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The mean sea level (MSL) of the Keelung tide gauge is used to define
the zero orthmetric height of Taiwan''s height system. The relationship
between the MSL of Keelung and the vertical datum''s original point of
Taiwan has been calibrated using a electronic device, differential leveling
and tide records. According to the changes of yearly averaged tide records
during 1971-1989, a trend of 0.16mm/yr (uplift) of Keelung MSL was
found. Over that period the original point of vertical datum of Taiwan has
been changed secularly at 1.34 mm/yr (uplift). The rate of vertical crustal
motion must be considered when re-measuring Taiwan''s first order
leveling net.
Using the GPS data from the IGS station in Taiwan and the IGS orbit,
the geocentric coordinates in ITRF of the six key tide stations around
Taiwan Island were determined. Satellite altimetry data, GPS and a geoid
model have been used to determine the sea surface topography (SST) of
the six tide stations. The mean SST at the five tide stations relative to
Keelung were also computed. The relative SST reveal the fact that the
MSL at these tide stations do not lie on the same equipotential surface. The
relative SST at tide stations were then used as weighted constraints in the
adjustment of Taiwan┬ first-order leveling net. and tht result reflects the
vertical crustal motion of some benchmarks. The model of weighted
constraints is recommended for furture adjustment of Taiwan''s leveling
net.
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