Geoscience Reference
In-Depth Information
To determine the MSL, a tidal station will usually be established at the coast
where it will be appropriate to record sea levels over time. The sea level changes
due to changing external conditions and factors from inside the Earth, particularly
the variations in positions of the Moon and the Sun. Such variations are periodic
and the long period is the astronomical tide cycle, which spans approximately
18.61 years. Statistics have shown that the long periodic average sea level is by
and large invariable and can be considered as the MSL for a particular area. From
analysis of long-term sea level data we can see that there has been a several
centimeter increase in global mean sea level each year, resulting from the rising
temperature over the last 100 years. Two main factors have contributed to observed
sea level rises. The first is thermal expansion of ocean water. The second is the
contribution of land-based ice due to increased melting of glaciers and ice sheets.
Therefore, corrections need to be applied to the vertical datum because of the long-
term changes in mean sea level.
As the level surface to which heights are referred, MSL is often defined as zero
height for a local area, also known as the zero point. To mark the position of the
reference surface for heights (zero point) with clarity and stability, one also needs to
establish a permanent benchmark and connect it to the MSL by precise leveling as
the reference point for height measurements in a national or local control network.
Such a benchmark is known as the leveling origin.
China's leveling origin is located at Qingdao Observatory Hill. One origin and
five subordinate points constitute the leveling origin network. The subordinate
points are used to monitor stability of the origin and to secure precision of the
leveling connection. The leveling origin is established in a stone house to ensure its
security. The inside of the house is built of granite from Mount Lao. At the center
and four corners of the rooftop are five stone columns. The stone columns are
exquisite and unique, chiseled and carved with delicate touches. The stone house
covers an area of 7.8 m
2
. Outside the stone house are very high double stockade
fences and, inside, the doors are locked fast with threefold locks. The architecture is
of Russian style and was built in 1954 (cf. Fig.
3.12
). The interior wall is inlaid with
a black marble stele on which the inscription “Leveling Origin of People's Republic
of China” is engraved. Within the house there is a dry well roughly 2 m deep. At the
bottom of the well is a priceless yellow agate as big as a fist. On the agate identifier
is a double-layered cap made of brass and stone. A little red dot on the agate is the
mark of the leveling origin (Fig.
3.13
).
In China, the Huanghai (Yellow Sea) MSL obtained from the tidal station at the
west end of pier No. 1 in Qingdao Large Port is taken as the reference surface from
which heights are measured. The tidal station is geographically located at
120
18
0
40
00
east longitude and 36
05
0
15
00
north latitude. Inside, there is a tide-
gauge well with a diameter of 1 m at a depth of 10 m. Three inlet tubes 60 cm in
diameter have direct contact with the open sea. The tide gauge originally used was
the German-made float automatic tide gauge and the tidal records began in 1900. It
was destroyed during World War II. In 1947, the tide gauge was renewed and
resumed work. After the founding of P.R. China the building was renovated and the
facilities upgraded. The instruments currently in use are the HCJ1 tide gauges (also
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