# Technological Development and Application of Photo and Video Theodolites
## 4 Application of the State-of-the-Art Video Theodolites--IATS in Monitoring  
Monitoring of artificial or natural structures, i.e., civil engineering structures or parts of the Earth's surface, involves periodic or continuous observations to estimate the object's general current state regarding its usability and stability, as well as a determination of the need for structure remediation, reconstruction, or destruction. The process involves performing different kinds of measurements using different sensors. The measurements and results must be precise and reliable, i.e., accurate, and tested for significance [8]. The results of the measurements represent an important parameter in assessing the condition and safety of the structures, and this is especially important for structures used beyond their designed lifetime. Any kind of damage or significant deformation affects the safety of the constructions, e.g., bridges, dams, towers, or skyscrapers, and this can result in their closure or even collapse. Monitoring of artificial or natural structures is one of the key tasks in engineering geodesy, next to site surveying and setting out. Geodetic monitoring is one aspect of monitoring systems in general. There are two subtypes of geodetic monitoring [8]:  
* Structural monitoring refers to the measurement and evaluation of civil engineering structures such as bridges, tunnels, dams, railways, towers, or skyscrapers, i.e., generally manmade objects. * Geo-monitoring, in contrast, is used as a term for the determination of changes, movements, or deformation of natural structures, such as landslides and slopes. The main aim of geodetic monitoring is to determine statistically significant geometric changes in size, shape, and position between two or more measuring epochs. According to the monitoring data, action can be taken on the construction to prevent material and nonmaterial damages. Vibration-based monitoring, i.e., structural monitoring, has become common recently. Vibration-based monitoring consists of determining the dynamic displacements and natural frequencies of objects from different epochs of measurements. Any changes from the designed frequencies can be a sign of structural damage and a cause for alarm. Dynamic displacements and natural frequencies of objects can be determined by RTS and GNSS instruments, although their use in these projects has certain limitations. The limitation of the first RTS models was their instrument measurement frequency of 1 Hz, which was lower than the fundamental frequency of the bridge, as demonstrated in [65]. Possibilities of newer RTS models with measuring frequencies of 5-7 Hz were presented in [66; 67; 68; 69; 70], where RTS instruments were used for the measurement of simulated dynamic displacements to analyze the accuracy of dynamic measurements by RTS, as well as for the determination of dynamic displacements and natural frequencies of bridges in exploitation. The accuracy of an RTS instrument with 20 Hz measuring frequencies for recording changing 3D coordinates of a moving target was tested in [71] and for measuring dynamic displacements and natural frequencies of railway bridges in [72], where the RTS determined dynamic displacements of the bridge in vertical and lateral directions and the first five natural frequencies of the bridge. To overcome the limitations of RTS measuring frequencies, a new approach was shown in [6], where the authors demonstrated how to increase the measurement frequencies of an RTS instrument from 7-10 Hz to 20 Hz. A new approach was also demonstrated in a field experiment on a 74 m long footbridge for vertical bridge vibration measurements. Unlike RTS, GNSS instruments do not have limitations regarding measuring frequencies, since the newer GNSS instruments have a measuring frequency of up to 100 Hz.