What does the altimeter indicate if it's at a certain elevation 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
An altimeter is just an adjustable barometer with extra hands.

There is a barometric correction you dial in so that the altitude shown is always feet above sea level.  The correction is from the current barometric pressure reference you get with the "altimeter setting" or the weather report.  

You dial in the altimeter setting, the current barometric pressure reference for your location, and shows your true height above sea level.  In your example, if the altimeter setting is correct, it will show 10 ft above 0 ft.

Or, you can work it the other way.  If you know the elevation you are at, you can dial the altimeter so its altitude indication matches your known elevation.  Voila!; the current barometric pressure is shown in the setting window on the instrument's face.