AmbientWeather.com: Altimeter varies at the same location.
Altimeters measure pressure, which is a function of altitude and barometric pressure. Normal fluctuations in barometric pressure will change the measured altitude at a fixed location.
A change is barometric pressure at standard sea-level pressure (29.92 in Hg) of 1 inch results in a change in measured altitude of 925 feet. To a lesser extent, barometric pressure (altitude) is also a function of temperature.
Thus, you must calibrate the altitude on a daily basis from a fixed location.