Relatively new user here. It is my understanding that aircraft altitudes are given in MSL. I live at 980ft MSL, but was watching a plane with an altitude of 300ft displayed. Where I live is a flat area, there are no valleys or other geographical features to explain this. What is going on?
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It really depends upon context and who is reporting the altitudes.
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Transponders report altitude based on QNH 29.92 or 101.3. ATC correct this for local conditions. They can be out over 1000ft depending on local temperature and pressure.
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To elaborate on Jon’s answer – Mode S / ADS-B primarily reports barometric altitude based on standard atmospheric conditions.
That is: measure static air pressure, and interpret that as an altitude assuming atmospheric conditions are standard.
This has the advantage that everyone is exchanging the same numbers (so two aircraft in the same area reporting, say, 3000ft are probably actually at the same altitude, which is important to know) but it makes it harder to translate into an altitude MSL or AGL. You need to know the local conditions at the time to be able to do that.
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Thank you for the responses and information. After posting this question, I could not find this forum again, no matter where I looked in the app for a link of some sort, and finally only got back here using google. Anyway, it seems the altitude data is just not very reliable, and also I have encountered many times when an airplane should be literally 1000 feet away from me, as I have a business right near the final approach to a runway, but there is no airplane in sight. I can watch it on the app, but there is no plane, which should be several hundred feet AGL and right in front of me, yet there is no plane. The only thing I can think of is there is a delay of many, many minutes or more. Still useful and fun to play with, but not very reliable IMO. Anyway, thanks again for the responses.