It is mentioned in Geoid that in order to establish the Geoid as a reference for heights, the ocean’s water level is registered at coastal locations over several years and averaged to eliminate variations in sea level over time. The resultant water level represents an approximation to the Geoid and is termed mean sea level. Local mean sea levels (also called local vertical datums) are parallel to the Geoid but offset by up to a couple of metres to allow for local phenomena such as ocean currents, tides, coastal winds, water temperature and salinity at the location of the tide gauge. Several definitions of local mean sea levels appear throughout the world.
Explain the relevance of reference surfaces, coordinate systems, and coordi-nate transformations in mapping (level 1 and 2).