ZHVelo

Plus the 2% you mentioned is a lot I would propose. That is 2km in a 100km ride. I think that is too much.

If I remember back when I used Wahoo on my phone and my bike computer they were always just a few hundred metres off after such a distance. I.e. less than 1%. 2% is quite a lot if you think about it.

njkayaker

The wheel sensor will be more accurate over longer distances too (as long as the circumference is accurate).

The GPS distance will be shorter (by a bit) for a curvy path than the wheel sensor because the GPS is measuring straight lines every 1 second while the wheel sensor is measuring straight lines every 0.33 seconds.

tomato coupe

That's one piece of the puzzle, but it isn't a serious issue unless the route is very curvy, like you might encounter mountain biking. On typical road routes, it doesn't introduce a significant error.

njkayaker

There's still no reason to expect that GPS will be more accurate (over longer distances).

Even though it's "not a serious issue" (I said it was a bit), it means GPS won't be more accurate.

GPS also has issues with reception, which has more opportunity to happen over longer distances.

Garmin prefers the distance and speed from the sensor over that from GPS. They aren't doing that because the sensor is less accurate.

tomato coupe

You have to look at how errors accumulate. The errors for a speed sensor accumulate differently than the errors for GPS, and this gives GPS the advantage for long distances.

That's a bit like saying there are more opportunities for your sensor magnet to fall off over longer distances.
They use it mainly because it offers better speed measurements.

njkayaker

The effect of curviness gets worse for GPS over longer distance.

And the error for a wheel sensor is much less (centimeters versus feet for GPS).

The weakness of the wheel sensor is the need for an accurate circumference and for the circumference to be stable.

No. Losing reception is fairly common (it's a normal issue).

Losing the magnet isn't normal. It's rare that that happens. It's more like losing the GPS.

​​​​​​No. Because they don't have to use it for distance when they are using it for speed.

tomato coupe

Yes, a wheel sensor has lower error, for a single measurement. But you don't seem to understand that the errors are very different in character, so they accumulate differently. It is incorrect to simply say "this has less single-shot error, therefore it is better at all distances."

A similar situation exists with the measurement of elevation changes. Pressure transducers work much better than GPS in the short term, but GPS works better in the long term. Why? Because they suffer from different types of errors, and those errors accumulate differently.

pdlamb

Are you trying to explain that systemic and random errors accumulate differently? The wheel sensor has systemic error which is dependent on calibration, and is highly repeatable. For example, my wheel sensor may read 0.5% low. Over 10 miles, it will read 0.05 miles short; over 50 miles, it'll read 0.25 miles short. That won't be affected by route, obstructions, multipath, or anything else. It'll only be affected by how far you skid or how far you're airborne with your wheel stopped, or by wobbles in the wheel path -- but even then, it'll be an accurate measure of how far the wheel travels.

The GPS has both systemic and random errors. One sigma random error on GPS location is about +/- 30 feet at an unobstructed (no trees or overpasses) or multipath (no tall buildings or steel bridges nearby) location. It's possible to do some post-processing to affect how that random error accumulates, but that processing can result in a computed path that's either longer or shorter than the real path taken by the device.

The GPS also has systemic error. That happens when your route includes turns or curves, and the magnitude depends on where the periodic location measurements are take, the error in location for each points, and how the post-processing smooths out the computed path. If you ride where there are trees, buildings, bridges, or tunnels, you've got another systemic error of unknown magnitude. As a practical matter, it's a systemic error that's random, and sometimes significant in magnitude.

The GPS result for many of my rides, at least, is that measurements of the distance traveled (and subsequently the speed) is usually smaller than the actual, although the systemic multipath error can make it larger than the actual distance. If I don't have a wheel sensor measuring how far I really ride, the GPS only measurement error is unknown, and the rate of error accumulation is unknown. But I can be fairly certain the GPS measurement is wrong, I just don't know whether the error is positive or negative, or how large it is.

Elevation measurement for a GPS is similar to location measurement, only it's about three times larger. The only errors you'll see for pressure transducers are mis-compensated temperature differentials or barometric pressure changes with weather. If you're on tour, you can turn on the GPS and let is settle for perhaps 5 minutes before riding to get a good starting altitude, if you care. For my money, I'd bet on barometric sensors for more accurate climbing readings any time except when a major front comes through during the ride. But when a front comes through, the weather usually concerns me much more than the climb measurement accuracy.

njkayaker

You failed to provided any detail about how they accumulate differently.

Since tires lose pressure over time, the circumference gets smaller. So, there's that issue with wheel sensors. But it's not clear how much that matters.

Nothing you've said is any support for GPS being better than using a wheel sensor. Keep in mind that we are talking about cycling (something with low speeds and, generally, short distances). And, we are talking about GPS as it's used in devices used by cyclists (which don't use things like WAAS).

This doesn't even make sense and it's vague.

Elevation has other issues. GPS favors horizontal location and errors at the level of 30 feet don't matter that much for that. For elevation gain (which is what most cyclists are interested in), GPS is a poor way to measure it.

===========================

Garmin prefers the distance and speed from the sensor over that from GPS. They aren't doing that because the sensor is less accurate. They use if for distance and speed (when they have the option of using one or the other separately).

You have to explain why this company (presumably, having some expertise) does this.

njkayaker

Garmin rates barometric data as better for elevation (gain) than GPS.

The Edges let you "calibrate" the barometer but I think that only affects absolute elevation numbers (which cyclists are not typically interested in) and doesn't affect gain (since that's the result of a difference).

Measuring gain has other issues beyond issues with measuring absolute elevation.

I've tried but haven't been able to determine how much it changes the gain value.

billridesbikes

This thread has made me think about my Garmin. Ever since the 705 I’ve used ‘auto-calibration’ for the wheel size. I assumed that it counts wheel revolutions and compared it the to GPS and used this number as the circumference. But does anyone know exactly how this feature works? Does it do it continuously, once an hour, once a ride, only durning a setup? How many wheel revolutions does it use?

Garmin says entering it manually can be more accurate for speed and distance, but I honestly don’t really care that much. I can always find the real speed and distance post ride if I’m that interested.

njkayaker

It does it once at the start of the ride for about 0.1 of a mile (from what I recall reading about it a while ago).

It gets the distance from GPS and calculates the wheel circumference based on the number of wheel rotations that occur during the time of the measurement.

tomato coupe

First of all, let's use the correct terminology. We're dealing with physics and systematic errors, not physiology and systemic errors.


Yes, it's as accurate as the initial rollout measurement, assuming the rollout doesn't change over time.

Yes.

Let's ignore post processing for now.

It's really only an issue for very curvy paths, like you might encounter mountain biking. Not really an issue in road cycling.


Multipath errors will not, in general, increase the distance.

You also don't know what the wheel sensor error is, or whether the error is positive or negative.


Elevation measurements via pressure transducer are prone to long term drift, whereas GPS is not. The breakpoint where GPS becomes the better option is usually at times longer than the average bike ride, however, which is why GPS-based computers universally use a pressure transducer for elevation measurements.

Actually, the correct number is about half of that, at 5m. But, this is for absolute accuracy, which is not the important figure for a bike computer.

Errors in the absolute position come from several sources, including ionospheric effects, timing errors, multipath signals, reference clock drift, ephemeris errors, etc. Luckily, however, the largest contributors to the error budget are slowly varying effects, which means they essentially become common mode errors for closely spaced measurements. In other words, two adjacent positions may be off by 5 m, but they're both off by the same amount in the same direction. That's why your GPS computer does not show you bouncing around ± 5 m every second when you're sitting at a stop light. It might put your position a few meters off the road, or partially into the intersection, but the point-to-point position is relatively stable. As a consequence, the distance calculated between two adjacent points will have an uncertainty much less (about 0.5 m) than the absolute uncertainty of the GPS readings.

You can easily see this lower error if you look at the raw data from a GPS file. Below is a screen shot taken from the last ride I did, showing a ~ 0.5 km segment where I turned from a southwest heading and rode almost due west. (The latitude and longitude data are in degrees, and I've yellow highlighted the last three digits of the latitude data.). Once I was riding west (at about 16:30), the latitude should have remained unchanged, which it pretty much did. What's more interesting though, is how low the scatter is. You can see that the point-to-point differences are about 0.000001 degrees, which corresponds to 11 cm in distance, almost 50x better than the absolute accuracy of GPS measurements.

If you assume the uncertainty of the longitude measurements are roughly the same as the latitude measurements, the relative distance uncertainty from point-to-point is about 1.7%, as the distance between points is about 9 m. This is not nearly as good as a wheel sensor, which can reach about 0.1% relative uncertainty. But, the wheel sensor will have the same relative uncertainty over the full 0.5 km segment, whereas the uncertainty of the GPS system drops to about .03% over that distance.

tomato coupe

Yes it is, because the time frame where GPS becomes the better alternative is longer than a typical bike ride.

njkayaker

No, the amount of error in GPS measurements of elevation is too large for gain. It's poor for elevation gain.

The drift is slower than the changes being measured for elevation gain. Anyway, what is the magnitude of the drift?

And issues outside use on a bicycle are something else (not really relevant to what's being discussed).

njkayaker

https://www.bicycling.com/bikes-gear...s-lies-to-you/

https://www.runnersworld.com/news/a2...-your-mileage/

philbob57

I ride in suburban Chicago's North Shore - that is, on fairly straight roads and MUPS with lots of trees. The GPSes I've used have been much less accurate than wheel sensors. I say that based on riding a few routes again and again over many years, using 3 sets of computers - Cateye (failed), Sigma (fell out of mount when I was not paying attention), & Lezyne w/Wahoo sensor. The 3 sensors all report essentially the same distances for my routes, and they are essentially the same as Strava's distances for the routes. Using a GPS in a Samsung Galaxy, I saw almost 10% less distance reported. The Lezyne was a good deal better, but it's better still with the sensor.

I KNOW I'm only one data point. I could be wrong, but I'll stick with what the sensor tells me. If I rode along a coast or in a desert, I'd expect the GPS to approach the accuracy of a sensor.

If I had a military GPS, I might find different results ... or are consumer GPSes no longer hobbled?

tomato coupe

Sorry, we have our wires crossed, and it's all on me. My original comment was meant to be about measuring elevation, not elevation gain. But, I typed elevation changes, which makes it incorrect. Pressure transducers are definitely better at measuring elevation changes in almost every situation.

wphamilton

This is what I thought your reasoning would be, but I didn't want just assume it. It's a mistake IMO. Distance actually travelled is the length of the actual path, not the difference between the end points. It's actually the GPS that accumulates errors linearly, because 1) it disregards small path variations, and 2) errors in an instant's position result in fictional movements. The second depends on the programming of the GPS of course, but you can see that it's true by examining the GPS track.

In contrast, there is no accumulated error from the wired bike computer other than calibration. That error is, as you implied, linear with respect to the total line distance. If it's off by .1% for instance, the entire distance is off by .1%. I'm not sure if that's what you meant by "accumulated linear error" (to me that implies a linear error accumulated geometrically)

I submit that using the wired wheel sensor on a GPS is not technically different than a using a wired wheel sensor on a bike computer. That it improves distance and speed calculations indicates to me that it generally is used for that purpose, and that the wired sensor is superior in accuracy and precision.

BlazingPedals

If distances are the same but average speeds are way different, it's likely due to how the 'time stopped' is computed. Sometimes it counts anything below a certain speed as 'stopped' for instance if you go below 2 mph. Or, it may continue to add time for several minutes after you've stopped. Once I parked my bike next to an electric fence and the pulses in the fence were sensed as wheel rotations by the computer and compiled about a quarter mile over 20 minutes. That really screwed up my average!

tomato coupe

No accumulated error other than the calibration? That is the source of error.
If you double the distance, you double the error. That's a linear relationship.

No. When you double the distance, the errors do not double, i.e. it is not a linear relationship.

njkayaker

Yes, the wheel sensor is completely dependent on having an accurate circumference.

Yes, the difference from the real value will accumulate.

People have pointed out the first thing. And the second is obvious.

I suspect people thought you were talking about something else.

In any case, it’s still not clear which is generally more accurate for a bicycle ride.

The Garmins determine the circumference at the start of every ride (which is, I suppose, what you were getting at with the “GPS and wheel sensor is more accurate” thing).

wphamilton

I think yes, it does double the error with double the distance when you use GPS only. GPS smooths the small curving path into more straight lines. That does add up linearly with distance. I'm talking about both the serpentine path that your bike takes, and the larger radius curves. GPS only estimate that. In addition, many GPS errors accumulate from other sources as well, and you'll see jagged paths for instance.

pdlamb

Except if you’re riding through a city or suburb, and going around corners. Or you’re on a bicycle ghetto, aka MUP, which has lots of twists and turns. Or your road route has a lot of turns at intersections. Oh, like one of my typical road bike rides.

Say what? You take a path, throw in a few points way off the true path, and it doesn’t add distance?? OK, perhaps your filtering/post-processing knocks that down – but that filtering is at the cost of accuracy going around corners or curves.

After I get through calibrating the wheel sensor, I do know the magnitude and sign of the error.

I’ll accept the correction if you can direct me to a specification for a consumer-grade GPS with that figure. There are a few caveats, naturally:
1. Spec sheet, not advertising blurb. It’s as easy for a manufacturer to make an unsupported advertising claim as it is for me to claim to be the most ruggedly handsome, charming, and erudite poster here. Only the latter claims are true.
2. Settle time less than, shall we say, 2 minutes to meet the specified accuracy?
3. No differential GPS allowed. I don’t ride my bike around a golf course.
4. Consumer grade implies no aerospace, military, or surveying instruments. Let’s keep the price tag under $1,000.

As noted above, you can filter out those point to point variations, but that’s at the cost of filtering out accuracy on curves.

tomato coupe

GPS has several sources of error. Even if some of the errors increase linearly, the overall error will not, in general, increase linearly.