The 735XT doesn’t have a barometric altimeter. So, altitude stuff would have either come from GPS or post-activity correction.

Elevation gain/loss measured by barometric changes is really just a SWAG. On rides with less elevation change, say 400ft or less, I've seen variance of more than 50%. I have a 20 mile loop around my house that I ride a couple times each month. The last 3 rides showed 725ft, 860ft, and 909ft. The info is nice to have, but not worth a whole lot.

I've got a Fenix 3. It's been giving me a few hundred extra meters of vertical gain on each of my long runs lately. I find this super annoying, TBH.

I've had long runs along a lake (virtually no elevation change) show as thousands of feet on my 935. It's completely useless and wish it used GPS/elevation map data instead

My Fenix 5 used to be fairly accurate (it seemed) but the last 6 months or so it is almost always exaggerated by thousands of feet. Totally useless. No luck troubleshooting with Garmin.

Yesterday I did intervals on the track and it said over 2000ft of elevation haha.

I think this is caused, or at least exacerbated by the little hole where the barometer measures the air, being clogged up.
Which sweat, dust, etc can do over time.

Or maybe that’s an Escherian track? 😂

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float , hammer , and jog

I regulaly see on strava where a cyclist has stopped at a cafe the air pressure has change and their altitude changes by several hundred feet.

The barometric sensor in the garmin devices is quite sensitive, and quite often breaks. If that happens you'll also notice that the temperature readings are inaccurate. If they don't record at all the sensor is truly toast. I've had it happen on a 920XT and a 935. Both times it happened when the watch was a little less than a year old and both times Garmin replaced the device under warranty.

Hey ,
How do you like the 520 ? I just purchased one and I’m pleasantly surprised on the amount of setting options .
Steve Ward -Milton .

Very interesting. Since you said that, I went back and looked at my data. Sure enough, it hasn't recorded a temperature since mid April. Before it stopped recording, it was either a bogus reading for each activity (90° always), or way off (e.g. avg 135°). So yeah the sensor is definitely toast.

Unfortunately my Fenix is over 2 years old at this point. But I'll try Garnin/REI again anyways.