Barometr and altitude change on starting activity

Thx for your answer.

I've got this 3 options as you write. I wonder why my watch changing baro when starting activity - does it mean watch updates data only when starting activity?

I've got this 3 options as you write. I wonder why my watch changing baro when starting activity - does it mean watch updates data only when starting activity?

If you manually calibrate the Altimeter immediately before an Activity, does the Baro still change after you enter and start an Acivity?

Ill try tomorrow. I think it happen after long time. I.e. after night. Another activity just after previous one changes nothing.

Ill try calibrate at the morning and then start activity. 

Hi Tomek, first you need to understand how the barometric altimeter and the automatic altitude adjustment work, from there it's all obvious.

The altimeter is barometric add it calculates the altitude based on air pressure. In principle the air pressure descends as you ascend. Roughly you can say that change of 0,1 hPa means a change of 1 m. The correlation is indirect meaning when one value grows, the other declines and vice versa.

But the air pressure does not only change due to the elevation change but also as the weather changes. Sometimes it changes slowly, sometimes rather fast like for example before a storm. To check this you can use some detailed weather forecast service online or within an app.

When the altitude auto adjustment is on, the watch uses the GPS information to correct the altitude which might be wrong as the air pressure changes.

When you start an activity the watch uses the GPS information to adjust your altitude. So you should first wait for GPS fix before you push the start button as this not only gives you your (relatively) accurate position but also (relatively) accurate altitude. Why relatively? You might have noticed that the "2D" GPS position even in ideal conditions is usually not more accurate than +- 3m; in a forest or under a cloudy sky the accuracy can be +- 20 m very easy... For the GPS based altitude this is even much less accurate - we're taking some +- 10-15 m under ideal conditions.

So if you want maximum accuracy you need to calibrate the altitude manually during the activity in a frequency which corresponds to the weather conditions - the faster the air pressure changes the more often you should adjust the altitude (not worth it if you ask me). The only reasonable way how to achieve a pretty accurate elevation information is to allow altitude adjustment of the particular activity in web Connect - when you review the activity in web Connect the option is at the very bottom close under your device image.

After manual calibration starting an activity makes no changes.

Thank you for long and complete answer.

I wonder why watch auto calibrate altimetr from GPS when I have settings:

During activity is set to off (I thought calibration will  happeni if I chose option at start?)

Also I wonder how accurate is altimetr without entering in activity and calibrating it via GPS? And how accurate is barometer/can I base on storm alert?

I too have the same problem. I have Auto Cal off  for the altimeter, yet every time I start an activity with GPS the altitude jumps to some ridiculous value +/- 20-100 meters. With Auto Cal off it should stay no matter what GPS says, I think it's a bug.

No, with autocalibration off the altitude always has to change when you start an activity. These are two different issues. Autocalibration means the watch checks the GPS altitude time to time and adjusts its value. But the initial GPS fix always sets the current altitude. The reasonability of the jump depends on what the watch say about the altitude at the start of the activity right after the GPS fix. It should show your current altitude based on the GPS position. A "jump" per se is no bug. Think of it this way: if you have the autocalibration switched off at all, the watch never try to autocalibrate but at the moment when you start the activity. And between one and the next activity the weather including the barrometric pressure can change dramatically. As said 0,1 hPa is roughly 1 m of altitude. So a change of 2 hPa is 20 m. As the barrometric pressure changes continuously between one and the next activity the watch can easily think you've changed your altitude by tenths of meters. And the mandatory setting during the GPS fix while starting an activity is such that it sets not only your 2D but your 3D position, so including the altitude. Hope I shed some light into your doubts...

A start of any outdoor activity with GPS on always triggers the GPS fix in 3D, this simply is obligatory and has nothing to do with the autocalibration. At certain models this can be overwritten by manual calibration, typically when you know your exact altitude but the GPS calculated altitude is wrong (as said there can easily be 10 - 20 m error). The best way is to read the altitude from a good map as often the altitude information on trail signs is also wrong... ;-)
I do not understand the last question. The barrometric altimeter is in principle more accurate than a GPS based one if this is what you are asking. But of course it can be affected by the weather changes. As said if you want an accurate information you would need to adjust the altitude information based on a good map regularly when there are fast changes in the air pressure... ;-)