The Altimeter shows the aircraft’s height (usually in feet or meters) above some reference level (usually sea-level) by measuring the local air pressure. It is adjustable for local barometric pressure (referred to sea level) which must be set correctly to obtain accurate altitude readings.  This requires the altimeter to be set prior to every flight, and during flight as barometric pressure in your flying area changes.  The Altimeter measures the Altitude or height of the aircraft above Sea Level. Remember, ground elevation varies widely, so the Altimeter reading does not measure height about the Ground, but instead above Sea Level.

Similar to a clock, an Altimeter has three hands. The fastest moving hand reads in Hundreds of Feet. The shortest hand reads in Thousands of Feet. The longest hand, which moves the slowest, reads in Tens of Thousands of Feet (on some altimeters, the Tens of Thousands of Feet is represented with the shortest hand, instead of the longest hand).