Arcadian

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.

CliffordK

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.

Arcadian

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.

Steve B.

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.

Eggman84

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.

WizardOfBoz

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.

kingston 

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)

AnkleWork

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

Andrew R Stewart 

[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

dsbrantjr

A Man with One Watch Knows What Time It Is; a Man with Two Watches Is Never Quite Sure.

WizardOfBoz

I recall designing resilient engineering systems. We'd use either one really reliable sensor or three pretty reliable sensors.

HillRider

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.

pdlamb

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.

AnkleWork

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.

Arcadian

"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.

WizardOfBoz

That's over 500 feet! That would drive me a bit crazy.

pdlamb

Off by a factor of 10, more like 53 feet. It'll still drive you crazy if you let it.

AndreyT

GPS computers and wheel-circumference computers measure two rather different trajectories and therefore two different distances. They are not supposed to be synchronized. Any attempts to synchronize them will fail one way or another.

WizardOfBoz

0.1 mile x 5280 feet/mile is... ?

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".

pdlamb

Yup, you're right. I was confusing 1% from higher up the thread.

WizardOfBoz

Yup And there's a difference in fractal dimensionality too.

woodcraft

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!

AndreyT

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.

woodcraft

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.

AndreyT

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.