I cannot stop to be amazed by just how smart these modern altimeter wrist watches really are. Here's a watch which last altitude calibration was more than three months ago. Meanwhile, since then, I've used it sparsely during my work and on my escapades into the mountains.
Today, in the Pico Ruívo mountain hut (at an altitude of 1775 meters), this was the error of the instrument: fifteen meters.
I repeat once again: the altimeter in these watches is a sub function of their barometric sensor. This means that the equipment "guesses" the altitude by the variation of the atmospheric pressure (lower pressure means higher altitude and higher pressure means lower). However, for any given place on Earth, the atmospheric pressure varies with time, due to changing meteorological conditions. In a certain moment in time, in any given place, you might have depression conditions (low pressures) or anticyclonic ones (high pressures). So, the atmospheric pressure, in any place on Earth, is never the same. Never constant.
This factor alone should be a cause for permanent errors in measurements from these equipments.
So, how do they do it? What kind of complex calculation algorithm do they use on these equipments, that narrows the error to 15 meters (in almost two kilometres of altitude)?
In the picture: the reliable Casio G-Shock Riseman Radio Controlled Watch, ref. GW-9200-1ER.