GPS derived elevation is not nearly as accurate as the 2D location. This is most likely a matter of the sensitivity of your phones GPS receiver how accurate it is.I've noticed that GPS elevations on serveral apps are questionable, including Positional. Is this just a problem with GPS, or worse at lower elecations?
Yesterday, I was showin off the App and was at a known 18ft. It showed minus 128. It was rainy and cloudy, if that's a factor.
GPS elevation can get pretty good if you have a good antenna, the software can obtain and use the augmentation satellites that provide correction algorithms, and so forth. Survey grade GPS and the units used in aircraft are capable of obtaining a elevation figure you can sort of trust. That said aircraft all have barometer based altimeters.
Some phones also have barometers, my Samsung S7 does. I would not know if Positional uses that, but I doubt it as barometer based altimeters require calibration, and all apps that use it will have a interface to calibrate at a known altitude.
My handheld GPS has a barometer, which tells you how confident they were in the reliability of GPS derived elevation....
Another thing to keep in mind is GPS signal is a variable thing, depending on the number of satellites above the horizon, the spread of them in the sky relative to the receiver, and solar activity, a locks accuracy can vary. For most people it's good enough most of the time, but there are odd times if all factors are working against you the accuracy can get dicey.