JonnyUtah75

So I ride in South Florida where the only hills are bridges. I've written to Garmin that I think the elevation may be off on my unit, and they told me to calibrate it from a known elevation. So this morning I set it at 6ft, which is the average elevation in my area. After my ride, it logged me in at 299 feet. When I uploaded to Strava and enabled elevation correction, it had me at 0 feet.

So is it possible that the Garmin is correct, and the 4 bridges I went over accounted for the elevation, or is Strava's elevation map correct (or they don't account for bridges)? My ride from this morning (below) is after calibration. Before, my normal 60-mile loop with a few more bridges would come in over 1,500 feet of climbing.

https://connect.garmin.com/activity/286728799

Looigi

Garmins use a atmospheric/barometric pressure sensor for elevation and in my experience always show more elevation gain/loss than what mapping websites show. The Garmin records all the small ups and downs that likely are not in map databases so wouldn't show up as elevation gain/loss in Strava or on other sites.

Also, changes in atmospheric pressure (aka barometric pressure as used to predict weather) will show up as changes in elevation so this can throw off accuracy. Changing winds or buffeting of the senor can cause altitude variations too.

Just a 0.003% change in pressure is equivalent to 1 foot change in elevation, so it doesn't take much pressure difference to show up as a change in elevation.

With mine I've had the pressure ports in the bottom get clogged by water while riding in the rain and the altitude suddenly jump up 1000 ft.

Pug

I was riding into a strong headwind on a flat road and I picked up about 60 feet of elevation.

Beachgrad05

Interesting...for me I have found that Strava will give me more elevation by significant amount once I use "correction". Garmin uses barometric.

example of differences:
Strava shows 3238 feet after I had Strava use what they use
https://app.strava.com/activities/44685412

same exact ride oh Garmin connect shows 2293 feet using the barometric measure within unit
https://connect.garmin.com/activity/285161218

johnny99

Strava is probably using digital map elevation, which uses terrain features only, not elevation due to bridges. Barometric altimeters are not 100% accurate, but should give you a better idea of what you actually rode.

Beachgrad05

This is from Strava's site

How Altitude data is collected:
Barometric altimeters determine altitude by measuring atmospheric pressure. Measurements can be affected by changes in the weather or the sensor holes getting blocked by water or other debris. Device manufacturers may provide basic calibration procedures. For example, Garmin devices with barometric altimeters allow a certain number of manual elevation points to be set; starting an activity near an elevation point causes the device to use the known elevation as the starting point. Consult the manual for your device to learn about calibration procedures.


GPS altitude measurements are derived from GPS signals and can have relatively high vertical error. They also depend on how many satellites are being used and where the satellites are located in the sky.


We generally consider the data from barometric altimeters to be of higher quality than the data derived from GPS signals and prefer that when processing activities. For devices without barometric altimeters, we consult elevation databases to determine the elevation at each point in the activity. The resolution of these databases can vary based on location. For example, the USGS National Elevation Dataset (NED), which we consult for activities located in the US, generally has a 10 meter resolution though some small areas have a 3 meter resolution. For activities located outside of the US, we consult the ASTER and SRTM databases which have a resolution of 30 meters and 90 meters respectively.


We discard outliers and smooth the elevation data when calculating gain to reduce noise. The amount of smoothing depends on the source of the data - data from barometric altimeters is smoothed less while data looked up from elevation databases is smoothed more. When computing gain, we use a threshold to determine whether or not to include a change in elevation; in this way, small variations in elevation change are ignored while larger changes are included.

achoo

Well, the Garmin 500 has had elevation issues since the 3.0 firmware version was released. Here's just one thread of MANY you'll find:

https://forums.garmin.com/showthread...installing-3-0

Seattle Forrest

Wind = air rushing from a high pressure zone into a low pressure zone. Barometric altimeters measure pressure, and are bound to get fooled by this.

kv501

For every ride I upload I look at the website's (Garmin Connect, Strava, or both, depending on how much time I have) elevation data, compare it to the unit's data and go with whichever is more reasonable.

Here is what I have non-scientifically found to be the case:

For rides where there is very little change in barometric pressure, i.e. relatively unchanging weather...the Garmin unit is very accurate at recording elevation. On days where you start out in nice weather and it gets worse or vice versa, I get sketchy data from the Garmin at best. Then I go with the website's elevation profile. To illustrate, my driveway is truly at 1407' in elevation, and on a nice day the Garmin unit is always within 10 or 15' of that. Last summer I went on a ride starting at 65 degrees, sunny, and hardly any wind. 50 miles later I rolled back into my garage at about 50 degrees, wind, and drizzle. The elevation when I got back read 1515' and the profile from the unit was wacko. said I climbed something like 1,700' and I live in dead flat northwest Iowa.

Bottom line is, look at both, pick the one you think is most accurate. If the weather conditions didn't change much during your ride, go with the Garmin.