how is sea level rise measured
How exactly is sea level measured? THE STANDARD method of measuring is by means of instruments called tide gauges. Gauges exist in many ports and harbours around the world, and record the height of the rising and falling tide relative to a reference level, called a benchmark, on the land nearby. A time-averaged value of sea level so recorded is often used to define a national geodetic datum. For example, the height of Ben Nevis or Snowdon or anywhere else in Britain above sea level means the location's height above average sea level measured at Newlyn in Cornwall during 1915-1921. This historical reference level is called Ordnance Datum Newlyn and is referred to on many maps and charts.