This question in its self is a quandary of relativity. To claim a height requires a base line from which to measure the height. As hydrographers we would recognise a gauge line and gauge zero as a means to determine a height at any one point. People measuring groundwater would look for a reference point by which to measure the Standing Water Level.
Water level is one of the primary parameters measured by water monitoring staff. It has the advantage of being able to be measured, recorded and reported continuously for every day operations and historical record. Height is used as a continuous surrogate measurement for other parameters by the use of correlations and typically, water availability.
We record stream height both historically and real time by a variety of methods. Technology and instrumentation enable us to measure these relatively accurately and disseminate the information quickly. Irrespective of equipment used at a monitoring site, the operator typically adjusts the recorded data to that observed at the site. This may be post processing of recorded data and/or the instrumentation will be reset to the observed value at the time of the service visit.
This observed water level is, by one way or another, an observation from a datum level. For bores is may be the top of the pipe or casing. Typically for gauging stations it is the datum or gauge zero of the staff gauge.
To maintain a sites datum requires accurate surveying practises. To verify the reliability of the staff gauge, the gauge is periodically surveyed from a datum control or bench mark. The datum control is supposed to provide continuity between all surveying activities related to a site. These surveys include gauge levels, cross and long sections, bed and hydraulic slope, storage capacity and inundation and more recently as control points for remote sensing programs.
Historically the datum control has been referenced to a local “assumed” datum, a “state” datum or the national “Australian Height Datum” AHD. This was the scenario in our part of the world until recent years.