An altimeter is an instrument for determining altitude attained, especially a barometric or radar device used in an aircraft. Seeing that we will soon have access to the whole world, there will be major terrain elevation differences in many parts of the world. The process of using this altimeter would be fairly simple. You would read the meter and find out the airport pressure elevation for that airport, then you would adjust the altimeter accordingly and it would correspond with our altitude indicator.
As you can see, there is a knob on the bottom left which controls the numbers in the Kollsmon Window.