Mean sea level (MSL)'s the average (mean) height of the sea, with reference to a suitable reference surface. Defining the reference level', however, involves complex measurement, and accurately determining MSL can prove difficult. Measurement To an operator of a tide gauge, MSL means the "still water level"—the level of the sea with motions such as wind waves averaged out—averaged over a period of time such that changes in sea level, for example, due to the tides, also get average… (More on Sea-level)