# Technological Development and Application of Photo and Video Theodolites
## 4 Application of the State-of-the-Art Video Theodolites--IATS in Monitoring
GNSS instruments are widely used for monitoring of dynamic displacements of large and flexible structures such as long-span bridges, towers, and high-rise buildings, which are charac terized by large displacements (10 cm or more) and lower natural frequencies (less than 1 Hz). Results of GNSS monitoring of dynamic displacements of large-scale structures were shown in [66; 73; 74; 75; 76], where dynamic displacements, i.e., natural frequencies, of structures were successfully determined from GNSS data. Determination of dynamic displacements of more rigid structures, which are characterized by smaller values of dynamic displacements, is limited due to the achievable measurement accuracy, which is on a centimeter level. Currently, many studies are focusing on applying GNSS instruments for monitoring the dynamic displacement of more rigid structures with dynamic displacements in the millimeter range and higher oscillation frequencies [67], as well as on improving the GNSS measurement accuracy due to the limitation of ephemeris, multipath, and atmospheric errors and receiver measurement noise [77]. The most commonly used geodetic instruments for structural and geo-monitoring include levels, RTS, and GNSS. Geodetic monitoring of engineering structures is usually done using a GNSS in combination with RTS. A GNSS offers great coverage, while an RTS offers high-precision measurements. The implementation of geodetic measurement sensors, i.e., instruments in complex construction monitoring systems, enables the so-called preventive maintenance of engineering structures and a transition from reactive maintenance, when damage has already occurred and possibly caused human casualties, to preventive maintenance [78]. In recent years, there has been a general trend toward the development of automated and autonomous monitoring. In addition to economics, which has constantly been the \"problem\", another reason for the development of automated and autonomous measuring is the increased demand for continuous 24/7/365 monitoring systems. Densely built buildings and complex and construction demanding object projects require a continuous control of predefined thresholds. This also includes constructions in hazardous locations, which have not been used in the past. Likewise, today's cities are expanding, and, due to a lack of land, tall buildings are being built, e.g., Burj Khalifa and similar skyscrapers all around the world. Precise 3D point measurement is either automated using an RTS with reflectors or using a GNSS. However, reflectors and GNSS cannot be used in every situation, because they cannot be placed on every point or object of interest. Furthermore, in the case of GNSS, a power supply must be ensured in sometimes hazardous and inaccessible places. These issues mean that we must use IATS with image sensors to collect spatial, spectral, and radiometrical information of the target point of interest and its environment, whereby, with the use of image analysis methods, RTS and GNSS methods can be replaced for determining precise 3D-point measurements of signalized points and unsignalized points [11]. Considering the aging of traffic infrastructure, a high number of bridges in particular are in need of rehabilitation around the world, which provides a huge opportunity for geodetic monitoring sensors (RTS, GNSS), especially IATS. Unfortunately, almost every 3 months, we are witnessing the collapse of bridges around the world. Any kind of damage or significant deformation affects the safety of the bridges, which can result in their closure or even collapse, thus disintegrating traffic systems [78]. In the next section, we give a few examples of IATS applications for the purpose of structural and geo-monitoring.