Geoscience Reference
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Fig. 3.11 Monumentation of the first- and second-order trig points
3.2 The Vertical Datum and Vertical Control Networks
A vertical datum (height datum) is realized by a vertical control network that
provides the basis for determining the height of a point on the Earth's surface.
The vertical control network is also known as the leveling network, which is
established using leveling as the primary method, supplemented by trigonometric
leveling. Leveling can be classified into four orders: the first- and second-order
leveling is known as precise leveling, and the third- and fourth-order leveling is
referred to as ordinary leveling. Correspondingly, the vertical control network can
also be divided into four orders.
3.2.1 The Vertical Datum and Leveling Origin
A vertical datum provides the reference surface relative to which heights are
measured. All leveling heights in the national unified vertical control network are
calculated and determined with respect to the vertical datum, which consists of a
zero elevation surface and a permanent leveling origin. Theoretically, the geoid is
usually used as the reference surface for heights, yet practically the mean sea level
(MSL) determined by averaging the level of water at a tide gauge over time is often
used as the level surface to which heights are referred. Overwhelmingly, the
majorities of countries and areas worldwide have chosen the MSL to be the
reference surface as this average position actually exists, is very stable, and can
be determined precisely. Another benefit is that the global MSL also approximates
to the physical surface of the Earth.
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