What is mean sea level? What does the sign about height above mean sea level at a railway station mean?
The ‘Mean sea level’ (MSL) is a reference level from which vertical elevation is measured. It is fixed by observations over a long time of the surface of the open ocean, averaging out the changes due to tides, etc. The level is then extended in an imaginary surface (an ellipsoid, or more accurately a geoid) to cover all points of the earth, including land areas far from the sea. Heights of locations can then be given with reference to the mean sea level computed for that latitude and longitude. This allows heights of different places to be compared in a uniform manner. Note that modern methods of geodesy use computer models of the earth’s surface to compute reference geoid surfaces which provide the vertical datum for measurements by instruments such as GPS receivers; observation of the ocean surface are not necessarily part of these models.
