Interesting. So would poor GPS have an impact on altitude values?
Resets after every pause means that it reads the altitude from GPS and then takes the barometric to update it?
I think this explains some weird readings that I got.
Poor GPS would indeed affect the absolute altitude metric, but only that metric. It would not affect the relative altitude or the ascent / descent figures because those metrics all come from the barometric sensor. The reason that the absolute metric uses the GPS is because the barometric sensor can only detect changes in air pressure and use that to determine your altitude relative to when it started taking air pressure readings. So to know the absolute altitude you need to use the GPS.
However because the barometric sensor is more accurate than GPS the app only uses the GPS altitude at the start (and after each pause) and then calculates the absolute altitude using that baseline and the relative change from the barometric sensor in the period since. Otherwise there could be an inconsistency between the relative and absolute altitude figures.
The problem with the barometric sensor is that changes in air pressure that are not due to altitude can also affect the readings. For example when a weather front comes in. So if you were to pause for a while as the air pressure changed then your absolute readings could get more inaccurate. This is why the app resets the baseline used to calculate the absolute altitude on resuming after a pause.
None of the figures will be perfect, because the sensors are not perfect. GPS is inaccurate for vertical positions and the barometric sensor is affected by weather changes. But the app tries to do the best it can with the available data.
Apologies for the long explanation!
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Apple has a support document on calibrating the watch for improved workouts, and ot continously calibrates every time a workout is done (
https://support.apple.com/en-us/HT204516).
Since WorkOutDoors is using official workout APIs I assume it should continously calibrate when using WorkOutDoors as well? Otherwise I would have to use Workouts every once in a while, and that would kinda suck.
I wish I knew if workouts in 3rd party apps are used to calibrate the watch. Unfortunately I haven't found anything anywhere that confirms or denies it.
One thing I can say is that you should be careful when choosing the activity type for a workout. I have heard of people using the running activity type with Apple's Workout app when they are actually cross country skiing or when they are roller blading. This has caused the watch to think that they have incredibly long strides and therefore affected the distance estimates when they do actually go running and use WorkOutDoors.
So if anyone does a similar sport then in theory they could determine if 3rd party apps like WorkOutDoors do affect the calibration. Although if you try this then make sure to reset the watch calibration before relying on the distances!