Over the years I have had several Garmin units. In the past, I almost entirely used them for navigation, but recently gave my current unit, a GPSMAP 66, to someone to do some locations. I thought that the heights/elevations stored for a waypoint were GPS derived and corrected using a geoid model like EGM96 or EGM08.?ÿ
However, I just took a reading on a known point, and the difference between the ITRF2014 ellipsoid height and the displayed height is about 89 meters, whereas geoid separation here is about 30 meters. And the two readings, taken an hour apart, agreed to about 7 cm.?ÿ
This leads me to believe that it is using the barometer on the unit. I checked the settings, and it was set to calibrate once, meaning the barometer was using whatever calibration was there for the last few years.?ÿ
Looking on the web site for Garmin, it says that calibrating to a known elevation is ?ñ50 feet, while using GPS to calibrate is ?ñ400 feet.?ÿ
Curious as to why they wouldn't use GPS and a model??ÿ
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I thought they used the pressure reading for short-term corrections but slowly adjusted the baro calibration from the GPS. I was guessing it did have a crude geoid model, but can't confirm that.
Their primary market doesn't need particularly good elevation data.
@bill93 I found a setting for auto calibration...Off/Once/Continuous. This unit was set to Once.