Reporting an altimeter issue.

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I don't own a 67i. In general, however, the ONLY time the altimeter will be fully accurate is right after you calibrate it  manually (either to a known altitude or to a known pressure). If you enable continuous correction via GPS, the reported altitude will slowly become somewhat inaccurate. There is more inherent error in GPS vertical measures than in horizontal (lat/lon) measures.

That said, even the auto-updated measures are usually good enough for practical purposes.

Understandable. Are garmin watches more accurate? Reason I brought it up is because I’ve hiked the same handful of trails over the years with Fenix 5/6/7 and Tactix. All pretty close, 67i over estimates 200-300ft of elevation gain. That doesn’t seem like “somewhat inaccurate”. Just seeing if anyone else is having the problem. 

Probably not inherently more accurate. However, for some devices, the maps in use may influence this. IIRC, some devices make use of DEM data if the maps have it. I don't know if fenix or Tactix devices do this, let alone the 67i. I no longer recall where I saw the reference to the use of DEM data, nor do I know exactly how/if it interacts with the altimeter settings.

That said, it's also possible that the devices use different sensors, so...

It looks to me like a weather issue.  I see this daily.  Here is a weather station within 1/2 mile of my house which is at sea level.

https://www.weather.gov/wrh/timeseries?site=AR684

This morning I calibrated my Fenix watch to the pressure provided by this sight and elevation of sea level at 6:30.  Notice that the pressure increased by 0.07 inHg at 11:30.  This was equivalent to an elevation of -50 ft.  No, I was not 50 ft below local PC warer.

Hello, Antonb I would recommend checking out the following articles on the accuracy of the altimeter readings for this device. Barometric Altimeter Accuracy of Outdoor Products and Calibrating the GPSMAP 66 Series Barometric Altimeter. I would also recommend that you have verify the device software is up to date Installing Software Updates to Outdoor Handhelds.