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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.

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    There is no clear answer as to which one is the "correct" hight, it depends on what you want. Make yourself comfortable with the concept of "geoid height" - a quick internet search will give you plenty of information, incliuding graphics.
    – Babel
    Nov 1, 2021 at 15:59

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It appears that the altitude values from your bird-mounted GNSS unit are returning heights in WGS84 geoid heights. Here is a nice tutorial on vertical datums. I see from the link you posted that the unit might also be returning 'elevation' in addition to geoid height. This elevation would be referencing the WGS84 ellipsoid. While that ellipsoid height will not solve your problem it will reduce and simplify the conversion since you will not need to figure out which geoid the unit is referring to.

You can use vdatum to convert between vertical datums, for example from your geoid to NAVD88 if you are in the USA, Candada, or Mexico. ArcGIS Pro also has the ability to do vertical datum conversions but you may need to download additional files. You may have limited output choices based on your location outside of the USA. A vDatum sea level conversion would require a close proximity to sea water.

Perhaps post a portion of the CSV data and clarify your question for more help.

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