it with the time taken and thus working out speed. I don't know how often it does this, which is the critical bit.

If, between fixes, you have ridden a route other than a straight line, it will do the speed calculation using the straight line distance, indicating a lesser distance and, therefore, a lower speed. (If you imagine going round a hairpin bend, you could, if the 'fix interval' is long enough, only register a distance of a few metres, but have travelled some tens of metres around the curved path of the road. Your odometer will, of course, record your actual distance travelled over the ground (however innaccurately ).

This may be complete bollox, but would explain the variation in accuracy you report (in that the roads you ride will vary, as will the accuracy factor of the GPS, depending on how many satellites it can 'see').

Certainly I was told to only accept the speed indicated on a GPS after the bike had been travelling in a straight line for a while.

Mike

 

[Dave Hall]

If, between fixes, you have ridden a route other than a straight line, it will do the speed calculation using the straight line distance

Yes this seems to make sense when you view a track on Mapsource...when you travel slowly you get more points and less points when going for it on an A class road. Also the track never follows the road accurately but draws straight lines between the points.

I suppose the difinitive way to check would be to do some straight M/way several times and see if I can reproduce consistant results

Thanks for the input

 

[Mick Fagan]

The other problem is altitude changes. Apparently the sharp altitude change of a quick up or downhill movement, also throws out the distance configuration.

Height above sea level appears to be the achilles heal of cheap GPS units. By cheap I mean the normal ones that people are buying.

We recently had a talk from an expert on GPS systems and units and their applications. I was staggered to find out that a very accurate GPS system to run road manufacturing machinery on course costs upwards of $50,000 and some of the really good units are priced around $100,000. The apparent real difference is their ability to be really accurate on height above sea level. Something in the vicinity of 10mm either way is what they start to call accurate.

I think that the cheapest way out for odometer reading sis a push bike computer.

Mick.

 

[Howard Millichap]

With regard to the comparison between the track log and the speed calculation.
The track log interval can be altered on some GPS's. My E-Trex Vista can be either time or distance. So when trail riding I used time. ('coz your not going very fast) and when on the road I used distance. ('coz you are) The SP3 cannot be altered.
The general GPS positioning and speed calculations are fixed at 1 second. It will say so in the booklet. e.g. SP 3 Page 64 "Performance"
"Update Rate : once per second,continuous"


So I would say that the distance is pretty accurate. I'd certainly trust it more than the BMW (Veglia) speedo.

 

[MikeO]

So I would say that the distance is pretty accurate. I'd certainly trust it more than the BMW (Veglia) speedo.

Howard,

so how does that explain the variation in the difference?

Mike

 

[Howard Millichap]

Howard,

so how does that explain the variation in the difference?

Mike

As has been said, the rolling diameter alters according to lean angle. So, the diference would alter depending on the type of road being riden.

Of course, one thing that f#cks up the GPS measurements is a tunnel. The reading stops when satelite reception is lost and restarts when it is regained.

 

[Dave Hall]

The reading stops when satelite reception is lost and restarts when it is regained.

I wondered about this as when travelling through wooded lanes I would loose reception.
When this happened I noted the odo reading on the GPS and when the signal returned the odo reading would jump to approx. the distance you travelled without reception.
I assumed that the GPS would plot the route from where you were when the signal stopped and where you are when the signal resumed and adjusted the odo accordingly.

Wodjafink?

 

[MikeO]

I wondered about this as when travelling through wooded lanes I would loose reception.
When this happened I noted the odo reading on the GPS and when the signal returned the odo reading would jump to approx. the distance you travelled without reception.
I assumed that the GPS would plot the route from where you were when the signal stopped and where you are when the signal resumed and adjusted the odo accordingly.

Wodjafink?

Probably - but it would do it in a straight line - the bike would almost certainly have been further - these differences would add up (Dartford tunnel etc?)

Mike

 

[Dave Hall]

Probably - but it would do it in a straight line - the bike would almost certainly have been further - these differences would add up (Dartford tunnel etc?)

What if it referred to its map.? Either a loaded mapset or base map to plot the route you took and accurately guage how far you`d travelled.
Or am I giving it too much credit.

 

[MikeO]

What if it referred to its map.? Either a loaded mapset or base map to plot the route you took and accurately guage how far you`d travelled.
Or am I giving it too much credit.

I think you're giving it WAAAY too much credit LOL!

This is not a scientific lab instrument you've bought, but a fairly low-level navigation aid. I think you'll find the new 3610 (or the other one, whatever it's called) has a 'dead-reckoning' facility in the programming, but the SPIII doesn't. The kind of accuracy you apparently want in recording distance travelled isn't catered for (broadly because it's not required by most customers) by units at this level/price....

Mike

 

[Dave Hall]

I`ve been for a run out in the car today with my new magnetically roof mounted active ariel and did 50 mile through forests where I would normally loose signal.
It performed brilliantly and on returning home I found less than a tenth of a mile difference in odo`s. The car and GPS seemed to agree within about 0.1%...I find that impressive.

I`m going to fit it to the GS somehow and see if things are different.