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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
positioning technique has become to use in deformation
measurements (Erol, 1999).
GPS technique has benefits of high accuracy and simultaneous
3-D positioning; however there are handicaps about vertical
positioning using this technique. Because, the height
component is the least accurately determined GPS coordinate,
predominantly due to inherent geometric weakness and
atmospheric errors (Featherstone et al., 1998; Celik et al.,
2001).
Therefore, using GPS measurement technique in deformation
measurements with millimeter level accuracy requires some
special precautions that increase the measurement accuracy in
GPS observables via eliminating or reducing some error sources
such as using forced centering equipments, applying special
measuring techniques like rapid static method for short
baselines or designing special equipments for precise antenna
height readings (Erol and Ayan, 2003).
In some cases, even these special precautions might be
insufficient to reach the necessary accuracy level; at that time to
support GPS measurements with another measurement
technique would be very useful as an improving solution.
In this study, 1D and 3D deformation analysis of a large viaduct
using GPS and Precise Levelling measurements are
implemented. The control network points were positioned with
GPS measurement technique and height differences were
supported with precise levelling measurements. As the result of
measurement campaigns, the X, Y, Z cartesian coordinates and
height differences were determined from the GPS
measurements and precise levelling measurements respectively.
Later on, deformation analysis using the height differences
according to provided data from the GPS and the data from the
precise levelling were carried out separately. Then, the 3D
deformation analysis using the GPS measurements data was
carried out too. Explanation on used analysis methods and
founded results will be given in addition to general review of
deformation analysis methods in the following sections.
2. OVERVIEW OF DEFORMATION MEASUREMENT
TECHNIQUES
As it is mentioned in the introduction part, measurement
techniques were divided mainly into two different groups as
geodetic and non-geodetic techniques. These main techniques
can also be divided sub-techniques. In the following, it can be
found short descriptions of the used techniques in deformation
measurements.
GPS; Global Positioning System offers advantages over
conventional terrestrial methods. Intervisibility between
stations is not strictly necessary, allowing greater flexibility in
the selection of station locations than for terrestrial geodetic
surveys. Measurements can be carried out during night or day,
under varying weather conditions, which makes GPS
measurements economical, especially when multiple receivers
can be deployed on the structure during the survey. With the
recent developed rapid static positioning techniques, the time
for the measurements at each station is reduced to a few
minutes (Anonym, 2002).
Photogrammetry; 1f an object is photographed from two or
more survey points of known relative positions (known
coordinates) with a known relative orientation of the camera(s),
623
relative positions of any identifiable object points can be
determined from the geometrical relationship between the
intersecting optical rays which connect the image and object
points. Aerial photogrammetry has been extensively used in
determining ground movements in ground subsidence studies in
mining areas, and terrestrial photogrammetry has been used in
monitoring of engineering structures. The main advantages of
using photogrammetry are the reduced time of field work;
simultaneous three dimensional coordinates; and in principle an
unlimited number of points can be monitored (Anonym, 2002).
Tilt and Inclination Measurements; The measurement of tilt is
usually understood as the determination of a deviation from the
horizontal plane, while inclination is interpreted as a deviation
from the vertical. The same instrument that measures tilt at a
point can be called either a tiltmeter or an inclinometer
depending on the interpretation of the results (Anonym, 2002).
The some of new techniques for deformation monitoring can be
listed as follows. :
Insar; Elevations can be determined from Synthetic Aperture
Radar (SAR) images by interferometric methods. This involves
the use of two antennas, displaced either vertically or
horizontally, installed on the same satellite or aircraft platform.
One of the antennas transmits the signal, but both receive it,
resulting in two images being created. The most accurate form
of interferometric measurement is differential interferometry
(InSAR), which involves the determination of elevation
differences between two epochs of terrain measurement. In this
case, the variations in the radar signal phases are determined
between the two epochs, which reveal terrain surface
deformations that may have occurred between the two
occasions when the images were recorded. It is claimed that
height differences as small as one centimeter can be detected by
this method. Such a technique therefore has the potential of
being a cost effective, near-continuous, remote method of
measuring terrain subsidence due to mining, and ground
movement due to land subsidence, earthquake or volcanic
activity, etc.
(http://www.gmat.unsw.edu.au/snap/work/insar.htm, accessed
May 2004).
Pseudolite; It is well known that for GPS-based deformation
monitoring systems, the accuracy, availability, reliability and
integrity of the positioning solutions is heavily dependent on
the number, and geometric distribution, of the satellites being
tracked. However, in some situations, such as in urban
canyons, monitoring in valleys and in deep open-cut mines, the
number of visible satellites may not be sufficient to reliably
determine precise coordinates. Furthermore, it is impossible to
use GPS for indoor applications and due to limitations of the
GPS satellite geometry; the accuracy of the height component is
generally 2 or 3 times worse than the horizontal components.
These factors make it difficult to address GPS deformation
monitoring applications in areas where the number of visible
satellites is limited or satellite geometry is poor, especially
where real-time high accuracy height component monitoring is
needed, as in such applications as ground subsidence or
deformation monitoring of man-made structures. Therefore, in
order to improve the performance of GPS-only deformation
monitoring systems, the integration of GPS with other
technologies needs to be investigated.
Pseudolites (pseudo-satellites), which are ground-based
transmitters of GPS-like signals, can significantly enhance the
