Height above mean sea level is a reference for a measure of a location's vertical distance (height, elevation or altitude) in reference to a vertical datum based on a historic mean sea level.
The zero level varies in different countries due to different reference points and historic measurement periods.
Common techniques are:[8][9] Accurate measurement of historical mean sea levels is complex.
Land mass subsidence (as occurs naturally in some regions) can give the appearance of rising sea levels.
Conversely, markings on land masses that are uplifted (due to geological processes) can suggest a relative lowering of mean sea level.