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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
However, at a first glance, it was surprising to found 
deformations in point 2 and point 4 that had been chosen as 
reference points, after geological and geophysical 
investigations, the origin of these results was captured. 
According to that the area is a marsh area that this characteristic 
might widen also underneath of these two reference points, 2 
and 4. The uppermost soil layer in the region is not seemed to 
be stabile and the foundations of the constructions of the 
reference points are not founded as deep as the piers of the 
viaduct and so they are affected the environmental conditions 
easily. And these conditions supported the results of the 
analysis that in the object points on the viaduct, there were not 
exposed any significant movements. On the other hand, it is 
possible to mention about the correlation between vertical 
movements in two reference points, 2 and 4, and wet/ dry 
seasons, because the uplift and sinking movements of these 
reference points seems very synchronic as related to seasonal 
changes in amount of water (see Figures 2, 3 and 4). 
The results of this study, experienced with measurements of 
Karasu viaduct, are thought to be very important remarks for 
deformation analysis studies using GPS measurements. As the 
first remark, GPS measurement technique can be used for 
determining deformations with some special precautions for 
eliminating GPS error sources. These include using forced 
centering mechanisms to avoid centering errors, using special 
equipments for precision antenna height readings, using special 
antenna types to avoid multipath effects etc. Also, during the 
data processes it is necessary to clear the cycle slips from the 
data, and to consider insufficiencies of the used tropospheric 
and ionospheric models. 
However, even though these precautions, to provide better 
results in deformation analysis, GPS measurements have to be 
supported with Precise Levelling measurements. 
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