The technology for altimeters has changed over time. Some use pressure measurements and others use satellites (GPS technology) or radar to determine the altitude of a given place (you can use sea level as the base against which you are measuring as in your example). Properly calibrated equipment (particularly pressure measures) can be pretty accurate. GPS accuracy depends on a lot of variables (number of satellites hooked in, satellite position relative to the horizon, etc.). I haven’t ever measured Denver, so I can’t speak to the accuracy of the mile-high claim. I imagine pilots that fly over the Rocky’s have a pretty accurate sense of just how high Denver is so one might be able to shed some light . . .