# Elevation Correction - Strava/Garmin Connect



## kragu (Jun 14, 2011)

I recently purchased a Garmin 500 after years of using my iPhone to track my rides. Yesterday, I ran them side by side for the duration of my ride. On the ride, my two devices logged different climbing data.

iPhone Strava: 2,627 
Garmin 500: 2,215 
My friend's Samsung S4 Strava: 2,675

I then deleted my iPhone ride and uploaded my Garmin data to Strava. Curious about the discrepancy, I went to both the Garmin Connect and Strava websites and found "elevation correction" options, applied both and, working from the data from the Garmin 500, got the following:

Strava: 2,560
Garmin Connect:2,984

What gives? Both claim to use information from professional surveys in this "elevation correction" process, yet they still wind up pretty far apart. Anyone have any advice as to which is most accurate for them?


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## challybert (Sep 5, 2014)

Subscribed. I always use the elevation correction on my 510. It just seems more accurate to me. For me, the correction is always downward. It can be as much as 50% less than what the non-corrected Garmin readings suggest. To me the adjusted figures "feel" more accurate. Anyhow, looking forward to reading what others think.


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## Harold (Dec 23, 2003)

When using elevation corrections, positional accuracy matters. Strava does positional corrections also with their slide system. Look it up in strava labs. It is a really cool process. Your data can be garbage but strava will fix it for you. Garmin connect does not do this. GPS receivers do some processing of data on the fly but this is not the same as what strava does. Strava is essentially crowdsourcing gps corrections.

My last ride climbing data ranged from 1000ft to over 3,000ft depending on where I put it and analyzed it. Tis the nature of the beast. Measuring elevation is a helluva lot harder than you might think. What is 0ft elevation?


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## ewarnerusa (Jun 8, 2004)

Is the elevation provided by the Edge 500's barometric altimeter suspect? I thought the default was to rely on the barometric altimeter data (no elevation correction applied) because it is considered the most accurate?


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## Harold (Dec 23, 2003)

Barometric altimeters aren't perfect. They absolutely provide a more fine-grained elevation profile. The "corrections" applied by most sites/services make use of digital elevation model data and most of that has a 30m resolution. So this alone gives barometric altimeters more accuracy.

HOWEVER, the major limitation of the barometric altimeter is that it uses a barometer to measure elevation. Barometers are affected by weather. Sometimes strongly so. You'll see this if you go ride, stop somewhere for half an hour or more to eat lunch or something (pause the GPS), and then head out again. I've done this where my elevation changes by a rather large amount that's CLEARLY visible on the elevation profile.

If you go out for a ride that lasts all day, you can see the effects of this, also. Especially if your start/finish location is the same place (not a point-to-point), and even moreso if you do an out-and-back where the elevation profile mirrors itself at your halfway point. The longer you are out, the more the elevation will "drift" because the weather is changing. Do this when there's a frontal system moving through and it can really exaggerate.

On my Oregon 450, this is definitely a thing. To keep the barometer accurate, I need to calibrate that device to known elevations periodically. It's not important for me, so I don't worry about it. My Forerunner doesn't have a barometric altimeter, so I use the elevation corrections. Say what you will about the accuracy, but at least it's more consistent (though elevation accuracy does depend on positional accuracy). Many of the newer Edges do run some automatic calibrations of the barometric altimeter using satellite-calculated elevation. I haven't used any of these devices enough to be able to say how I like the system, but it's an effort to minimize problems caused by barometric drift that my Oregon 450 has that it requires periodic manual calibration to address.


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## ewarnerusa (Jun 8, 2004)

I recognize the potential pitfalls on relying on ambient air pressure for elevation, I've also confirmed the impacts based on changes in ambient pressure not related to climbing/descending as you've described. I have my Edge 510 set up to do the calibrations when I leave from home or from work, but the precision of that value isn't very important to me so I've never looked into if I'm happy with it. 

It just seemed to me that the OP and poster of the first response are considering the elevation correction from Strava or Garmin Connect to be the best representation of the true elevation profile. Strava and Garmin Connect use different correction algorithms, so it doesn't seem peculiar to me that they report different elevation profiles. I have elevation corrections turned off on both Strava and GC and I find that my elevation readings are identical between the two sites.


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## Harold (Dec 23, 2003)

Yeah, I'm suspect of the elevation corrections of both sites.

Here's the recent 30mi ride I was talking about:











Running that same file through Topofusion's Climbing Analysis yields the following table:
TopoFusion: 2430 ft
Straight GPS: 2463 ft
Straight DEM: 3205 ft
Maptech: 1004 ft
TopoUSA: 3360 ft
Training Center: 1577 ft
Custom Algorithm: 2430 ft


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## Harold (Dec 23, 2003)

When it comes to elevation, you can't get caught up in the absolute numbers. You just can't.

The earth is not a perfect sphere. It's an irregular ellipsoid, and any elevation "corrections" have to make some kind of simplified assumption about the shape of the earth. Then, what is 0ft/m elevation? Mean sea level is often quoted, but what about rising sea levels? What about geological activity that causes mountain building, and erosion? Then you get into finer grained issues with inaccuracy. Sampling intervals, measurement/calculation type (think about it, nobody is DIRECTLY measuring elevation - it's all calculated based off the measurement of something else).


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## ozzybmx (Jun 30, 2008)

I don't "correct elevation", some of the algorithms around here are massively wrong, you are relying on everyone else's data to determine your elevation.

There is one bike track here that I could "everest" in approx 8-9hrs if I used the correction. The correct elevation on this 3.5km (7km up and down) of trail is about 165m, Garmin is basically spot on +/- 10m, a mobile phone (uses correction data automatically) or doing a correction gives you approx 400m of elevation.

Here are the same rides on that trail, the top one is Garmin data reading 814m climbed, the bottom one is correction data giving me 1925m of elevation for the same ride.

We have aptly named it, Scam hill !


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## BigLarry (Jul 30, 2004)

*Elevation and Total Climb*

To figure out which of all the disparate climb measures are correct, I once strapped three biking altimeters on my bars, and did a uniform ~1043' climb up a dirt road. This road was a uniform 10% grade with no dips. I had accurate elevation of the start and end points from topo maps.

What I discovered was surprising. All three units (Vista, GPSMap60CSx, and Cateye bike computer) had barometric altimeters and if calibrated at the bottom, gave the correct elevation at the top within several feet.

Interestingly, the older Vista and CatEye computer got the the total climb accurately within a few percent, but the newer 60CSX had about a 10% lower climb registered. I often see it being up to 15% lower. Basically, the GPSMAP60CSx knew the correct altitude at the top, which was 1000' feet above where I started. Yet, it said I only climbed 941' which is mathematically impossible using it's own accurate altitude data.

The point is that the newer units (Edge, GPSMAP60 and above) are consistently reporting 10-15% lower climb than actual. The older units like Vista didn't do this.









Likewise, the Garmin Motion Based used to report the correct climb. But about three months ago, they changed the algorithm and it now reports about 10-15% lower than actual, more in agreement with the (inaccurate) new units readings.

I also used a spread sheet and manually totaled all the positive differences in altitude or climb reported by the typical GPS collection mode of every few seconds with a resolution of 1'. The total matched the older Garmin Motion based and the reading on the older units. And it matched the actual total on the uniform climb. But it didn't match the new units or the latest revision by Garmin.

The point is that Garmin is doing something odd to reduce the total climb by 10-15%. I'm not sure how or why. I suspect they're trying to reduce the errors from drift with the newer units that don't use manual altimeter calibration, and instead get altimeter drift from the GPS vertical readings that's relatively inaccurate.

BTW, I typically get an altimeter drift of 10'/hour upon return to the same point, or say 80' on a long 8 hour ride. I suspect the drift is mostly from temperature, cold in the morning versus warm in the afternoon. The drift can be larger if a weather front is passing by, but that's somewhat obvious and rare.


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## BigLarry (Jul 30, 2004)

Harold said:


> HOWEVER, the major limitation of the barometric altimeter is that it uses a barometer to measure elevation. Barometers are affected by weather. Sometimes strongly so. You'll see this if you go ride, stop somewhere for half an hour or more to eat lunch or something (pause the GPS), and then head out again. I've done this where my elevation changes by a rather large amount that's CLEARLY visible on the elevation profile.
> 
> If you go out for a ride that lasts all day, you can see the effects of this, also. Especially if your start/finish location is the same place (not a point-to-point), and even moreso if you do an out-and-back where the elevation profile mirrors itself at your halfway point. The longer you are out, the more the elevation will "drift" because the weather is changing. Do this when there's a frontal system moving through and it can really exaggerate.


For most barometric altimeters, I generally see a drift of about 10' per hour of ride, based on the difference in altitude when I return. This could be weather or temperature related. (Start in the morning, end in the afternoon.)

The worst drift I ever saw was ~200' difference on a long 12 hour ride with a weather front coming in. Usually, it's much lower, less than 100' drift on a long ride.

Those GPS without a barometric altimeter, such as the Edge 200, will get drift in the GPS altitude that will keep your altitude within a couple hundred feet, but adds up to 200' of total climb per hour with the up and down drift in the error. I would never use a unit without a barometric altimeter for climb data.

The total climb calculations using digital elevation based on position (DEM, Garmin "Elevation Correction") are lucky to have better than 25% error, with greater errors in hilly terrain where position matters. But they may still be better than using the GPS elevation without a barometric altimeter.

The barometric altimeters are fairly accurate, and consistent total climb can be achieve with their use. In calibration I find they work well, although Garmin typically under-reports the total climb by ~15% as mentioned above.


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## ewarnerusa (Jun 8, 2004)

Edge 500 has a barometric altimeter. Edge 200 does not


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## BigLarry (Jul 30, 2004)

ewarnerusa said:


> Edge 500 has a barometric altimeter. Edge 200 does not


Thanks. Will correct.


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## Harold (Dec 23, 2003)

I have been on rides where the barometric drift over the course of a half hour to hour-long rest stop has been greater than the largest climb on the entire ride. This time of year would be a good example of something like that. Systems move in FAST and can be quite strong this time of year.


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## BigLarry (Jul 30, 2004)

Harold said:


> I have been on rides where the barometric drift over the course of a half hour to hour-long rest stop has been greater than the largest climb on the entire ride. This time of year would be a good example of something like that. Systems move in FAST and can be quite strong this time of year.


And you must ride in a fairly flat area to have that much relative effect? How much altitude change did you see during that period?

In such cases, I'd recalibrate altitude after the rest stop. Flatter areas with fast moving systems might be one of the rare situations where the elevation models might be better than barometric altimeters.


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## Harold (Dec 23, 2003)

BigLarry said:


> And you must ride in a fairly flat area to have that much relative effect? How much altitude change did you see during that period?
> 
> In such cases, I'd recalibrate altitude after the rest stop. Flatter areas with fast moving systems might be one of the rare situations where the elevation models might be better than barometric altimeters.


Over the course of a ride, I can accumulate a couple thousand feet of climbing, but individual climbs top out at a few hundred feet, and most of the time are less than 150ft. The steepest ones are usually less than 50ft.

I haven't dealt with that sort of thing for a long time at this point, so digging up a particular ride to illustrate my point would have me digging into my archives pretty deep. For one, I've been using a GPS without a barometric altimeter for quite some time now. Second, Garmin Connect doesn't make it easy for me to see which device I recorded with in the general Activities table. Strava doesn't really give me the option to switch between elevation corrections or not. I suspect it's doing some processing of the elevation data even when there's barometric altimeter data, because Strava seems to process just about everything.


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## ghettocruiser (Jun 21, 2008)

I can't speak to the "professional surveys" they use in the USA

But in Canada, the elevation correction data is highly suspect, and I think they are just using the 5m +/- elevation model built into google.


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## Harold (Dec 23, 2003)

ghettocruiser said:


> I can't speak to the "professional surveys" they use in the USA
> 
> But in Canada, the elevation correction data is highly suspect, and I think they are just using the 5m +/- elevation model built into google.


Typical digital elevation models (commonly available on a global scale) have a 30m pixel size (30m^2). Elevation accuracy varies depending on the source data.

http://pubs.usgs.gov/of/2014/1008/

More accurate and higher resolution elevation models are available, but coverage is much more limited and these datasets are enormous where they are available. LIDAR elevation data is top notch stuff but using it would increase processing times significantly. I have used it on occasion. And there is not 100% coverage even in the US.

And for gps data where there is horizontal error, an elevation dataset with a higher spatial resolution can actually increase the error of elevation gain totals simply due to an increase in the noise from the data.


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## ghettocruiser (Jun 21, 2008)

I do recall the old 30-m DEM squares. I think around the city it's a bit better than that, but it's hard to say how much.

I know that the resolution is coarse enough to "round off" a cliff into a slope, for instance... if you wanted to rack up some bogus elevation totals, you can just turn circles on the level trail at the top of the bluffs, and it will think you're going halfway down them and climbing back up on every lap.


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## Harold (Dec 23, 2003)

ghettocruiser said:


> I do recall the old 30-m DEM squares. I think around the city it's a bit better than that, but it's hard to say how much.
> 
> I know that the resolution is coarse enough to "round off" a cliff into a slope, for instance... if you wanted to rack up some bogus elevation totals, you can just turn circles on the level trail at the top of the bluffs, and it will think you're going halfway down them and climbing back up on every lap.


I don't think any of the elevation corrections use a variety of dem data sources to do the job. That would really complicate the calculations.


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## redcon1 (May 9, 2008)

DC Rainmaker has an excellent Blog and tests a lot of Fitness products including GPS devices. Here's his excellent resource on GPS elevation issues:

Understanding Sport Device GPS Elevation Issues | DC Rainmaker


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## PlutonicPlague (Jan 19, 2014)

Being a gps newbie, I have found this thread to be educational. Mine has a barometric altimeter, and I have been having to reset it prior to every ride. I live right on the WA coast, within hearing distance of the surf, and the barometer goes thru changes here all day long! Sometimes when I turn it on, standing in my backyard, the first reading I get might be -25'. Other times, it might be +34'! 
My backyard patio is at 10' elevation. Holding my hand-held unit, I manually set the elevation at 14' and then I'm good to go. I just need to know distance, time riding, time stopped, max speed, av speed and total elevation gained. 
When I'm at some high point on my ride, or at a turn-around point, I'll pull it out of my frame bag and check the trip data, and maybe mark a waypoint. I've got a Map62s, and since its a bit bulky, I don't have it mounted on my bars.

Ha ha! I just turned my GPSmap62s on a few minutes ago, and let it sit upright out on my back porch for a bit. Had to give it time to adjust to the barometric pressure. There's a front going thru right now, with gusty winds and rain. It finally settled on +18' after about 5 or 6 minutes of sitting there, and the actual elevation of my back porch is +12'.


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## PlutonicPlague (Jan 19, 2014)

I left the unit sitting there on my porch with the power on, and just went out and checked it again. It read 14 feet. Now its reading is fluctuating between 14 feet, 16', then 18' then 20' ! Now up to 21'/22' and holding! Like I said, a low pressure system is moving through my area right now.


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