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Title
International cooperation and technology transfer
Author
Mussio, Luigi

Also in this test, scattered point positions are
derived from real measurement campaigns.
Figure 4: Test area (B)-Geoid contour lines (m) and
GPS/leveling points
The target data set is a 5’x5’ grid in the area
44.16 <

when using input gridded 3’x3’ data and a 5’x5’ grid
in the area
44.20 <(p <44.62 11.68 <À< 12.3
when using input scattered data. This reduction of
the target grid is done to account for the distribution
of the scattered data which are placed in a quite
restricted area
The same testing scheme presented before was
applied to this test area; the results are summarized
in Table 3 and Table 4.
n
E(m)
a(m)
Min(m)
Max(m)
AN T
81
0.00
0.01
0.01
0.14
-0.02
-0.08
0.02
0.71
an r
81
0.00
0.01
0.01
0.05
-0.02
-0.08
0.02
0.30
AN r
81
0.00
0.01
0.01
0.03
-0.01
-0.07
0.01
0.15
Table 3: Statistics of the residuals computed with
gridded input data (bold types for weighted mean
interpolation)
n
E(m)
a(m)
Min(m)
Max(m)
AN T
40
0.01
0.08
0.14
0.13
-0.43
-0.14
0.28
0.37
ANr
40
0.01
0.01
0.06
0.06
-0.18
-0.12
0.15
0.17
AN r
40
0.02
0.02
0.06
0.06
-0.11
-0.08
0.18
0.19
Table 4: Statistics of the residuals computed with
scattered input data (bold types for weighted mean
interpolation)
As in the previous test, better statistics are obtained
when using input gridded data and the residual
geoid values N r . It must be also underlined that,
contrary to the Alps test area, there are no sharp
differences between N r and Nr residuals. This is
expected since no relevant topography is present in
the test area (B).
Finally, also for this second test, the results obtained
with the two interpolation methods are practically the
same.
In the end, following a commonly used practice,
another test was performed. A plane has been fitted
to the scattered N T data to get the geoid in the area.
This is usually done in field survey and it is accepted
as a reliable procedure, at least in flat topography
areas such as the one under investigation.
Residuals have been then computed on the 5’x5’
restricted target grid; their statistics are listed in
Table 5.
n
E(m)
a(m)
Min(m)
Max(m)
ANx
40
0.04
0.26
-0.34
0.81
Table 5: Statistics of the residuals computed by
approximating the geoid with a plane
Poor results are obtained with respect to weighted
mean and krigging. It must be concluded that this
procedure can introduce relevant distorsions in the
geoid estimate and that it must be avoided.
3. CONCLUSIONS
The tests that have been presented proved that the
interpolation of geoid values should be carried out
carefully.
Krigging and weighted mean give nearly the same
results and can be thus used to get reliable geoid
estimates. The best results are obtained when
interpolating the residual geoid N r , applying a
“remove-restore” procedure. On the other hand,
fitting the observed geoid data with a plane gives, in
general, poor geoid estimates , even in flat
topography areas.