Synchronize GPS and cyclocomputer
#1
Newbie
Thread Starter
Join Date: Oct 2019
Posts: 3
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 2 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Synchronize GPS and cyclocomputer
I have a technical question I've been unable to find an answer for Online. My Garmin Foretrex GPS shows slightly higher mileage than my Cateye cycling computer. I have the Cateye circumference chart available, but I don't know if I need to increase or decrease the circumference to synchronize.
#2
Senior Member
Join Date: Nov 2014
Location: Eugene, Oregon, USA
Posts: 27,547
Mentioned: 217 Post(s)
Tagged: 0 Thread(s)
Quoted: 18354 Post(s)
Liked 4,502 Times
in
3,346 Posts
The GPS numbers often varies slightly from a wheel computer, or even from one GPS system to another, even on the same device.
Get a long tape measure and lay it out on a flat spot of your driveway or walkway.
Then start with your valve down (or a chalk mark on the tire), and roll out 1 or 2 wheel revolutions and measure. I usually like to try to walk the bike with some weight on it. Convert to centimeters or millimeters if necessary.
I'd go by your actual walkout measurement rather than the GPS.
Get a long tape measure and lay it out on a flat spot of your driveway or walkway.
Then start with your valve down (or a chalk mark on the tire), and roll out 1 or 2 wheel revolutions and measure. I usually like to try to walk the bike with some weight on it. Convert to centimeters or millimeters if necessary.
I'd go by your actual walkout measurement rather than the GPS.
#3
Newbie
Thread Starter
Join Date: Oct 2019
Posts: 3
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 2 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Thank you CliffordK, I already knew how to do that. What I haven't been able to figure out is how to sync the gps and computer when one shows a greater mileage. Please note original question.
#4
Senior Member
Join Date: Jul 2007
Location: South shore, L.I., NY
Posts: 6,863
Bikes: Flyxii FR322, Cannondale Topstone, Miyata City Liner, Specialized Chisel, Specialized Epic Evo
Mentioned: 18 Post(s)
Tagged: 0 Thread(s)
Quoted: 3225 Post(s)
Liked 2,056 Times
in
1,174 Posts
I wouldn’t sweat this as well. GPS distance tracks are typically a bit shorter then a calibrated cycle computer. The nature of the track being recorded is that theres a location point recorded about every second. If you ride a route that has turns, the GPS cannot record every foot of the route, thus comes up short. Might be 1/2 mile in 100, might be a bit more. If there’s a lot of tree coverage, the signal can suffer dropouts as well. This is the reason a lot of GPS users add a speed sensor, especially if mt, biking, riding in wooded areas, etc... makes for a more accurate track.
Only solution outside a speed sensor would be to modify the cycle computer wheel diameter setting until they match.
Only solution outside a speed sensor would be to modify the cycle computer wheel diameter setting until they match.
#5
Full Member
Join Date: Nov 2016
Location: SoCal
Posts: 489
Bikes: 2014 Bruce Gordon Rock&Road, 1995 Santana Visa Tandem, 1990 Trek 520, 2012 Surly LHT
Mentioned: 3 Post(s)
Tagged: 0 Thread(s)
Quoted: 211 Post(s)
Likes: 0
Liked 43 Times
in
35 Posts
If the distance on the GPS is larger than the cycling computer, then increase the circumference on the cycling computer. To be "exact', rather than guessing how much to change it, calculate the difference in distance between the 2 units, then divide this by the trip distance on the either unit (wont really matter). This gives you the percent difference. Then multiply this by the current circumference in the computer. Than add (or subtract) this value to the current circumference in the computer. Of course, the next ride, they probably wont agree.
#6
Generally bewildered
Join Date: Aug 2015
Location: Eastern PA, USA
Posts: 3,037
Bikes: 2014 Trek Domane 6.9, 1999 LeMond Zurich, 1978 Schwinn Superior
Mentioned: 20 Post(s)
Tagged: 0 Thread(s)
Quoted: 1152 Post(s)
Liked 341 Times
in
251 Posts
I'll answer the question. Your cyclometer doesn't measure distance, it measures revolutions of the wheel, right? And one revolution gives you one circumference of distance. That is, the cylometer just multiplies revolutions times the circumference. If you take the GPS as gospel truth, and your cyclometer registers LESS distance, you need to increase your circumference on the computer. If the cyclometer registers MORE distance than the GPS, you need to decrease the circumference on the computer.
Clifford K gave you one way to get a more accurate circumference out of the Cyclocomputer. I'd advocate a slightly different one, because the circumference is slightly different when you are actually on the bike and riding over a real road with the wheels bearing a rider. Find someplace you can measure out a very accurate mile (from maps, mile markers, or perhaps the distance between 7 and 8 mile road). Then just set a new circumference as the product of the old circumference and the actual (map) distance, divided by the distance indicated by the Cyclocomputer.
For right now, if you want GPS and your Cyclocomputer to match, with the computer showing a smaller than GPS distance, you should increase your circumference. Use the current circumference times the GPS distance divided by the Cateye measurement. Then things should agree a little better. No guarantee that they will be correct, but if they're wrong at least they'll be consistently wrong.
Clifford K gave you one way to get a more accurate circumference out of the Cyclocomputer. I'd advocate a slightly different one, because the circumference is slightly different when you are actually on the bike and riding over a real road with the wheels bearing a rider. Find someplace you can measure out a very accurate mile (from maps, mile markers, or perhaps the distance between 7 and 8 mile road). Then just set a new circumference as the product of the old circumference and the actual (map) distance, divided by the distance indicated by the Cyclocomputer.
For right now, if you want GPS and your Cyclocomputer to match, with the computer showing a smaller than GPS distance, you should increase your circumference. Use the current circumference times the GPS distance divided by the Cateye measurement. Then things should agree a little better. No guarantee that they will be correct, but if they're wrong at least they'll be consistently wrong.
#7
Jedi Master
They'll never be exactly the same on every ride, but keeping track of this will help you get them acceptably close.
New Circumference = Current Circumference * (GPS Distance/Computer Distance)
New Circumference = Current Circumference * (GPS Distance/Computer Distance)
#8
Senior Member
Join Date: Jan 2013
Location: Llano Estacado
Posts: 3,702
Bikes: old clunker
Mentioned: 13 Post(s)
Tagged: 0 Thread(s)
Quoted: 684 Post(s)
Likes: 0
Liked 105 Times
in
83 Posts
Neither GPS nor cycle computers measure distance, they calculate it by very different methods. And since the methodologies are different it will be impossible to synchronize them in any meaningful way.
The obvious question is: which is closer to truth? but most people strenuously avoid truth. HTH
The obvious question is: which is closer to truth? but most people strenuously avoid truth. HTH
#9
Senior Member
Join Date: Feb 2012
Location: Rochester, NY
Posts: 18,056
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4195 Post(s)
Liked 3,837 Times
in
2,295 Posts
[QUOTE=AnkleWork;21172108]Neither GPS nor cycle computers measure distance, they calculate it by very different methods. And since the methodologies are different it will be impossible to synchronize them in any meaningful way.
The obvious question is: which is closer to truth? but most people strenuously avoid truth. HTH[/QUOTE]
And ain't that the truth! Andy
The obvious question is: which is closer to truth? but most people strenuously avoid truth. HTH[/QUOTE]
And ain't that the truth! Andy
__________________
AndrewRStewart
AndrewRStewart
#10
Senior Member
Join Date: Sep 2010
Location: Roswell, GA
Posts: 8,319
Bikes: '93 Trek 750, '92 Schwinn Crisscross, '93 Mongoose Alta
Mentioned: 30 Post(s)
Tagged: 0 Thread(s)
Quoted: 1438 Post(s)
Liked 1,092 Times
in
723 Posts
A Man with One Watch Knows What Time It Is; a Man with Two Watches Is Never Quite Sure.
Likes For dsbrantjr:
#12
Senior Member
Join Date: Aug 2005
Location: Pittsburgh, PA
Posts: 33,656
Bikes: '96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '20 Surly Midnight Special, All are 3x10. It is hilly around here!
Mentioned: 39 Post(s)
Tagged: 0 Thread(s)
Quoted: 2026 Post(s)
Likes: 0
Liked 1,095 Times
in
741 Posts
When I rode with both a Cat-Eye cyclometer and a Garmin GPS watch I noticed in town riding with a lot of turns and stops, the GPS read slightly less than the cyclometer for distance. Once out of the congestion and onto long uninterrupted stretches, the GPS read further than the cyclometer. The differences were small (less than 1%) but predictable.
As noted, the distances are measured using different methods and can't be expected to be identical. Unless you are doing land surveying or have to locate the exact spot the treasure is buried, the difference is insignificant.
As noted, the distances are measured using different methods and can't be expected to be identical. Unless you are doing land surveying or have to locate the exact spot the treasure is buried, the difference is insignificant.
#13
Senior Member
Join Date: Dec 2010
Location: northern Deep South
Posts: 8,895
Bikes: Fuji Touring, Novara Randonee
Mentioned: 36 Post(s)
Tagged: 0 Thread(s)
Quoted: 2599 Post(s)
Liked 1,924 Times
in
1,208 Posts
This will get you close (or closer), perhaps. But it's unlikely even an exact fix on one ride will carry over to the next. Fix 0.5% low, next ride will be 0.1% high. Best solution is to get them "close enough" and then change your expectations so "that close" is acceptable.
#14
Senior Member
Join Date: Jan 2013
Location: Llano Estacado
Posts: 3,702
Bikes: old clunker
Mentioned: 13 Post(s)
Tagged: 0 Thread(s)
Quoted: 684 Post(s)
Likes: 0
Liked 105 Times
in
83 Posts
Here's an excellent exercise for the OP:
Mount two "identical" cycle computers next to each other, one reading from the front wheel and one reading from the rear. Calibrate and synchronize them the best you can then ride and observe. Prepare for insanity.
Clue: neither will report the actual distance traveled by the bike. One will read high and one will read low.
Mount two "identical" cycle computers next to each other, one reading from the front wheel and one reading from the rear. Calibrate and synchronize them the best you can then ride and observe. Prepare for insanity.
Clue: neither will report the actual distance traveled by the bike. One will read high and one will read low.
#15
Newbie
Thread Starter
Join Date: Oct 2019
Posts: 3
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 2 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
"For right now, if you want GPS and your Cyclocomputer to match, with the computer showing a smaller than GPS distance, you should increase your circumference."
This was the information I requested, and the only information I needed. Using it, I sync'd my Garmin and Cateye so they varied less than .1 mile on my 25 mile ride today.
This was the information I requested, and the only information I needed. Using it, I sync'd my Garmin and Cateye so they varied less than .1 mile on my 25 mile ride today.
#17
Senior Member
Join Date: Dec 2010
Location: northern Deep South
Posts: 8,895
Bikes: Fuji Touring, Novara Randonee
Mentioned: 36 Post(s)
Tagged: 0 Thread(s)
Quoted: 2599 Post(s)
Liked 1,924 Times
in
1,208 Posts
#18
Full Member
I have a technical question I've been unable to find an answer for Online. My Garmin Foretrex GPS shows slightly higher mileage than my Cateye cycling computer. I have the Cateye circumference chart available, but I don't know if I need to increase or decrease the circumference to synchronize.
#19
Generally bewildered
Join Date: Aug 2015
Location: Eastern PA, USA
Posts: 3,037
Bikes: 2014 Trek Domane 6.9, 1999 LeMond Zurich, 1978 Schwinn Superior
Mentioned: 20 Post(s)
Tagged: 0 Thread(s)
Quoted: 1152 Post(s)
Liked 341 Times
in
251 Posts
Granted, the total error is about 0.4% between different sensors.
PS One of my Profs went to the Swedish Royal Academy as an undergrad. One perquisite is that you got to listen to the Nobel Laureate lectures. He was there for Richard Feynman's lecture. Feynman was writing out equations on a chalk board for the assembled dignitaries when he stopped and walked off the stage. About 30 seconds later, he walked back onto stage and wrote down a number. He then turned to the audience.
"No human", he said, "should do units conversion in public".
Last edited by WizardOfBoz; 10-22-19 at 02:11 PM.
#20
Senior Member
Join Date: Dec 2010
Location: northern Deep South
Posts: 8,895
Bikes: Fuji Touring, Novara Randonee
Mentioned: 36 Post(s)
Tagged: 0 Thread(s)
Quoted: 2599 Post(s)
Liked 1,924 Times
in
1,208 Posts
#21
Generally bewildered
Join Date: Aug 2015
Location: Eastern PA, USA
Posts: 3,037
Bikes: 2014 Trek Domane 6.9, 1999 LeMond Zurich, 1978 Schwinn Superior
Mentioned: 20 Post(s)
Tagged: 0 Thread(s)
Quoted: 1152 Post(s)
Liked 341 Times
in
251 Posts
Yup And there's a difference in fractal dimensionality too.
#22
Senior Member
Join Date: Apr 2012
Location: Nor Cal
Posts: 6,016
Mentioned: 17 Post(s)
Tagged: 0 Thread(s)
Quoted: 1814 Post(s)
Likes: 0
Liked 923 Times
in
569 Posts
GPS measures along the ground, so to get identical distances,
you need to bypass bridges, fording all rivers, crossing freeways, and avoid tunnels by going over what it tunnels through.
It'll be worth it, 'tho!
you need to bypass bridges, fording all rivers, crossing freeways, and avoid tunnels by going over what it tunnels through.
It'll be worth it, 'tho!
#23
Full Member
However, GPS always heavily "smooths out" any direction changes in the trajectory, because it does not have enough precision to tell the real changes in direction from positioning imprecisions inherent in the system (i.e noise). If you ride your bike in a zig-zagging trajectory from point A to point B, the wheel-magnet computer will meticulously measure the length of the whole zig-zag, while the GPS computer will most likely assume that you rode in a straight line from A to B. The difference in the measured distance will be drastic.
#24
Senior Member
Join Date: Apr 2012
Location: Nor Cal
Posts: 6,016
Mentioned: 17 Post(s)
Tagged: 0 Thread(s)
Quoted: 1814 Post(s)
Likes: 0
Liked 923 Times
in
569 Posts
That's not true. GPS mesaures the exact 3D trajectory to the best of its ability. It doesn't measure "along the ground". It does not know (and doesn't need to know) where the "ground" is.
However, GPS always heavily "smooths out" any direction changes in the trajectory, because it does not have enough precision to tell the real changes in direction from positioning imprecisions inherent in the system (i.e noise). If you ride your bike in a zig-zagging trajectory from point A to point B, the wheel-magnet computer will meticulously measure the length of the whole zig-zag, while the GPS computer will most likely assume that you rode in a straight line from A to B. The difference in the measured distance will be drastic.
However, GPS always heavily "smooths out" any direction changes in the trajectory, because it does not have enough precision to tell the real changes in direction from positioning imprecisions inherent in the system (i.e noise). If you ride your bike in a zig-zagging trajectory from point A to point B, the wheel-magnet computer will meticulously measure the length of the whole zig-zag, while the GPS computer will most likely assume that you rode in a straight line from A to B. The difference in the measured distance will be drastic.
I live near San Francisco. If I plot a route with ridewithgps, it shows the elevation across the Golden Gate bridge as 0 feet.
At least it doesn't plot the ground under the water- that would be tough. I also see negative elevations on occasion.
#25
Full Member
That's strange. I rode over Golden Gate using my Garmin Edge computers (510 or 830) as well as using just my phone to record the ride in Strava. In all cases the elevation of the bridge was recorded properly. One could argue that the Edges used their barometric sensors to determine the elevation. But the phone did not have such sensor.