sea level, the level of the sea, which serves as the datum used
for measurement of land elevations and ocean depths.
Theoretically, one would expect sea level to be a fixed and
permanent horizontal surface on the face of the earth, and as a
starting approximation, this is true. However, a number of
factors operate to cause variations in sea level ranging up to
several meters from place to place and to cause long-term global
variations, often severe enough to cause flooding and damage to
coastal zones. Sea levels vary greatly from one location to
another, i.e., between Nova Scotia and Florida sea-level heights
differ at about 16 in (40 cm). Locally the levels of the surface
of the world's oceans are disturbed by wind-driven waves and
tides. Sea level therefore fluctuates in periods ranging from
seconds to a year as a result of these factors. Thus for some
purposes it is necessary to know the mean sea level (MSL) in a
particular area, determined by averaging the elevations of the
sea's surface as measured by me