Geoscience Reference
In-Depth Information
Table 11.1. From observations to the geoid
observations referred to
(
L
i
T
)
Geodetic Reference System 1980
↓
reduction
TI, EM
L
i
(
T
TI
+
T
EM
−
T
TI
)
−
↓
(
L
i
δT
)
↓
collocation
δT
↓
inverse reduction
TI, EM
+
T
TI
+
T
EM
−
T
TI
)
T
↓
N
=(
T/γ
)
0
,ζ
=(
T/γ
)
h
The standard error of this model is on the order of 100 m. Investigations have
shown that, in spite of its poor accuracy, the model is reasonably adequate for
reduction of deflections of the vertical; it is, however, totally inadequate for
gravity! In fact, the reduction error for
ξ, η
is approximately proportional
to the terrain inclination; it is thus very small if the deflection station is
situated in an area of inclination zero. This is the case not only if the station
lies in a horizontal plane but also if it lies on the top of a mountain, as most
deflection stations do.
Results
Itturnedoutthatalmostallofthesignal(
T, N, ζ
) comes from the topo-
graphic-isostatic model and the
N
= 180 gravity model used. This part,
TI + EM, lies between 41.5 m and 49.5 m. The contribution of collocation
(
γ
−
1
T
) lies between
−
0
.
5 m and 1.5 m, after removal of a pronounced trend
on the order of 3 m.
The eciency of topographic-isostatic reduction can also be seen from
the fact that it has reduced the variance of the deflections of the vertical in
