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How Accurate is the Odometer on my 60CSx?


murs

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I've been riding my bike with my 60CSx and comparing the trip odometer on my bike computer. I've determined that the GPS measures about .25-.3% more than my bike computer. Before I re-calibrate my bike computer, I wanted to know if anyone has verified the accuracy of their GPS.

 

FWIW, I determined today that the distance between 9.99 miles and 10.1 is only .6 miles meaning that when it goes from 1/100th mile precision to 1/10th mile, it rounds up to the nearest .1 mile.

 

-murray

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I've been riding my bike with my 60CSx and comparing the trip odometer on my bike computer. I've determined that the GPS measures about .25-.3% more than my bike computer. Before I re-calibrate my bike computer, I wanted to know if anyone has verified the accuracy of their GPS.

 

FWIW, I determined today that the distance between 9.99 miles and 10.1 is only .6 miles meaning that when it goes from 1/100th mile precision to 1/10th mile, it rounds up to the nearest .1 mile.

 

-murray

Do the current speed readings match up when you're riding at a steady pace? If so, I wouldn't worry about it. But it they're different, I'd adjust the bike computer to match the GPS.

 

When you set up the bike computer, did you do a roll-out test? Since tires and rims vary from manufacture to manufacture, that the only way to do it. It's best done with two people and a good tape measure or yard stick. Position whichever tire has the sensor on it so that the stem is at the bottom. Mark that spot on the pavement. Sit on the bike, and roll straight ahead exactly 10 revolutions, so that the stem is again at the bottom. Mark that spot. Measure the distance between the the two spots, and divide by 10, and you'll have your true rolling tire circumference. You should be able to look that number up in your computer's owner's guide to find the correct size setting for that tire.

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Do the current speed readings match up when you're riding at a steady pace? If so, I wouldn't worry about it. But it they're different, I'd adjust the bike computer to match the GPS.

 

No offense, but looking at the current speed readings seems like the least accurate method I could think of to confirm the accuracy of either unit.

 

When you set up the bike computer, did you do a roll-out test? Since tires and rims vary from manufacture to manufacture, that the only way to do it. It's best done with two people and a good tape measure or yard stick. Position whichever tire has the sensor on it so that the stem is at the bottom. Mark that spot on the pavement. Sit on the bike, and roll straight ahead exactly 10 revolutions, so that the stem is again at the bottom. Mark that spot. Measure the distance between the the two spots, and divide by 10, and you'll have your true rolling tire circumference. You should be able to look that number up in your computer's owner's guide to find the correct size setting for that tire.

 

Yeah, I've done a "roll-out" calibration where I put a paint dab on the tire and measure the distance between 3 marks it leaves on the floor as I ride.

 

I'm just wondering if anyone has done anything to confirm the accuracy of the odometer. I'll likely try this myself between mile markers on the highway, though that is at a steady speed without any turns.

 

As for trusting the GPS more than the bike computer, what would that be based on? I know the bike computer is very precise and repeatable (within a couple hundredths of a mile over 20 miles), but the accuracy depends on the calibration.

 

Bottom line, it's not really that big of a deal, I was just curious.

 

-murray

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I've been riding my bike with my 60CSx and comparing the trip odometer on my bike computer. I've determined that the GPS measures about .25-.3% more than my bike computer. Before I re-calibrate my bike computer, I wanted to know if anyone has verified the accuracy of their GPS.

 

FWIW, I determined today that the distance between 9.99 miles and 10.1 is only .6 miles meaning that when it goes from 1/100th mile precision to 1/10th mile, it rounds up to the nearest .1 mile.

 

-murray

You do know that the GPS has an error when it reports your position, right? Everybody seems to get this concept. Why is it that people expect an accurate number when the GPS adds up the distance between these positions to make a pseudo odometer? Tire measurements are extremely accurate compared to a GPS.

 

Also, if you go horizontally one mile and climb 100 feet the GPS ignores the extra distance you travel because you are climbing.

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Of course if you only climb 100 feet in a mile your bike computer will also ignore it because the extra distance will be less than the precision of a wheel based unit which measures in hundredths of a mile can deal with.

If you compare all the data from both your GPS and your bike computer (odometer, ave speed, max speed, etc) and everything is within a couple of percent then you are probably ok.

The GPS will cut off corners on the odometer reading, the wheel sensor will skip a rotation occasionally. Neither is perfect 100% of the time.

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Of course if you only climb 100 feet in a mile your bike computer will also ignore it because the extra distance will be less than the precision of a wheel based unit which measures in hundredths of a mile can deal with.

If you compare all the data from both your GPS and your bike computer (odometer, ave speed, max speed, etc) and everything is within a couple of percent then you are probably ok.

The GPS will cut off corners on the odometer reading, the wheel sensor will skip a rotation occasionally. Neither is perfect 100% of the time.

 

The GPS odometer is always a bit more than the bike computer. From what you're saying, the GPS should be slightly less due to cutting corners, not counting ascent, etc.

 

As I said, it's no big deal, just wondering if anyone had checked the accuracy. Being an engineer, I key in on things like accuracy :P

 

-murray

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...You do know that the GPS has an error when it reports your position, right? Everybody seems to get this concept. Why is it that people expect an accurate number when the GPS adds up the distance between these positions to make a pseudo odometer? Tire measurements are extremely accurate compared to a GPS.

...

 

Bike Tire Error is systematic. Over time it's always off exactly the same. (Ignoring tire wear).

GPS error is more random. That means over time it averages out. I'm not sure if there is a systematic error to GPS error or not.

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Do the current speed readings match up when you're riding at a steady pace? If so, I wouldn't worry about it. But it they're different, I'd adjust the bike computer to match the GPS.

 

No offense, but looking at the current speed readings seems like the least accurate method I could think of to confirm the accuracy of either unit.

My point was that the if you're speeding up or slowing down, the GPS reading will always lag behind the computer. So under fairly common circumstances, it would appear that the GPS is giving faster or slower readings. You need to be going at as steady a pace as possible to really check them against each other. If your computer has a cadence read-out, that would help maintain a constant speed.

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...You do know that the GPS has an error when it reports your position, right? Everybody seems to get this concept. Why is it that people expect an accurate number when the GPS adds up the distance between these positions to make a pseudo odometer? Tire measurements are extremely accurate compared to a GPS.

...

 

Bike Tire Error is systematic. Over time it's always off exactly the same. (Ignoring tire wear).

GPS error is more random. That means over time it averages out. I'm not sure if there is a systematic error to GPS error or not.

Exactly! The systematic error can be determined to a very high degree of accuracy by riding a calibrated distance(the longer the better). The round off error per revolution can be determined much much more accurately than the rough plus or minus .01 mile used to set the wheel circumference in the computer.
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Here's one method of checking the GPSr that comes to mind. Find somewhere where you can go straight and level for as long as possible. Reset the GPSr odometer and create a waypoint when you start, then ride some distance (a mile would be great). When you stop create another waypoint. The distance between the waypoints should match the odometer reading, if not, the odometer reading is wrong. Now that would be a best case scenerio - the GPSr should be most accurate in a straight line where corners, curves and hills aren't cut. You should expert even worse accuracy of the odometer in real terrain. You can also compare the bike computer to the waypoint distances, and calibrate your bike computer accordingly. Basically, the GPSr odometer will accumulate errors - it has to add up the differences between many samples. The error between waypoints is never more than twice the GPSr accuracy, no matter how far apart they are (talking level ground of course).

 

Dan

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Murray;

Are there any "serious" road running events in your neighbourhood? The course will most likely be measured quite accurately. If you know the exact location of the start and finish line for say a 10K run, biking through the course would give you an /indication/ of computer and GPS accuracy. As Dan points out, the straighter the course the better.

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I've done a number of hikes over well documented routes and I always find my CSx says I've walked a few more miles than is documented, even if I know the route well and don't vere off at all. I've always assumed that this is due to the inaccuracy of GPS. If you walk from point A to B the GPS may register you being 6' left of point A and 9' right of point B. It therefore thinks you have moved further than you actually have. I would have expected the altitude discrepancies mentioned above to have compensated for this, as I tend to walk in pretty hilly areas.

If the distance is exagerated then the speed will also be exagerated, as speed = distance/time. The faster you travel the less significant the error will be though.

If this is true then the inaccuacy in speed will be greater for less accurate GPSr's.

Someone will probably tell me that the GPSr is coded to compensate for this inaccuracy. :laughing:

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I've done a number of hikes over well documented routes and I always find my CSx says I've walked a few more miles than is documented, even if I know the route well and don't vere off at all. I've always assumed that this is due to the inaccuracy of GPS. If you walk from point A to B the GPS may register you being 6' left of point A and 9' right of point B. It therefore thinks you have moved further than you actually have. I would have expected the altitude discrepancies mentioned above to have compensated for this, as I tend to walk in pretty hilly areas.

If the distance is exagerated then the speed will also be exagerated, as speed = distance/time. The faster you travel the less significant the error will be though.

If this is true then the inaccuacy in speed will be greater for less accurate GPSr's.

Thanks. You said it better than I could.
Someone will probably tell me that the GPSr is coded to compensate for this inaccuracy. :laughing:
Some coders use the acronym GIGO. garbage in => garbage out. IMHO on the trail, there is no compensation in the GPS. In the car, snap to road might help.
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My point was that the if you're speeding up or slowing down, the GPS reading will always lag behind the computer. So under fairly common circumstances, it would appear that the GPS is giving faster or slower readings. You need to be going at as steady a pace as possible to really check them against each other. If your computer has a cadence read-out, that would help maintain a constant speed.

 

I was trying to point out that a) Current speed is a dynamic number and thus more difficult to check :laughing: As you point out, it lags on both the GPS and bike computer c) The precision is is lower (0.1 mph on the GPS, 0.5 mph on my computer) than distance measurements.

 

FWIW, I've done several rides up to 46 miles and the GPS consistently shows around .25-.3% more miles traveled than my bike computer. I've compensated for the lower precision of the GPS by watching for it to click over a 10th of a mile and checking the bike computer mileage.

 

-murray

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Something that might be interesting to try is to change the tracklog settings to record by time. I changed mine to do some accuracy tests and set it for every 5 seconds. When I took the same walk last night that I've done a lot recently, the distance was about 1.20 mi, all my previous readings have been about 1.38 mi when the tracklog was set to auto. I'm going to try it again to see if it was just a fluke.

 

When I checked the distance on Google Earth, I came up with 1.18.

 

VT

Edited by VTtrader
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...You do know that the GPS has an error when it reports your position, right? Everybody seems to get this concept. Why is it that people expect an accurate number when the GPS adds up the distance between these positions to make a pseudo odometer? Tire measurements are extremely accurate compared to a GPS.

...

 

Bike Tire Error is systematic. Over time it's always off exactly the same. (Ignoring tire wear).

GPS error is more random. That means over time it averages out. I'm not sure if there is a systematic error to GPS error or not.

Exactly! The systematic error can be determined to a very high degree of accuracy by riding a calibrated distance(the longer the better). The round off error per revolution can be determined much much more accurately than the rough plus or minus .01 mile used to set the wheel circumference in the computer.

True, however random error is self correcting. That's the GPS.

 

Someone else made a point about a documented course. If you follow the exact trail yes, it's an exact distance. If you cut corners, extend corners, meander back and forth...you won't be exactly on course and your milage will vary. Nobody rides in a perfectly straight line.

 

It's like someone walking with dogs. You walk a mile. The dogs 2 because they meander more.

 

The only way the GPS should be off is that it's got a systematic error that day. Anyting else should average out. (Changed my mind somewhat about this, see below).

Edited by Renegade Knight
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...Someone will probably tell me that the GPSr is coded to compensate for this inaccuracy. :laughing:

Inaccuracy in a GPS is from error introducted by variations in the GPS signal. The signal is distorted for variouse reasons making the assumptions used in the calcuations the GPS makes to determine it's possition wrong to a small extent. That extent is 10-20' normally. WAAS can help correct for these errors by supplying a correction factor for the GPS to use. WAAS uses a base station. The base station knows where it is. WAAS does the calcuation checks the difference and supplies the correction to the GPS. The GPS then uses that information for a more accurate location.

 

When you stand still, you can see the error as "bounce" as the GPS bounces around. This is especially noticable when you are standing on ground zero looking for a cache and watching the pointer.

 

Bounce can be seen by the GPS as movment. The faster you go the less this is a factor in your GPS speed and odomoter function. Since the error is random and since the bounce would follow a pattern over time, a GPS maker could factor in a correction to your speed and distance.

 

That said, it's probably time I retract a bit of what I have said before. Bounce in a GPS can impact what you are reading if you are looking at something to the 0.25% range. So can the meander off the line on a bike compared to a measured course. The question becomes how accurate do you need to be? The bikes wrong due to meander, the GPS is wrong and both are pretty freaking close. Better than your car odomoter.

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That said, it's probably time I retract a bit of what I have said before. Bounce in a GPS can impact what you are reading if you are looking at something to the 0.25% range. So can the meander off the line on a bike compared to a measured course. The question becomes how accurate do you need to be? The bikes wrong due to meander, the GPS is wrong and both are pretty freaking close. Better than your car odometer.
I'm getting kind of picky now, but the odometer on the bike is "right" because it tells you how far you traveled. If you compare the odometer to the calibrated course length they may be different because of meander, which shoots down my calibration method.
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I've done a number of hikes over well documented routes and I always find my CSx says I've walked a few more miles than is documented, even if I know the route well and don't vere off at all. I've always assumed that this is due to the inaccuracy of GPS. If you walk from point A to B the GPS may register you being 6' left of point A and 9' right of point B. It therefore thinks you have moved further than you actually have. I would have expected the altitude discrepancies mentioned above to have compensated for this, as I tend to walk in pretty hilly areas.

If the distance is exagerated then the speed will also be exagerated, as speed = distance/time. The faster you travel the less significant the error will be though.

If this is true then the inaccuacy in speed will be greater for less accurate GPSr's.

Someone will probably tell me that the GPSr is coded to compensate for this inaccuracy. :(

 

I have had similar experiences with my GPS 60csx. Several months ago I was on a hike with my son, a hike I had done many times, my GPS gave me a total travel distance that was 3.8 miles longer for a 6.5 mile hike. My conclusion at the time was that due to the accuracy issue and the fact that the GPS was tracking ALL motion (forward and back) not just A to B. In fact, while we sat and ate lunch we managed to go almost a quarter mile sitting on our butts. This past weekend though I was on a hike with two other people with GPS's. For a 5.5 mi hike (based on park maps and trail markers) I had over 8 miles of travel. The other fellas has numbers closer to the stated length of the trail but the spread between the three was really surprising. I would have thought that if sats were the cause of the inaccuracy that it would have been more consistent across the machines. My GPS was the most expensive of the three but it certainly wasn't the most accurate or, maybe it was... Is it possible that it is so accurate, that it is measuring travel from arm swing and other "ambient" motions? Finally, I though that GPS (at least the 60csx) did account for vertical travel, I suppose I could test this by taking readings at the bottom of a building then ridding the elevator to the top and getting a reading. the area we were hiking is very hilly (cliffs actually - Smith Rock State Park) if only horizontal distance is measured then we were all really off, we did over 1400 ft of elevation change in less than a mile!

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