From Simple English Wikipedia, the free encyclopedia
The term
above mean sea level
(
AMSL
) is the
elevation
or
altitude
used by
engineers
to determine the
coverage
area
a station will be able to reach. It is also used in
aviation
, where most heights are recorded and reported in AMSL (see
flight level
), and in the
atmospheric sciences
.
A "mean sea level" is imaginary. It is not possible to know the average sea level for the entire planet. The sea level also changes over time when measured in the same place. The sea moves, high and low pressure zones affect the height, tides change the sea level, and local difference in gravity all affect the sea level. People can only pick a spot and calculate the mean sea level at that point and use it as a
datum
. For example, the British
Ordnance Survey
uses a datum based on the measurements of mean sea level at a gauge at
Newlyn
,
Cornwall
from 1915 to 1921
[1]
for their maps of Great Britain. This datum is actually about 80 cm different from the mean sea level reading on the other side of the country. An alternative is to base height measurements on an
ellipsoid
of the entire earth.
GPS
and other satellite systems do this. In aviation, the ellipsoid known as
World Geodetic System
84 is increasingly used to define mean sea level. Another alternative is to use a
geoid
based
datum
such as
NAVD88
.