I own the following device - https://interrex-tracking.com/lego-small/ the device is attached to a bird, and every day I receive information about time and date, X and Y locations, altitudе and geoid altitude, speed and more. My question is related to height measurement. Every day I have negative values, such as - 30, -100, -200 meters.
What caused this ?
Also what is the difference between altitude and geoid altitude and wich quantity is from sea level?
In other words, I need the exact height at which the bird flew.