The purpose of the altimeter is to show the pilot the altitude and the pressure above a fixed level. It uses barometric pressure and is 29.92Hg in ISA conditions.
What I dont understand is, how can this be extrapolated to be used in a situation where there is an elevation. For example, say the airplane is in a static position in an airfield at code EGNX, 52°49′52″N 001°19′40″W that has a 10 foot elevation. What would the altimeter indicate in such a situation.
Edit: Added a starting position