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Distance...what Is Correct?


pepperblues
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I have been using my GPS on my bike rides.

I wanted to use the tracks to calibrate my bike

odometer....

When I down load the track to my map software

and compare it to the

odm reading on the GPS they are different.

...and ofcourse the bike odm does not match.

 

Thoughts/comments appreciated.

pepperblues

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Actually just drove my van over 1500 kms and the GPS consistently registered less.....no idea why?

 

Mike

Does your car think you are going faster than your GPS does?

my car always thinks its going faster than my gps. i wonder why that is

For liability reasons most car speedos read faster than you are actually going. This way you can not blame the car manufacture for your speeding tickets.

 

Paul

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Even though they are really cheap, most bicycle speedos (odometers) can be fairly accurate.

 

The thing is, comparing one to your GPS is like apples and oranges. I would think that in most cases that the bike would show more mileage than the GPS. Normally riding a bike will include quite a bit of wobble and meander which would increase mileage. However the gps normally tracks it's mileage (as well as it's tracklog) by tracking waypoints at either set or variable intervals. Doing this will miss most of the wobble or even average out the track.

 

I first found this when I tried to map some of the trails I rode on my MTB. Really curvy sections of trail would appear to be nearly straight sections. To fix this I would set the acquisition interval to a really low time (~1 sec). After this the distance got closer.

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Actually just drove my van over 1500 kms and the GPS consistently registered less.....no idea why?

  With my eTrex, anyway, it only mesures distance while it has a satellite lock.  If there is a temporary loss of satellite lock, then any distance traveled during that time is not counted.

 

 

For liability reasons most car speedos read faster than you are actually going. This way you can not blame the car manufacture for your speeding tickets.

  I bet that the odometer is also off by the same amount as the speedometer, in the same direction.  So if the speedometer reads a higher than actual speed, then the odometer will almost certainly record greater miles than are actually travelled.

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my car always thinks its going faster than my gps. i wonder why that is

I have no actual known knowledge on this subject, but it is something my Dad said to me once. Feel free to call me an idiot on this one:

 

Another reason might be non-standard tire size. If the car is rated for a certain size wheel (14") and you use a different size (15") the rotation for the axel will turn a different computation for the odometer and speedometer.

 

Found this on the Candadian Tires Yahoo Site.

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Another reason might be non-standard tire size. If the car is rated for a certain size wheel (14") and you use a different size (15") the rotation for the axel will turn a different computation for the odometer and speedometer.

That's why, if you go to a bigger wheel, you generally go for a tyre with a smaller aspect ratio.

 

For example, a Mazda MX-5/Miata is approved for 185/60/14 or 195/50/15. That's a 14-inch or 15-inch wheel, with a tyre wall height of (185x0.60) or (195x0.50) millimetres. Turns out that (195x0.50x2)-(185x0.60x2) is 27, and one inch is 25.4mm, so they're pretty close (especially since you have 6-7 mm of wearable rubber x 2).

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I have been using my GPS on my bike rides.

I wanted to use the tracks to calibrate my bike

odometer....

When I down load the track to my map software

and compare it to the

odm reading on the GPS they are different.

...and ofcourse the bike odm does not match.

 

Thoughts/comments appreciated.

pepperblues

Ahhhh... Do you suppose that odometer on the bicycle registers the distance going up and down hills, where the gps considers everything to be flat?

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Do you suppose that odometer on the bicycle registers the distance going up and down hills, where the gps considers everything to be flat?

Yes it does, but if the hills are steep enough for this to be a significant source of error you're going to be walking the bike instead of riding. [The extra distance from including the slope when climbing the Kaibab trail out of the Grand Canyon only adds about 1% to the distance.]

 

Bicycle odometers are inherently very accurate since they just count wheel revolutions and calculate the distance from that - same principle as used by surveyers on trails who use measuring wheels. But you need to enter an accurate distance for the bicycle wheel circumference. One way is to put a drop of ink on one spot of the tire and then ride the bike a few revolutions of the wheel. Measure the distance between marks and use that to enter the right calibration number into your odometer.

 

Instructions for most common models and also circumference values for most tire sizes can be found at:

http://www.sheldonbrown.com/cyclecom_calibration_proc.html

 

When properly calibrated and installed, the bicycle odometer should be accurate to a fraction of a percent making it more dependable than either a GPS receiver or a car odometer.

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Another reason might be non-standard tire size. If the car is rated for a certain size wheel (14") and you use a different size (15") the rotation for the axel will turn a different computation for the odometer and speedometer.

 

Actually you don't need a non-standard tire size. The actual distance per rotation is calculated with a dynamic tire radius. This radius depends on wear of the tire, tire pressure, weight and CG of the vehicle and even the speed (centrifugal forces). As the speedometer must not show less speed than the true speed it's likely that it shows too much most of the time. The GPSr has a more direct way to get the distance and the systematic errors tend to decrease the distance reading. Hence the distance reading is expected to be smaller for the GPSr.

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Simple geometry. You GPSr computes distqance as a series of "small" individual straight lines. Therefore the odometer is most likely closer

 

Try this. Draw an arch. Draw a series as line connected points along that arch. Compare the length of the two. The length of the arch will always be longer.

 

Now consider that the above is a flat (no elevation) example. Add changes in elevations, causing a second set of in dependent arches. The error just compounded on you.

 

Forget the mechanical erro in the size of a tire. Yes this may be important if your are doing properity surveying (andyes it is at that pint) but for what we do compaired to the error on how the distance is computed (not arch length but a sum f chord lengths) is far grater.

 

Wake up, smell the what ever and realize that this error is still not bad adnask your self are they acceptable.

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I use my gps for auto routing among other things (like geo caching) and what I have noticed is the GPS always shows things as further than the other estimates. As an example I will be driving down the highway and the sign says my exit is 1.5 miles away and my GPS says it is 2 miles away.

 

I have learned to trust my GPS.

 

I can sit in my backyard and save the location as a waypoint and later return to it and have my GPS tell me I am < 10ft away. I can't say the same for my speedometer which I know is intentionally made inaccurate by the manufacturer to show me as going faster than I am. I can't say the same for the road signs which are placed by people who couldn't care less about what they are doing. I can't say the same for the folks submitting data to the map makers like census workers. A GPS using WAAS is accurate to within less than 10ft under good conditions. I know this to be true from personal experience as well as the literature. As such I trust it above any other source.

 

The GPS isn't motivated by legal concerns nor is it subject to laziness or human error. Nearly everything else is.

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There seems to be more votes for the bike odm.

 

I have used both the set calibration number for my

bike odm and used the dynamic wheel rotation distance

to calibrate. Not a big difference.

I'm going to accept the bike odm, and just get on

with riding. { ;-))))

As with most things in life,

none of this is an exact science.

 

As to GPS speed calculation and car speedometer,

on my Honda Civic, they both report the exact same

speed.

 

Thanks for your thoughts/comments.

pepperblues

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Before I purchased my GPS, I changed the tires on my Truck to larger tires and a gear to adjust my Speedometer, never got around to changing the gear. When I bought my GPS I was surprised that my Speedometer and GPS always agreed. Well, 5 years and 100 000Km later it was time to change my bald large tires, due to money and wearing out my offroad tires on the highway, I went back to my original tire size. Now my Speedometer is 5Km/hour slower then the GPS, go figure. When I checked the label for the recommended tires for my truck, it calls for even smaller tires then what was on it when I bought it (I find the current tires look small on my 4X4). I figure they were the original tires and was an "at the dealer upgrade" since I had 76 000km on almost worn out tires upon purchase with a matching new spare and a date code matching the year of the truck. I trust my GPS speed more then my vehicles speedometer, our car is the opposite if crusing at 110Km/h, your really almost doing 125 - 130 which will get you a ticket so I use the GPS to set my speed.

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I am surprised that so many are dismissing the effect of hills??!!!? even a low hill of 10 foot height above surrounding terrian over 1/10 of a mile is going to make the gps give a different (lesser) reading than the bike. [draw a picture - you will travel over 2 legs of a triangle - by definition that is longer than traveling over any 1 leg] Also many others have pointed out that track logs are points in time that tend to straighten out a semi-wandering path.

 

Having said this, my Garmin 60C uses a 3D tracklog to take into account elevation changes but still uses a series of fixed points.

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I am surprised that so many are dismissing the effect of hills??!!!? even a low hill of 10 foot height above surrounding terrian over 1/10 of a mile is going to make the gps give a different (lesser) reading than the bike.

Well, lets do the math. According to the pythagorium theorum, a rise of ten feet over a run of 528 feet (1/10 mile) will give you a slope length, which IS the actual distance travelled of about, oh, let's see, 528.095 feet. Just a little more than an inch in a tenth of a mile. Sounds pretty inconsequential to me. B)

 

GeoForse

Edited by GeoForse
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I've found that both of my cyclometers (one on my road bike, one on my mtb) agree fairly well with both GPS and car odometer (eTrex yellow and 2002 Dodge Grand Caravan)

The GPS agrees with the car odometer to within about .1 miles per 100... (yeah, I was bored, it was a long car trip!)

I've discovered that when I've got my differental beacon reciever hooked up to the gps, the gps mileage goes up... ever so slightly, maybe about .01 miles per 25

 

Happy Caching

Jeff

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