ELEMNT Bolt V2 Elevation Question

[fourflys]

So, as I've been out riding more lately, I've been wondering about the elevation accuracy on my Elemnt Bolt...

Some examples from Strava (fed from the ELEMNT) vs RideWithGPS (which doesn't seem to be fed from the ELEMNT, at least elevation).

Today's ride- Strava 59 ft, RWGPS 205 ft
Yesterday- Strava 460 ft, RWGPS 599 ft
19Feb- Strava 220 ft, RWGPS 402 ft
and it goes on like this for all the rides...

I will say it feels like I'm doing more climbing than what my ELEMNT says most days...

Anyone have similar issues? I ELEMNT isn't very old, so should still be under warranty...

With this and the constant updates I keep having to do on the Wahoo, I'm seriously considering going back to Garmin...

Thoughts from the collective?

[RobJ]

Yes the Bolt v2 has a number of issues with the GPS and the elevation (IIRC). Read through the comments to Rays review and you will get the story...

https://www.dcrainmaker.com/2021/05/...-progress.html

[NHAero]

I have the Elemnt and also use RWGPS. Elevation is always more on RWGPS. I also have a Garmin Fenix 3 and it doesn't agree with either. It's clear that the GPS units aren't very accurate- I believe they rely on sensing barometric pressure, and to do that they need temperature. The Garmin is on my wrist so it never knows outdoor temp in cold weather. If these allowed manual calibration they would be more accurate. Almost all my rides start and end at my house, which will show different elevation at start and finish.

[Mark McM]

Originally Posted by NHAero

I have the Elemnt and also use RWGPS. Elevation is always more on RWGPS. I also have a Garmin Fenix 3 and it doesn't agree with either. It's clear that the GPS units aren't very accurate- I believe they rely on sensing barometric pressure, and to do that they need temperature. The Garmin is on my wrist so it never knows outdoor temp in cold weather. If these allowed manual calibration they would be more accurate. Almost all my rides start and end at my house, which will show different elevation at start and finish.

For real-time measurements of altitude gain, barometric altimeters are generally better than GPS. If this weren't the case, GPS manufacturers wouldn't include barometric altimeters into their units. Systems like RWGPS don't measure altitude gain directly, but instead rely on databases of altitude profiles - but these profiles are based on a lot extrapolations, interpolations, and assumptions, so these aren't exceptionally accurate, either. Unfortunately, attempting to measure altitude gain is similar to the Coastline Paradox, so all methods of altitude gain measurement are inherently fraught with errors, and there's probably no one correct value for any route.

For real-time measurement of altitude, why are barometric altimeters more accurate than GPS? Because measuring altitude gain often requires accumulating many small changes in altitude over time, and barometric altimeters can detect smaller changes in altitude than the GPS receivers found in cycling computers. It is true that barometric altimeters are subject to changes in weather (temperature and sea level pressure changes), but these changes tend to be gradual drifts over time, whereas altitude changes cause more rapid changes in barometric pressure. This means that the contribution of altitude are much larger then the contributions of weather for changes in pressure measurements, so weather introduces only small errors to altitude gain measurements. For example, if you rode a closed loop with 3,000 feet of altitude gain, and found that the barometric altitude at the start/end point differed by 100 feet due to weather changes, then the error in the altitude gain measurement due to weather changes would only be 100 feet (less than 1%).

As far as the Fenix 3, you actually can adjust the barometer:

https://www8.garmin.com/manuals/webh...DBC06CDB4.html


I've used a number of different altimeters, but the Wahoo Elemnt is the only one I've used that does not have an altitude adjustment. Consequently, the Wahoo is the only one I've used that is never, ever right.

[rice rocket]

Originally Posted by Mark McM

For real-time measurement of altitude, why are barometric altimeters more accurate than GPS? Because measuring altitude gain often requires accumulating many small changes in altitude over time, and barometric altimeters can detect smaller changes in altitude than the GPS receivers found in cycling computers.

GPS can derive an altitude fairly closely from satellites, but back when I used my phone to record and upload to Strava, I found they just used the latitude/longitude coordinates and derived elevation through predefined maps.

That meant, when I traversed a bridge, the data would say I rode down into the valley and back up the other side.

[Mark McM]

Originally Posted by rice rocket

GPS can derive an altitude fairly closely from satellites, but back when I used my phone to record and upload to Strava, I found they just used the latitude/longitude coordinates and derived elevation through predefined maps.

That meant, when I traversed a bridge, the data would say I rode down into the valley and back up the other side.

Yes, one of the problems with Strava and similar systems is the accuracy of the database, including assumptions built into the database. For example, what if there is a 50 foot hill, and there had just been a 100 foot tower built on top of it, and you hiked up the hill and to the top of the tower. Many systems similar to Strava would indicate that you gained only 50 feet, since the map database likely only has the altitude of the top of the hill.

A note about GPS accuracy: Due to the positions of the satellites (usually at a low angle when observed from the ground), GPS altitude accuracy can be fairly low compared to its horizontal accuracy. And because the satellites are always moving accuracy is always moving. GPS altitude accuracy is typically off by between 50 - 100 feet, and can be off by as much as 400 feet (see this Garmin article on GPS accuracy).


An absolute altitude error of 50 feet is good enough for most things, really. If you are climbing a tall mountain, and you want to know when you should don your oxygen mask, an error 50 feet is good enough. But it's not good enough for measuring accumulated climbing over undulating terrain (particularly if the accuracy of the measurements are ever changing). For measuring accumulated climbing, you don't need absolute accuracy, you need to be able to measure small changes in altitude with good repeatability. Barometric altimeters are better at this than GPS.

It has sometime been asked, if GPS is so accurate, why do so many GPS computers use wheel based speed sensors and barometric altimeters? Because for speed and climb measurements, its not about absolute accuracy, its about being able to measure small changes.

Here's an example: Say you where dropped somewhere on the plane and you wanted to know you where, you had to choose between using a GPS and a yardstick, which would you choose? The best choice here is the GPS, of course. But what if you wanted to measure a horizontal distance of 1", then which tool would you choose? Here, a yardstick, because GPS is not suited to measuring very small changes in position. Measuring speed requires measuring relatively small distance between two points (at two different times), so a wheel based speed sensor is more accurate than a GPS based speed sensor.

The same is true for measuring small changes in altitude, and then accumulating these changes to calculate total amount of climbing. A barometric sensor is better at measuring small changes in altitude, so it is better than a GPS for measuring accumulated climbs.

[fourflys]

Re: Strava

Not sure what it did previously, but Strava seems to take the measurements from the Wahoo as it’s always exactly the same as my head unit…

With this issue, the issues others have had, my issues with the TICKR… pretty sure I’m headed back to Garmin…

Anyone know when REI has their 20% off coupon?

[old fat man]

Originally Posted by fourflys

Re: Strava

Not sure what it did previously, but Strava seems to take the measurements from the Wahoo as it’s always exactly the same as my head unit…

With this issue, the issues others have had, my issues with the TICKR… pretty sure I’m headed back to Garmin…

Anyone know when REI has their 20% off coupon?

I just use the strava elevation correction button. My gen 1 elemnt is almost always 20+% lower than the corrected elevation. I do the same route 2 or 3 times a week and wahoo reads between 900-1100 ft of elevation. Actuality it's closer to 1500 ft. That said, Garmin products are such a bad user experience that I'd rather live with the inaccurate elevation from wahoo than have to navigate a Garmin user screen of any sort.

[fourflys]

Originally Posted by old fat man

I just use the strava elevation correction button. My gen 1 elemnt is almost always 20+% lower than the corrected elevation. I do the same route 2 or 3 times a week and wahoo reads between 900-1100 ft of elevation. Actuality it's closer to 1500 ft. That said, Garmin products are such a bad user experience that I'd rather live with the inaccurate elevation from wahoo than have to navigate a Garmin user screen of any sort.

I wonder what other Garmin or Wahoo-like computers folks have used? I was on the Lezyne site last night poking around and noticed they had a computer… didn’t really look that close into it though.

[cgolvin]

Originally Posted by Mark McM

Unfortunately, attempting to measure altitude gain is similar to the Coastline Paradox, so all methods of altitude gain measurement are inherently fraught with errors, and there's probably no one correct value for any route.

…the Wahoo is the only one I've used that is never, ever right.

Well, based on the first point above (with which I agree), every device/method is never, ever right.

[fourflys]

Eh, maybe I'll just adjust to the Strava elevation after every ride... now the only annoying thing about the Wahoo is the constant updates... not bad per se, just annoying as I have to use my wife's phone as a hot spot to do them...