## 1 Introduction
Considering GPS coordinate differences between the campaigns after datum transformation, the deformations were investigated to a millimeter-level (relative) accuracy [29]. Using the pair of inclination sensors, the structural movements of the building were continuously monitored for 36 days between the second and third GPS campaigns [30]. Through monitoring the deformations of the building, besides testing the performance of precise inclinometers versus GPS in deformation monitoring, we sought to analyze the long-term elastic deformations of the building in addition to record the probable static deformations that occurred under the effect of environmental forces, such as earthquakes and strong wind load. In addition to the engineering structures, geodetic control network benchmarks are also subject to deformations under environmental effects and tectonic activities, and the distortions in a geodetic network affect the health and reliability of the geospatial infrastructure and cause crucial problems in every spatial data application. Therefore, we performed a second experiment with the inclinometers to monitor the movement of a geodetic monument, namely, a 1.20 m tall ISTA-IGS pillar. The pillar is located outdoors on the roof of a five-story reinforced concrete building on the campus of the Istanbul Technical University, and it carries a bullet type GPS antenna. Continuous monitoring of the pillar was required to explain the effects of movement from the host building, as well as atmospheric effects such as temperature, wind, and humidity. The inclination sensors continuously measured the movements of the pillar and the pier of the building, where the pillar is located, for two periods of approximately 30 days each in two different years, 2000 and 2002 [31, 32]. Along with the observations from the inclination sensors, the baseline solutions were also obtained between the two continuous reference GPS stations, ISTA and KANT (a station of the Marmara GPS Network-MAGNET, which was established for geodynamic and earthquake research in the Marmara region) and the displacements of ISTA pillar was investigated depending on these solutions [33]. The time series obtained from the inclinometers and continuous GPS data were processed using the Least Squares Spectral Analysis (LSSA) technique, which is useful for the evaluation of periodic, multi-parameter, and noisy time series that can be decomposed into their component signals [34-37]. This method is based on the least squares approximation technique, which is closely related to least squares parametric adjustment [34, 37], and it has certain advantages compared to classical Fourier methods, such as overcoming the blindness of former techniques to the existence of sampling irregularities or gaps in the data. It was applied in its original form by [38] and [39], and alternate forms have been used by a number of researchers in the fields of geodetic science [34, 37, 40, 41-45], astronomy [46, 47], geophysical applications (e.g., [48]), mathematics (e.g., [49]), microbiology (e.g., [50]), medicine (e.g., [51]), and finance. In the overall conclusions from the case studies, besides the high accuracy advantage of inclination sensors in monitoring the dynamic structural movements at submillimetre level, detecting static or quasi-static deformations of the structure are not easy using these instruments. The displacements along the plumb line, like uplift or collapse of the structure, cannot be recognized using them. However, these limitations of inclination sensors can be defeated by GPS technique. Using static GPS observation method, the static or quasi-static deformations of a structure can be determined at the centimeter level. In literature, certain strategies, applying effective mitigation algorithms to GPS observations, are reported and suggested to improve the reliability of the GPS results [52-54].