# Technological Development and Application of Photo and Video Theodolites
## 5 Discussion  
The conducted research regarding the technological development of photo and video theodolites showed that their development was slow and largely dependent on the development of distance-meters, image sensors, motorization, and their integration into the theodolites. Initially, a theodolite supported a photo camera for the purpose of determining the elements of the exterior orientation. These so-called photo theodolites were used for many mapping projects because surveying was faster and easier, while they also offered the possibility of surveying larger areas with satisfying accuracy. With the development of EDM and the principle of the difference in two currents in a photoelectric cell, static targets could be recognized and measured with high accuracy. Figure 16: Amplitude spectrum of vertical component and detected first and second frequencies by IATS prototype (excitation by train passing at 40 km/h). Figure 17: Amplitude spectrum of vertical component and detected first and second frequencies by IATS prototype (excitation by train passing at 60 km/h). The developed image sensors were then integrated into the theodolites and their purpose changed from mapping to object tracking. The demands of the automotive and aerospace industries regarding assembly and quality control in the 1980s forced companies to build electronic tachymeters with integrated image sensors into servomotorized theodolites. The greatest contribution from this development line was that image sensors were used for autofocusing and for determining the target position relative to the optical axis of the telescope. Although the technological development of commercial instruments slowed down in the 1990s, the academic community maintained research efforts and developed the applications of digital image processing methods and computer vision, which have been preserved to this day. Initial research focused on detecting artificial targets and accurate aiming with the help of cameras for the purpose of aiming and measuring unsignalized targets. Solutions based on artificial intelligence were developed. By implementing the expert knowledge of sighting points into software solutions, artificial intelligence was applied, and expert systems were developed. The developed algorithms were later implemented in commercial instruments. By automating the measurements, the level of achieved accuracy was increased. The year 2005 saw the introduction of IATS that we know today. Technological developments regarding the different sensors, i.e., hardware and software, enabled their integration into the instruments. Following their fusion, IATS have been more frequently used for the tasks of structural and geo-monitoring. The currently used sensors for structural monitoring and for determining a structure's vibrations and dynamic displacements are accelerometers, GNSS, and RTS. These sensors present some drawbacks, whereby access to the monitored structure necessitates them being installed on the structure with a power supply being ensured. Using accelerometers, it is only possible to evaluate the vibrations of the structures (Kal  
sighting axis, the IATS EDM can be used; alternatively, if available, IATS scanner data can be used, i.e., fusion of laser scans and camera data to RGB + D images. This approach combines the advantages of the two methods. This is also possible with the latest generations of IATS, i.e., IASTS, as explained in Section 3.3. At the moment, a few studies exist on this topic; however, further work is needed, because the possibilities and advantages offered by this approach for engineering tasks in geo-monitoring are extensive, especially in hazardous and difficult-to-access areas, necessitating thorough testing and analysis. With the integration of cameras into total stations, so-called IATS, these different sensor classes have been fused into one single universal instrument [5].