Accuracy - Wahoo Elemnt vs. Ipad Mini

[NHAero]

I picked up a slightly used Wahoo Elemnt and today rode to work (through the woods it's 1.7 miles, not a huge test :-) I also had Ridewithgps logging on my iPad Mini 5. I've been suspecting that the Ridewithgps app on the iPad has been over-estimating elevation, and indeed, this morning the RWG showed 63 ft of climbing and 69 ft of descent; the Elemnt showed 27 ft and 48 ft, respectively, which I think is a lot closer.

Any insights into accuracy of these two devices and apps? What drove my interest is riding some routes others recorded on RWG and recording more elevation than the routes were listed at.

[old fat man]

My Wahoo elemnt seems to under report elevation compared to my ride companions. I think it's due to a dirty barometric sensor on the underside of the unit but I don't know how to clean those tiny pin holes.

[NHAero]

So elevation is coming from an onboard barometric sensor, rather than from location info from satellites?

Originally Posted by old fat man

My Wahoo elemnt seems to under report elevation compared to my ride companions. I think it's due to a dirty barometric sensor on the underside of the unit but I don't know how to clean those tiny pin holes.

[kramnnim]

Originally Posted by NHAero

So elevation is coming from an onboard barometric sensor, rather than from location info from satellites?

Yes. https://support.wahoofitness.com/hc/...-My-Ride-Data-

[NHAero]

Thanks so much - now I know there is a lot of info on their website, which I should read!

Originally Posted by kramnnim

Yes. https://support.wahoofitness.com/hc/...-My-Ride-Data-

[Mark McM]

Originally Posted by NHAero

So elevation is coming from an onboard barometric sensor, rather than from location info from satellites?

Wahoo uses an odd system: It uses a barometric sensor for measuring changes in elevation, but uses GPS for adjusting elevation. Which is quite annoying, because the GPS does a poor job of adjusting elevation, and the unit does not allow manual adjustment of elevation. I start most of my rides from the same point (my back door), but my Wahoo Elemnt Bolt reports the elevation of this location as anywhere between 60 ft to 270 ft. I have had other bike computers with barometric altimeters (from Avocet, Garmin and Sigma), and all of them allowed adjusting the altimeter to a known altitude (or sea level pressure). It appears that Wahoo wants to add convenience by automatically adjusting altitude - but badly done automation is often worse than no automation at all.

I believe that the RideWithGPS app uses a topographic map database for elevation. But topographic map elevation is usually of lower resolution than barometric altimeters, so it is likely prone to a lot of round off errors. For changes in altitude that are large or occur over long periods of time, the topographic data probably provides more accurate measures of elevation change; but for small changes in altitude over short periods of time, the barometric altitude is probably better.

[dave thompson]

A change in the barometric pressure will cause a change in the elevation reading. In aircraft the altimeter which uses barometric pressure to read altitude has to be manually adjusted to compensate for this change to give a true altitude. The change(s) in barometric pressure while you ride explains the differences in your climbing/descending results of your rides.

[NHAero]

This is helpful. Since most of my rides also begin from home, and I'm near large bodies of water, which Wahoo lists as one of the things that can throw off the accuracy of the barometric sensor, I'll be interested to see the variation in what it calls my starting elevation. Today it was 100 ft, and my best guess is that I'm at about 87 ft.

I'm more interested in knowing the absolute value of the amount climbed than the elevation of various locations calibrated to sea level. But many of my rides get to sea level, so I do have that calibration!

Originally Posted by Mark McM

Wahoo uses an odd system: It uses a barometric sensor for measuring changes in elevation, but uses GPS for adjusting elevation. Which is quite annoying, because the GPS does a poor job of adjusting elevation, and the unit does not allow manual adjustment of elevation. I start most of my rides from the same point (my back door), but my Wahoo Elemnt Bolt reports the elevation of this location as anywhere between 60 ft to 270 ft. I have had other bike computers with barometric altimeters (from Avocet, Garmin and Sigma), and all of them allowed adjusting the altimeter to a known altitude (or sea level pressure). It appears that Wahoo wants to add convenience by automatically adjusting altitude - but badly done automation is often worse than no automation at all.

I believe that the RideWithGPS app uses a topographic map database for elevation. But topographic map elevation is usually of lower resolution than barometric altimeters, so it is likely prone to a lot of round off errors. For changes in altitude that are large or occur over long periods of time, the topographic data probably provides more accurate measures of elevation change; but for small changes in altitude over short periods of time, the barometric altitude is probably better.

[cgolvin]

Originally Posted by NHAero

I'm more interested in knowing the absolute value of the amount climbed than the elevation of various locations calibrated to sea level.

I'm more interested in seeing what your ride setup incorporating an iPad looks like.

[NHAero]

Hah!
In my camelback on MTB rides and on the Firefly road bike; in my handlebar bag on any of the other road bikes. So not useful really to follow a route, just to record the ride and get stats on distance, speed, and elevation. That's why the Elemnt at a used price tempted me.

Originally Posted by cgolvin

I'm more interested in seeing what your ride setup incorporating an iPad looks like.

[Mark McM]

Originally Posted by dave thompson

A change in the barometric pressure will cause a change in the elevation reading. In aircraft the altimeter which uses barometric pressure to read altitude has to be manually adjusted to compensate for this change to give a true altitude. The change(s) in barometric pressure while you ride explains the differences in your climbing/descending results of your rides.

I don't think the difference experienced by the OP can be explained by changes in barometric pressure. It is true that barometric pressure changes with changes in the weather (temperature, humidity, etc.), but this usually results in slow drift in elevation measurement over time. Typically, pressure changes due to altitude changes during a ride are much larger than changes in atmospheric pressure, so barometric pressure drift tends to produce climb/descent errors of only a few percent.* In the OPs case, the ride was only 1.7 miles, so there should have been very little barometric drift.

*As an example, I often do a ride from my house in Concord, MA to the top of Mt. Wachusett (the only mountain in Eastern MA) and back again, on a route that goes over all the hills in the middle. This 70 mile route has about 5,300 feet of climbing over a duration of 5 hours. Typically, my barometric altimeter shows a change of 100 ft or less between the start and finish of the ride, so the barometric drift error is only about 2% of the altitude gained measurement.

[benb]

It's definitely worth finding out the actual altitude at your house/apartment/office/whatever and then you can set the altitude at the beginning of the ride the same way you would when doing preflight in an airplane.

I don't think Edge units do it but some Garmin units in the past have allowed pulling an altitude off the GPS and using that as a one time calibration of the barometric altimeter.

A lot of newer iDevices I thought had a real barometric altimeter in them, the apps should be able to take advantage of that.

edit: Any iphone >= iphone 6 has an altimeter, iPad Air 2 and up, iPad Mini 4 and up

[Mark McM]

Originally Posted by benb

It's definitely worth finding out the actual altitude at your house/apartment/office/whatever and then you can set the altitude at the beginning of the ride the same way you would when doing preflight in an airplane.

Yes, that's generally the best approach (but see the comment below about adjusting the Wahoo altimeter). But this doesn't really address the OPs issue about differences in measured climb/descent.

Originally Posted by benb

I don't think Edge units do it but some Garmin units in the past have allowed pulling an altitude off the GPS and using that as a one time calibration of the barometric altimeter.

Unfortunately it's not quite as easy as that. Barometric altimeters have to be adjusted for the current atmospheric conditions, which is not a one time thing because atmospheric conditions change. However, the adjustment is primarily to set the absolute altitude, not the ratio between change in pressure and change in altitude. So even an unadjusted barometric altimeter can measure changes in altitude quite well, so it can measure accumulated climb/descent well.

I don't know about other Garmin Edge devices, but the Garmin Edge 800 and Garmin Edge 520 allow you to adjust the altimeter to the current altitude. Sigma computers allow you to adjust their altimeters in either of 3 ways: To a known current altitude; to the altitude of any of 3 preset locations; or to the adjusted sea level pressure.

Sadly, Wahoo barometric altimeters can not be adjusted by the user to a known reference point. They always automatically self-adjust to their GPS measured altitude - and unfortunately, their GPS altitude measurement is not very good (as with most non-WAAS GPS's). GPS systems are very good for some things, but it is not good at others. That's why the most accurate GPS bike computers still use wheel sensors for measuring speed and distance, and barometric altimeters for measuring climb/descent.

[rnhood]

Garmin units have been using a barometric altimeter for years to measure elevation gain, correcting it via satellite GPS about every 15 minutes. The correction is an indirect way of correcting for changing weather conditions (primarily pressure referenced to sea level).

The electronic barometric sensor used is extremely sensitive, which leads to variability between units, even the same model. But it's plenty close enough in it's aggregate measurement.

The Wahoo units should do the exact same thing, although being a bit odd, who knows.

RidewithGPS uses a formula to predict elevation gain based on available topographical data. Their formula changed somewhat over the years and, while not perfect its plenty good enough for an indication of a route's profile. It's not perfect, but it's generally in the ballpark. More often than not, it does over state elevation gain.

[Mark McM]

Originally Posted by rnhood

Garmin units have been using a barometric altimeter for years to measure elevation gain, correcting it via satellite GPS about every 15 minutes. The correction is an indirect way of correcting for changing weather conditions (primarily pressure referenced to sea level).

Are you sure about using GPS to correct the barometric altimeter? Why does it say nothing of this on the Garmin web page about barometric sensor accuracy? All it says regarding altimeter corrections is this: "Many Edges calculate elevation data using a barometric altimeter sensor inside the device. To ensure that elevation data is accurate at the beginning of an activity, you can calibrate the elevation manually." Following this are links to the user manual pages on manual adjustment. Under the heading "What are some of the factors that can cause an elevation reading to be wrong?" The web page also says this: "The barometric altimeter is not temperature compensated. Temperature changes in the measuring device will affect the barometric pressure sensor and altimeter readings." So it sounds like it does not automatically compensate for weather conditions.

Originally Posted by rnhood

The Wahoo units should do the exact same thing, although being a bit odd, who knows.

This Wahoo trouble shooting page says that is what it does - but from first hand experience, I know it does it very poorly. In my backyard, my Wahoo Elemnt Bolt typically reports an altitude of anywhere between about 50 ft and 250 feet, and occasionally it reports even higher or lower numbers.