How to stop altimeter from autocalibrating using GPS?

Find here some more information on altimeter calibration:

https://support.garmin.com/en-US/?faq=pCL0xvX9lj8hdtZcNf1ZAA

You can not fully disable Auto calibration. The nightly calibration is always active. 

Thanks for providing the link. Even though that page doesn't address my problem at all, it's useful for others.

This seems like a design flaw. It allows you to manually calibrate, but then auto calibration proceeds to override and ruin the manual calibration and you can't stop it.

I am quite disappointed as it renders my altimeter unusable once GPS is turned on, just because they left out the option to disable auto cal.

If it's one of Garmin's consumer-level watches, I can accept it, just as consumer DSLRs have limited feature sets. But for their professional military model, it's ridiculous.

You should try with nightly calibration. I am not sure how it will behave when you regularly calibrate manually. Ideally it will skip nightly calibration if you manually calibrated shortly before. 

Where do you get the elevation values that you use for manual calibration? Can you be sure that these are more accurate than the ones used by Garmin in the auto calibration process? Maybe you can omit manual calibration completely and rely on auto calibration. This should make the readouts more precise. 

The problem is that we are not sure how it will behave... If I know how it will behave, I can work around it although I will still be very annoyed. But if I don't know how it will behave, it is as good as useless.

The values I use for manual calibration are from a large-scale topographic map produced to high standards, ground-truthed with a well-known landmark whose elevation has been determined very accurately.

Furthermore, I calibrated it at one location and walked in a convoluted route up and down to the second location, and the altimeter accurately showed the elevation of the second location, which was incidentally the aforementioned landmark. Both locations were peaks, so I was sure I was at the elevations stated on the map. Of course, this was before I turned on the GPS.

I don't think the problem is caused by the watch using some inaccurate DEM for auto cal, as the wrong readings seem to arise only upon GPS activation.

A remote possibility is that the Global Positioning System uses a different sea level datum than what my government is using, and the difference is about 30 metres. But I seriously doubt it because our government tends to follow Western international standards for these things, and 30 metres seems absurd.

I'll try to find a chance to check if the altimeter is always the same amount too high. But even then, it may deviate by a different amount in another part of the world.

7269862 said:

I don't think the problem is caused by the watch using some inaccurate DEM for auto cal, as the wrong readings seem to arise only upon GPS activation

It would be interesting what a manual DEM calibration would give you after saving the location (which seems to trigger a calibration of some sort).

GPS altitude is quite inaccurate, that's why it is only the last choice that the watch should take. I is mostly DEM data that is used for altimeter auto calibration. The Garmin DEM is for sure not the most accurate, but 30m is indeed a lot. In front of where I live, Garmin gives me 5 meters too high altitude readings from DEM, compared to LIDAR elevation data.