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What to believe - computer or maps
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 2008-09-28 11:01 AM |
Elite  7783      PEI, Canada |  Subject: What to believe - computer or mapsI just got home from the longest ride I've done on my new bike (Cervelo Soloist, WOOHOO!). My question is this - my computer (Sigma wireless 1606) tells me that the total distance was 63.04 km but when I plot the route out on the maps on this site (tried another one too) it says the distance was 62.0 km? I double checked that my computer is set for the proper tire size. So, what do I believe and record in my logs? Is this normal for there to be that much of a discrepancy? |
| 2008-09-28 11:05 AM in reply to: #1701644 |
Champion 5575 Butler | Subject: RE: What to believe - computer or maps I would say your computer is more accurate. The problem with ploting a course on mapping software is small variances of where you turned etc. can cause small distance variances and over a long ride that can add to a difference (if that makes any sense. Either way you are talking less than 1km which is not that much differnce anyway. |
| 2008-09-28 3:16 PM in reply to: #1701644 |
Champion 6999 Chicago, Illinois | Subject: RE: What to believe - computer or maps well its a 1.6 percent error assuming the map is correct. That is not much of margin of error. chance are the course will be closer to 62 km but chances are you bike more about 63k because people can not bike/ run the tangents perfectly. Even with the tangents painted on the ground at bejing the elites at the beiling marathon were not following them very well. I would just log the 63 km. Ulimately it does not matter since the difference is so small. Edited by chirunner134 2008-09-28 3:17 PM |
| 2008-09-28 4:02 PM in reply to: #1701644 |
Pro 6767 the Alabama part of Pennsylvania | Subject: RE: What to believe - computer or maps I think you should go back out there with one of those wheels they use to measure distance for runners, and walk the route so you get the most accurate reading possible. Be sure to use a GPS unit as well to cross reference it. |
| 2008-09-28 4:53 PM in reply to: #1701846 |
Extreme Veteran 307 | Subject: RE: What to believe - computer or maps chirunner134 - 2008-09-28 3:16 PM well its a 1.6 percent error assuming the map is correct. That is not much of margin of error. chance are the course will be closer to 62 km but chances are you bike more about 63k because people can not bike/ run the tangents perfectly. Even with the tangents painted on the ground at bejing the elites at the beiling marathon were not following them very well. I would just log the 63 km. Ulimately it does not matter since the difference is so small. What a great explanation! Thanks. This answers a question I've been having when my hubby & I bike together. DUH! I guess I never thought it through very carefully. |
| 2008-09-28 6:29 PM in reply to: #1701644 |
| Extreme Veteran 422 | Subject: RE: What to believe - computer or maps Well, the tangents might be part of it. How did you calibrate your bike computer, do you roll out 2 or 3 full rotations of the front tire, while inflated to the pressure you normally ride at, with you on in your normal riding position? If not I would say your bike tire circumference has most of the error and not a straight line is a smaller part. Remember you need to be on the bike, cause your weight will cause the circumference to be a bit smaller. |
| 2008-09-28 6:31 PM in reply to: #1701644 |
| Elite 3650 Laurium, MI | Subject: RE: What to believe - computer or maps also remember that maps are vulnerable to errors from resolution and georeferencing. The map may or may not factor in elevation. If it doesn't, then that can introduce quite a bit of error as is. Think about a triangle where the base is what the map shows and the hypotenuse is the path you take. The steeper the grade, the larger the difference between mapped distance and actual distance. If the map does account for elevation, it is limited to a particular resolution. The elevation may have a grid spacing of 100m (arbitrary number). This means that elevation change for the grid cell will be the net change between the edges of the cell. If there is a big ascent followed by a descent which is less then 100m (or the grid spacing), it won't be fully accounted for. Then we have georeferencing, which is the crux of all mapping. A map is a flat version of a round object. To make it work, you have to project the curved map to a flat version. Remember the different map projections from highschool geography? What it amounts to is that the distance between two points on a map aren't necessarily the same distance as in the real world. This is also why google maps satellite imagery doesn't always match the map. The satellite image is of a round earth that was georeferenced to the map, and the process isn't perfect.
The only truely accurate way to measure distance is with a surveyors wheel. It's a wheel with known circumference and a counter..... The same thing as a well calibrated cycling computer. |
| 2008-09-28 6:40 PM in reply to: #1702087 |
| Extreme Veteran 422 | Subject: RE: What to believe - computer or maps Don't feel like doing the trig, but if you do the map, at most road grades, the lack of elevation is less than a 1% error. I'll still bet on a mis-calibrated bike computer. (ie just typed in the value given for your wheel or not rolling it out accurately enough) but as said by someone else a 1% error is pretty small. |
| 2008-09-28 6:49 PM in reply to: #1701644 |
Master 2491 | Subject: RE: What to believe - computer or maps Did you ride through a wormhole? That can really throw off the distances... ; ) |
| 2008-09-28 7:57 PM in reply to: #1702081 |
Elite 7783 PEI, Canada | Subject: RE: What to believe - computer or maps davhamm - 2008-09-28 8:29 PM Well, the tangents might be part of it. How did you calibrate your bike computer, do you roll out 2 or 3 full rotations of the front tire, while inflated to the pressure you normally ride at, with you on in your normal riding position? If not I would say your bike tire circumference has most of the error and not a straight line is a smaller part. Remember you need to be on the bike, cause your weight will cause the circumference to be a bit smaller. I just entered the number associated with my tire size from the computer manual. It never occurred to me that this might not be accurate. I will try rolling the tire out as you suggested to see if I get a different result. |
| 2008-09-29 10:51 AM in reply to: #1702106 |
Elite 7783 PEI, Canada | Subject: RE: What to believe - computer or maps davhamm - 2008-09-28 8:40 PM Don't feel like doing the trig, but if you do the map, at most road grades, the lack of elevation is less than a 1% error. I'll still bet on a mis-calibrated bike computer. (ie just typed in the value given for your wheel or not rolling it out accurately enough) but as said by someone else a 1% error is pretty small. Well, I measured the wheel size by rolling it out 3 times and I definitely get a different number than what is in the manual. The book says it should be 2133mm, I got 2085mm. When I do a rough calculation to convert what I would have gotten with that number in the computer yesterday (63.04km converted to mm, divided by the original wheel size, multiplied by the new wheel size), I get almost exactly what the maps say. The calculation gives me 61.9km, the maps say 62.0km. So, thank you davhamm! Incorrect wheel size seems to have been the culprit. I won't be trusting measurements I get from a book anymore... |
| 2008-09-29 11:12 AM in reply to: #1701644 |
Elite 2673 Muskego, WI | Subject: RE: What to believe - computer or maps I'd have gone with what chirunner said...
...unless the map came out longer than my bike computer. Then I'm all over the accuracy of the map vs. the bike computer. |
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