Geodetic Datum and Geodetic Control Networks - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 88

Fig. 3.14 Relations
between different height
datums and the leveling
origin in China
The data used to calculate the “Huanghai Mean Sea Level 1956” was obtained
from tidal observations between 1950 and 1956. As the time span was rather short,
the result was far from satisfactory. In May 1987, China implemented the “National
Height Datum 1985.” This new datum used the tidal observations from 1952 to
1979 at Qingdao tidal station, and the Huanghai MSL was obtained by averaging
heights of the sea levels in this area. The height of the leveling origin is 72.2604 m
by precise leveling connection, differing from the Huanghai Mean Sea Level 1956
by 0.0289 m (cf. Fig. 3.14 ).
3.2.2 The Sounding Datum
Concept of the Sounding Datum
In the domain of marine surveying and mapping, the concepts of depths and the
sounding datum are needed for describing seafloor surface features and carrying out
corresponding bathymetric surveys. Depth is the sounding value obtained through
bathymetric surveys of oceans and coastal sea waters starting from the sea surface
at the time of measurement (i.e., instantaneous sea surface). Due to such effects as
tides, sea waves, sea currents, and so on, the instantaneous sea surface height is
subject to temporal variations. Hence, the instantaneous water depth obtained at the
same sounding point at a different time also varies. Therefore, a fixed water surface
should be specified as the reference surface for depth measurement. All the sound-
ings measured at different times should be reduced to this reference surface, which
is known as the sounding datum.
The sounding datum is the datum to which depths on the charts and tide heights
are referred, and is also known as the chart datum. It is reckoned through long-term
tidal observations and chosen data models in designated sea waters, with the local