Measurement of the sea level at a tide station
A tide station is equipped with a well into which sea water is run through the inlet pipe (aqueduct) so that the height of the water surface in the well is regarded as the sea level.
The sea level is measured by a tide gauge through a buoy which is floating on the water in the well.
There is a fixed point as a reference at the upper end of the well.
The height of the fixed point above the local reference datum is defined by a datum constant. In other word, the local reference datum is defined as a level at the depth of the datum constant below the fixed point. The tide gauge is calibrated so that the sea level is measured as height above the local reference datum.