Altimeter creep

It doesn't sound like you have the alto/baro/temp issue that has beset some owners of F3 HRs (that issue has crazy large rapid swings in altitude). So it sounds like you've manually calibrated to 109m and then started a GPS activity (cycle, run or walk) and the altitude is not at the manually set altitude at the end of the activity. So the next thing to check is if your Altimeter Auto Calibrate setting (in the sensor settings menu) is On. If it's On then the watch recalibrates altitude to GPS altitude when you start an activity; so the error you see could be an incorrect GPS altitude calibration. If it's not already Off, it might be worth you doing a quick test with the Altimeter Auto Calibration set to Off.

Yes, you are describing the correct circumstance.
I will see how Auto Calibration set to Off effects it.
I suppose another way is to drive somewhere, ie no Activity, then see if the Alt reading is the same when back home.

This offset of 6 m at 150 m is around 4%.
Normally, you need 10 degrees temperature difference for this, which seems a bit high in this case.
Were you wearing the watch before or did you put it on and start? In this case it might be plausible.

Maybe next time wear it an a jacket pocket so that it doesn't get warmed by body heat or the sun. Just for this experiment's sake, I'm not suggesting you should wear it like that all the time :)
If the results are close, it might as well be the temperature.

What I'm saying is that in a couple hundred walks 150 -> 2 is always correct while 2 -> 150 is never correct and always ends up higher. It's not just one instance.

If the problem was related to the temperature I'd be getting the offset value at sea level too. It remains to be seen whether the Tempe can correct this by excluding the wrist's heat but we really don't know whether Garmin's software takes the heat into account in the first place and since Garmin needs 500 e-mails to give 1 reply, often inconclusive, we'll probably never know.

It's not really the end of the world for just a few meters but if this error cumulates (like it seems to do in the picture of the bike loops) then it's a big problem.

What I'm saying is that in a couple hundred walks 150 -> 2 is always correct while 2 -> 150 is never correct and always ends up higher. It's not just one instance.

My guess was when you go from 150 to 2, your body temperature doesn't increase much as it's easy walking or running downhill. When you go up, you overheat, i.e. your body temp goes up and hence the difference.
So the problem is not air temperature, in this case the offset should be the same in both directions, but warmer watch due to body heat.
Try it with the watch off your wrist.

This issue has zero to do with air temp...
My suunto core at 1/4 the price is infinitely more accurate in the ABC department than the F5, side by side tests as well as 3 previous years of wearing the Core prove this. The core is a tool, the Fenix is a novelty fitbit on steroids with it's accuracy issues.

There are undoubtedly more accurate devices available. If you need that accuracy, best to return the watch. If you want a watch that handles a large number of activities, is a great daily tracker, is loaded with metrics and customisation, while still looking damned good, then stay with the Fenix. It is a great, well rounded tool for many people who want a device that, while not the most accurate in some areas, still delivers a good result in a stylish package. I love the GPS accuracy of my Tomtom watch, but I loathe its lack of features, its cheap looks and inability to export to sites such as Myfitnesspal. However, for someone that just wants really good GPS in an economical package, it is a very good choice. We are all different and have expectations of our devices that reflect our personal requirements. Ignoring manufacturing defects (these happen with all manufacturers), if you find the features and performance don't meet your needs, buy something else. My needs are more general fitness, so trends are more important the perfect accuracy. That means I can live with my three devices being a couple of metres out (which is the most altitude difference I have seen between 2 x Tomtom watches, the Fenix 5 & 5X). That does not mean others do not have a valid issue. It just means that if you have an issue (once again, not just a manufacturing defect that can be resolved by swapping it under warranty), then you should be exchanging it ASAP for something that does make you happy. Life is too short, so go get that perfect watch.

There are undoubtedly more accurate devices available. If you need that accuracy, best to return the watch. If you want a watch that handles a large number of activities, is a great daily tracker, is loaded with metrics and customisation, while still looking damned good, then stay with the Fenix. It is a great, well rounded tool for many people who want a device that, while not the most accurate in some areas, still delivers a good result in a stylish package. I love the GPS accuracy of my Tomtom watch, but I loathe its lack of features, its cheap looks and inability to export to sites such as Myfitnesspal. However, for someone that just wants really good GPS in an economical package, it is a very good choice. We are all different and have expectations of our devices that reflect our personal requirements. Ignoring manufacturing defects (these happen with all manufacturers), if you find the features and performance don't meet your needs, buy something else. My needs are more general fitness, so trends are more important the perfect accuracy. That means I can live with my three devices being a couple of metres out (which is the most altitude difference I have seen between 2 x Tomtom watches, the Fenix 5 & 5X). That does not mean others do not have a valid issue. It just means that if you have an issue (once again, not just a manufacturing defect that can be resolved by swapping it under warranty), then you should be exchanging it ASAP for something that does make you happy. Life is too short, so go get that perfect watch.

To summarize what you're saying, "Pay $800 for mediocrity.. as it does a lot, but isn't great at anything" haha nailed it

To summarize what you're saying, "Pay $800 for mediocrity.. as it does a lot, but isn't great at anything" haha nailed it

To summarise, buy what you want, or if you don't like it, buy another watch, get off this forum and stop whinging? Your rudeness is amazing. If you have come here solely to gripe and offer nothing constructive, perhaps you may have more luck on the Suunto forum?
I do apologise for my response, but you have responded quite rudely to a post that was just my opinon, but you seem to be making a habit of that, judging by your other posts.
BTW, both of my watches work briliantly (I have an F5 and F5X, as I passed the F5 to my wife to get the 5X). GPS trace is not perfect, but it is close and the distance is always consistent. Altimeter is spot on (yes, I had an issue but that is resolved). All features work as advertised. Battery life is amazing. The app and web interface work brilliantly. It has a massive amount of metrics etc. I can customise the experience with great watch faces etc. Still sounding mediocre? Enjoy your cheap Suunto, but please do not criticise others for liking what they have purchased. Sure, talk about your own dislikes and likes. When you criticise other people for having an opinion that disagrees with yours, feel free to leave. Apparently you don't like anything about the watch, so why did you buy it? Oh sorry, does that make you mediocre, foolish, or just a troll?

To summarise, buy what you want, or if you don't like it, buy another watch, get off this forum and stop whinging? Your rudeness is amazing. If you have come here solely to gripe and offer nothing constructive, perhaps you may have more luck on the Suunto forum?
I do apologise for my response, but you have responded quite rudely to a post that was just my opinon, but you seem to be making a habit of that, judging by your other posts.
BTW, both of my watches work briliantly (I have an F5 and F5X, as I passed the F5 to my wife to get the 5X). GPS trace is not perfect, but it is close and the distance is always consistent. Altimeter is spot on (yes, I had an issue but that is resolved). All features work as advertised. Battery life is amazing. The app and web interface work brilliantly. It has a massive amount of metrics etc. I can customise the experience with great watch faces etc. Still sounding mediocre? Enjoy your cheap Suunto, but please do not criticise others for liking what they have purchased. Sure, talk about your own dislikes and likes. When you criticise other people for having an opinion that disagrees with yours, feel free to leave. Apparently you don't like anything about the watch, so why did you buy it? Oh sorry, does that make you mediocre, foolish, or just a troll?

You are far too easily offended/upset. Definitely a common problem in America today! I'm really not sure you know the difference between critiquing the watch from a consumer perspective and"criticizing others".. I never once criticized anyone for purchasing the watch, as I did the same thing. You come across extremely awkwardly as I'm not sure if perhaps you work for Garmin and are personally offended by negative comments regarding certain aspects of watch performance (certainly not all). I quote you when I say "when you criticize people for having an opinion that disagrees with your own, feel free to leave".

I am not American and maybe my expectations of respect exceed yours (sorry, no disrespect to Americans, who I typically find quite courteous).