I use two devices when flying: an Ipad Mini 2 (cellular) and a Samsung J5 2016. I realized flying over the coast, that the value indicated in the Samsung was always correct, as it was close the the one of the Altimeter, less than 20 ft different. On the other side, with the Ipad, the level of error was at the same time of 300 ft, and was not stable, it could change strongly in some minutes. The Samsung uses GPS+GLONASS.  and the Ipad only GPS. Does somebody know why the measure of the Samsung is better ? is the addition of GLONASS ?  or there is another reason ?