ELEVATION
The elevation of a geographic location is its height above or below a fixed reference point. Commonly,
elevation is measured using the center of the Earth as the reference point.
What
is height of instrument (HI)
In any particular set up of an instrument
height of instrument, which is the elevation of the line of sight, is constant.
The elevation of unknown points can be obtained by subtracting the staff
readings at the desired points from the height of instrument. This is the basic
behind the height of instrument method for reduction of level.
Image
example 