<P> Precise determination of a "mean sea level" is difficult to achieve because of the many factors that affect sea level . Sea level varies quite a lot on several scales of time and space . This is because the sea is in constant motion, affected by the tides, wind, atmospheric pressure, local gravitational differences, temperature, salinity and so forth . The easiest way this may be calculated is by selecting a location and calculating the mean sea level at that point and use it as a datum . For example, a period of 19 years of hourly level observations may be averaged and used to determine the mean sea level at some measurement point . </P> <P> 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, e.g., due to the tides, also get averaged out . One measures the values of MSL in respect to the land . Hence a change in MSL can result from a real change in sea level, or from a change in the height of the land on which the tide gauge operates . </P> <P> In the UK, the Ordnance Datum (the 0 metres height on UK maps) is the mean sea level measured at Newlyn in Cornwall between 1915 and 1921 . Prior to 1921, the datum was MSL at the Victoria Dock, Liverpool . </P> <P> In Hong Kong, "mPD" is a surveying term meaning "metres above Principal Datum" and refers to height of 1.230 m below the average sea level . </P>

Where is sea level measured in the uk