View source for Reference systems for GPS coordinates and altitudes

All of our UvA-BiTS GPS trackers use the Ublox Max-6G GPS receiver to collect vertical and horizontal position data. 

The GPS unit measures the distance from the satellites to the device and then it calculates the distance relative to the WGS84 ellipsoid (World Geodetic System WGS84). GPS coordinates in the database are calculated relative to the WGS84 datum.
 
The altitude measurements provided in the database are altitude above a geoid that approximates global mean sea level rather than altitudes relative to the WGS84 ellipsoid. (A good explanation about the difference between ellipsoids and geoid is available here: https://www.esri.com/news/arcuser/0703/geoid1of3.html)

Be aware that global mean sea level is not the same as the earth’s surface (i.e. land surface elevation). However, the SRTM-DEM data included in the database as land surface elevation data are also calculated relative to global mean sea level and can therefore be combined directly with UvA-BiTS movement data to calculate the bird’s altitude above land surface elevation. Different GPS tracker manufacturers may provide altitude measurements using different methods and/or reference systems, so use caution when combining GPS data collected with multiple tracking systems.