# GNSS-Based Vertically Monitoring of Super-Tall Buildings
## 1 Introduction  
A Global Navigation Satellite System (GNSS) is a positioning technology widely used for monitoring displacement of large civil engineering structures, such as dams [1; 2; 3; 4] and bridges [5; 6; 7; 8; 9]. In recent decades, a larger number of super-tall buildings have been built in modern cities worldwide; for instance, the 828-m high Burj Khalifa (Dubai) and the 438-m high Guangzhou International Finance Center (China). To maintain the safe construction and running of these super-tall buildings, monitoring deformation under varying environmental conditions is a very important issue. [PERSON] et al. [10], [PERSON] et al. [11], [PERSON] and [PERSON] [12], and [PERSON] et al. [13] applied GNSS to measure the displacement responses of super-tall buildings under the effects of wind, construction load, temperature, and sunexposure. In addition, [PERSON] et al. [14] have investigated real-time kinematic PPP (Precise Point Positioning) GPS for structure monitoring, and [PERSON] et al. [15] studied the use of low-cost GNSS receivers to monitor surface deformations, which may be an important topic for future experiments. The comparison between GNSS and acceleration by [PERSON] et al. [16] and [PERSON] et al. [17] reveals their high consistency, which also implies that GNSS can effectively monitor the motion of super-tall buildings. The real-time kinematic (RTK) positioning technique (e.g., network RTK GNSS) is widely used for obtaining the coordinates of monitoring points in real time [18; 19]. RTK has several advantages, such as flexibility and the real-time acquisition of coordinates.  
During a super-tall building's construction, a laser plummet is usually used to transmit the coordinates of reference points from the ground layer-by-layer, which effectively controls the verticality of super-tall buildings. However, motion of super-tall buildings in a horizontal plane will occur due to such external factors as tower crane operation, wind, construction load, temperature, and sun exposure during construction as shown in Figure 1. In this case, it is difficult to accurately transmit the coordinates of reference points by laser plummet, and the plumbing accuracy would be severely affected by the transmitted errors as the height of the building increases. Therefore, monitoring the motion of super-tall buildings in a horizontal plane and checking the result of a laser plumbing point based on GNSS technology is extremely important.  
In fact, the GNSS results refer in many cases to an ellipsoidal reference system while the laser plummet refers to a geoidal system. In order to check the result of a laser plumbing point based on GNSS technology, their results need to be matched into a unique datum with deflections of the vertical. For instance, the gravity vector has been considered during the construction of the 828-m high Burj Khalifa [20]. Therefore, the determination of the deflections of the vertical is very important work. Traditionally, the deflection of vertical components are determined from astro-geodesy [21; 22; 23], gravimetry [24; 25], GNSS/leveling [26; 27], etc. from which it is easier to calculate the deflections of the vertical with an Earth gravity field model and a geoid model.  
As a case study, this paper aims to monitor the verticality of the 438-m high Guangzhou International Finance Center based on GNSS technology. The paper is organized as follows: first, the data processing strategy for calculating the monitoring point's coordinate time series and deflections of the vertical are introduced, and we match the results of static GNSS Measurements and Laser Plumbing with deflection of the vertical components obtained from an Earth gravity field model (e.g., EGM2008) and a geoid model (e.g., GZGEOID). Finally, we analyze the horizontal motion of the Guangzhou International Finance Center and check its verticality based on static GNSS technology.  
Figure 1: Schematic of plumbing with a laser plummet.