International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
satellite geometry, and even replace the GPS satellite
constellation in some circumstances (such as deformation
monitoring indoors).
The geometry of the satellite constellation can be improved by
the careful selection of the pseudolite location(s). In the case of
GPS, the measurements with low elevation angles are usually
rejected in order to avoid serious multipath, tropospheric delay
and ionospheric bias. However, this is not necessary in the case
of pseudolites. The quality of the measurements with less than
half degree elevation angle (from the pseudolite transmitter to
the GPS receivers) is still very high. Therefore, high quality
pseudolite measurements with low elevation angles, when
included in data processing, can be expected to significantly
improve the ambiguity resolution performance and solution
accuracy, especially in the height component. The availability
is also increased because a pseudolite provides an additional
ranging source to augment the GPS constellation (Dai et al.,
2001).
Laser Scanning, Existing techniques (e.g., surveying, GPS)
used to monitor large structures such as buildings, viaducts,
dams and bridges, while very accurate, are greatly hindered by
their low point density. Data acquisition time limits monitoring
to only a few samples located at strategic points on the
structure. Ground-based laser scanning is a new technology
that allows rapid, remote measurement of millions of points,
thus providing an unprecedented amount of spatial information.
This in turn permits more accurate prediction of the forces
acting on a structure. As an emerging technology though,
several issues concerning instrument calibration, sensitivity
analysis, data processing and data filtering techniques require
investigation
(http://www.cage.curtin.edu.au/-geogrp/projlaser.html, May
2004).
For any particular application of deformation measurements,
the most appropriate technique (or combination of techniques),
which are going to be used, are determined as related to type of
the structure, required accuracy and also economical aspects.
3. NETWORK AND DATA
In this study, the deformations of the Karasu viaduct were
investigated using GPS and precise levelling data. Karasu
viaduct is 2160 m in length. As the longest viaduct of the
Turkey, It is located in the west of Istanbul in one part of the
European Transit Motorway. The first 1000 meter of this
viaduct crosses over the Büyükçekmece Lake and the piers of
the structure were constructed in to this lake (see Figure 1).
The viaduct consists of two separate tracks as northern and
southern and was constructed on 110 piers (each track has 55
piers). There is 40-meter width between two piers and also one
deformation point is constructed with in every 5 piers sequence.
The deformation measurements of Karasu involved four
measurement campaigns. The first campaign was carried out in
June 1996, the second in March 1997, the third in October 1997
and the last one in April 1998. These four campaigns include
GPS measurements and precise levelling measurements. With
the aim of investigating the deformations of this structure,
before carrying out the measurement campaigns, a well
designed local geodetic network had been established, and it
was measured using GPS technique according to designed
session plan. Also, precise levelling measurement technique
was applied between network points.
During GPS measurements, Trimble 4000 SSI and Leica
System 300 dual frequencies receivers were used. Leveling
measurements were carried out using Koni 007 precise level.
The network has 6 reference points, set around the viaduct and
24 deformation points, set on the ‘building of the viaduct and
they are established especially on the piers where expected to
be most stabile places on the structures (see Figure 1).
Karasu Viaduct Network
4553750 i
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x i3 f
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Easting
Figure 1: The configuration of geodetic network.
4. DEFORMATION ANALYSIS USING HEIGHT
DIFFERENCES
With the aim of determining the deformations in engineering
structures, landslide areas, crustal deformations ete, the
geodetic networks are built. The observations are carried out in
this geodetic network with certain intervals, and by this way
stabile network points and instable network points are verified.
This provides to determine the changes on the observed
structure or area (Demirel, 1987).
In general, the deformation analysis is evaluated in three steps
in a geodetic network. In the first step, the measurements,
which were carried out in t, and t measurement epochs, are
adjusted separately according to free adjustment method;
outliers and systematic errors are detected and eliminated in this
step. In the second step, global test procedure is carried out and
by this test it is ensured that if the network point, which were
assumed as stabile, stayed really stabile in the At = t; —t; time
interval or not. In the global test, after the free adjustment
calculations of the networks separately, the combined free
adjustment is applied to both epoch measurements. (Ayan,
1982; Ayan et al., 1991).
After determining a group of stabile points as the result of
global test, following step of the analysis is the localizing of
height changes. For doing this, Ty test values are calculated for
the every network points, except the stabile points, and they are
compared with F critical value that is given in the Fisher
distribution table (Erol and Ayan, 2003).
vi Py, +Y, Pay,
d=H;-H; Sÿ = [o f f
27 Hi 0 ff Qmd tio
i! Qa |
Tu Sa Qaa = Qu,H, + Qu,H, reto zie
rd
If the TH > Fr m aM is said that the height of the point
changed significantly. Otherwise, it is resulted that d height
624
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