Fendertele

We rode the same trail today with a large variance between results.
Wahoo ELEMNT Bolt. 220 ft
Apple Watch 7. 385 ft
i expect a difference between devices but this seems rather large. At a quick glance the elevation map profiles appear similar. Anyone know if they use different algorithms?

mschwett 

both devices have a barometric altimeter and GPS. what apps/software are you using to view the results? many map based systems (e.g. strava) will correct the device altitude readings based on map data. said map data varies in quality greatly, so you have three different potential altitude sources:

1) the barometric altimeter
2) GPS altitude - essentially useless
3) elevation from maps by correlating GPS location

different software will absolutely use different methods to reconcile the variation between these three sources.

Fendertele

Viewing on Strava. Apple Watch used Strava to record the ride. Wahoo app recorded the ride and showed 219 ft and was subsequently uploaded to Strava.

Camilo

Both were telling you the same thing - it was a flat ride.

Eds0123

Oh I am really shocked and I am very disappointed, apple watch not telling you the Truth?

70sSanO

My personal rule of thumb has always been to mix-n-match using the best numbers. One gives more elevation, the other more miles, use them both and toss out the lower numbers.

I would guess that for the vast majority the numbers really don’t matter. If it helps to make you feel better about yourself, who cares.

I remember when our boss wanted everyone in our dept to participate in recording daily steps. We all got clip on pedometers. I’d clip it on and drive to work. I was breezing through each day until I noticed my drive registered around 4000 steps each way. Oh well, I guess I deserve some credit for fighting traffic for almost an hour to work and an hour back. There were no prizes, but 10,000 steps were never so easy.

John

CAT7RDR

I gave up on recording with my Garmin Vivoactive. Always gave me wildly inflated #'s compared to mapping software like ridewithgps.
I want accuracy not a pat on the back.

fishboat

Lots of folks buying lots of electronics that are essentially random number generators and then spending their time trying to figure out why they make little sense. Unless you figure out how to calibrate/verify what these "instruments" (being generous in many cases) are indicating, there's no reason to assume they're offering useful data. They may be consistent within themselves (i.e. they have precision) and may be useful under that umbrella(only), but without verified accuracy they have little connection to reality..or other devices. Toss in the software that lays on top of all theses devices..yikes. (imho..as a retired chemist that is familiar with real instrumentation as well as gage R&R instrument capability analysis)

Iride01 

Who cares which is correct? If they are consistent in their reporting, then you'll know which route has more climbing and descending in it.

I wouldn't expect two different devices to report elevation gain loss the same way. If we really got to talking about it I doubt that even we would agree what constitutes elevation gain or loss.

If I'm on a fairly level road that goes up and down a foot 20 times over the course of a mile do I count that as 20 feet of gain/loss? What if it's 5 feet 20 times in one mile?

Flipper_

On relatively flat territory I don't think it's worth worrying about this stuff at all. There isn't any place within 100 miles of here where I'd even acknowledge an elevation gain/loss measurement as noteworthy, much less significant.

indyfabz

Where I see a big difference between map sources is between Google Maps and RWGPS. I’ll get, say a 49 mile route from the former and then plot in on tha latter. The former will show something like 2,500’. The latter has shown nearly 1,000’ more.

mschwett 

the number people seem to track is total elevation climbed, so yeah, your mile would be 100 feet. obviously 5 twenty foot rollers is a lot easier than riding up one 100' hill at the end of a mile, but we have power and heart rate to account for that i suppose.

if you really dig into it, it just gets worse. the underlying data that strava uses for elevation correction is not that great. i've compared it to known-good data (surveys and other directly-measured elevation datums) and it's pretty far off in places. 15 to 20 percent error in these stats is pretty regular, whatever the source.

mschwett 

yep. different underlying terrain models, different degrees of precision in those models and the way they interpolate the "2D" points onto the underlying model... and then just the inherent problems of that methodology. it took RwGPS about 10 years to not register riding across the golden gate bridge as dropping 200' to sea level, and then abruptly climbing 200' back up. an extreme example, but 400 additional feet of climbing per round trip.

indyfabz

I still see it doing that at some bridge crossings. Tunnels too. You should see a map of the Hiawatha Trail. It’s got at least 12 tunnels. The elevation spikes show grades of over 20% in some places. No way to really tell how much I climbed using RWGPS.

fishboat

RWGPS introduced a "Flatten Elevation" tool a couple-few years back to eliminate the up and down spikes from tunneled hills and gorges. It's a premium feature. I haven't used it(not a premium acct), but the functionality is pretty straight forward. They have a demo of it on their site somewhere.

https://ridewithgps.com/help/advance...tten-elevation

fishboat

Not to be too anal(or pedantic if the last word was aced), but when talking about accuracy and precision it's best to use specific/correct terms.

Precision is "repeatability"(in the science and engineering worlds). An instrument can be highly precise and yet be completely inaccurate at the same time. Accuracy is an instrument's ability to measure a correct or theoretically correct value. All measurements are estimates of some true value. A good estimate of a true value is an average of multiple measurements.

Maps or reported elevation estimates need to be accurate. With any luck, they were developed with accurate and precise instruments.

Camilo

Whenever I'm on a group ride, we always choose the device with the longest distance and/or most elevation gain. These numbers are only valuable for one's own personal "data" ("I rode 30 miles today" sounds better than "I rode 28 miles today")

mschwett 

yes. you're correct. the issue with the terrain models is actually not so simple - they are precise in the sense of repeatability - if you project the same 2d coordinate onto the model you will of course always get exactly the same result. but the frequency of sampling of the underlying data is very poor, which makes it inaccurate in a way that could also be described as imprecise. not in the sense of repeatability but in the sense of "lacking exactness and accuracy of expression or detail." (per the definition of imprecise.)

in addition to the lack of spatial resolution, when the data is generated by remote sensing (as it usually is) things like trees, buildings, etc can make the sampled point just completely inaccurate. combine that with the low resolution, possible inaccuracy of the placement of the road on the map, the user's GPS position on the map, which may or may not be snapped to the road... and it's actually surprising to me that they aren't off by even MORE!

here's an example of a couple block spin around my neighborhood. RwGPS terrain model thinks it's a 1.3 mile segment with 170 feet of climbing. it's actually 1.25 miles (no way to know if RwGPS knows that but is just rounding...) and 239 feet of climbing. the actual vertical gain is almost 41 percent more! i've seen errors like this all over the place, with both strava and RwGPS, but there aren't all that many places in the world that "good" data is easy to come by.

70sSanO

Excellent!

I have also found that the degree to which I round up is in direct proportion to my age.

So far I have not yet resorted to metric distances. However, I anticipate doing so sometime in the future.

I also think many non-cyclists will be more impressed with kilometers, it just sounds further.

John

Edit added: Or a metric half century.

fishboat

...imprecise..while I get your meaning..imprecise, precise and precision are very different words. The latter, in science, is basically a reserved term/professional jargon that only involves measurement repeatability. Detail and "precision" have no relationship. What the non-science/engineering world does with these words, in their own environment, including Webster's Dictionary, is up to them.

As for for my initial comments.. the random number generation applies to watts..calories burned, watches counting steps...... The devices may be useful in terms of individual unit precision, comparing output to other's results or reality will have limited success.

terrymorse 

How did you accumulate vertical gain from the municipal data? How often did you sample?

When calculating vertical gain, the length of your ruler has a significant impact on the answer. Even if your elevation data are perfect.

mschwett 

in this case just listing and then subtracting vertical components of the start and ends or climbs, which for this simple example are also all but one at changes in direction. no need to set a sampling frequency as it’s visually obvious where the three or four climbing segments start and end.

i’ve done it before by projecting the line onto the surface created by the contour data and the using a script to sum any positive differences between subsequent points on the line. the projection algorithm introduces vertices wherever there needs to be a kink last a certain degree of variance.

it’s worth noting that the contour data is itself created by interpolation from a (very, very large) number of points, so it’s not perfect either. but it’s within inches in most of the dozens of locations that i’ve spot checked it against surveys of actual pieces of property. data like this is very hard to come by at the scale needed for something like cycling.

indyfabz

Thanks. This is what BF is supposed to be good for, instead of all the trolling that has been allowed to proliferate.

mschwett 

they actually quite recently introduced the auto-flatten feature for bridges and tunnels, and at the same time made the manual flatten free for all users. good features.

https://ridewithgps.com/news/5657-br...a-improve?otu=

worth noting that the auto version doesn’t work for rides, just routes:

fishboat

I agree. It's getting harder to find good info here as trolls seem to be taking up more and more 'bandwidth'. Of course, the other side of the coin is that trolls remain more than well-fed. If folks would just ignore them they'd look for a bridge to crawl under somewhere else.

My ignore list is up to 88..and continues it's slow growth.