## 2 Description of the Observation Setup
### GPS Observations  
The satellite-based Global Positioning System (GPS) was originally introduced for navigation, but its high accuracy and reliability in positioning drove its adoption for structural deformation monitoring as well as other geodetic surveys, and subsequently for seismological studies [9,15,16,55,56]. With advantages in terms of accuracy and practicality, GPS proved its success for determining abrupt displacements and also very slow movements of structures, and this success motivated its application to studies of rapid displacements, for instance, recording the oscillations of bridges and other slender engineering structures [1,10,15]. There are, however, various measurement approaches for determining coordinates using geodetic GPS receivers, such as static (fixed GPS antenna in a certain position), kinematic (GPS antennas along a path), real-time kinematic (positioning in real time with receivers connected by radio or wire link), and post-processing kinematic. In each of these, the positioning principles remain the same: the GPS antenna represents the apex of an inverted pyramid, the base of which is usually defined by at least four satellites (with accurately-known coordinates), and information is continually received from the satellites during the entire observation session. Hence, the coordinates of the point (the receiver antenna) can be determined to average accuracy of a few millimeters after correcting for short-period errors that affect the GPS signals, such as atmospheric and tropospheric delays [10]. However, the accuracy and reliability of coordinates vary depending on certain factors that affect the quality of GPS measurements, namely, the satellite visibility, availability, and geometry; the quality of the transmitted signal and its delay by the ionosphere and troposphere; secondary reflections of the satellite signal by topographical objects (multipath signals); and personal errors like misreading the GPS antenna height or centering the antenna on a wrong benchmark [1,10,19]. To improve the positioning quality, therefore, special precautions and efforts may be required in GPS monitoring of deformations [52-54]. In any case, low accuracy is expected from observations taken in places where satellite visibility is poor, such as in canyons or streets surrounded by high buildings, as well as near reflecting surfaces where signals are noisy, such as water reservoirs and metal covers. The selection of receiver location and the duration of measurements, as well as software exclusion of low-quality measurements or signals (including \"cut-off angles\", _i.e._, satellites at very low angles relative to the horizon and signals subject to strong atmospheric effects) permit us to control these sources of errors. Figure 5: **(a)** Monitoring setup of the geodetic pillar [31], **(b,c)** Pictures from the campaign 2000. In addition to inclination sensors, GPS was also included in the monitoring setup of both experiments here. Hence, both techniques were compared, and their merits and limitations in deformation analysis were clarified. We can thus assess their individual and together contribution to the deformation monitoring. In each case study, a different GPS measurement and data analysis strategy was adopted. In monitoring the tall building, three GPS campaigns were completed at six-month intervals, and the static GPS measurement method was applied. The control network for GPS monitoring, established at the beginning of the first campaign, can be seen in Figure 4. The benchmarks of the control network were located on stable hard terrain around the building (numbered 101 to 106), and four of the deformation benchmarks were also located at the tops of the pairs of the building (S101, S102, N101, and N102) (Figure 4). Dual-frequency twelve-channel geodetic GPS receivers were used in the campaigns. The durations of the GPS sessions varied from 40 to 60 minutes, with 5-second sampling intervals.