Geodetic head refers to the actual physical difference in height between the liquid level in the pit and the highest point of the discharge pipe or water level in the outlet.
The geodetic head between a pumping station and the outlet from the pumping station will constantly change during a pump cycle. When dimensioning a pump, the average water level between start and stop is normally used for defining the geodetic head.
Here it is important to understand that the geodetic head also can be negative. This will result in the siphoning effect.