# Technological Development and Application of Photo and Video Theodolites
## 2 Technological Development
### Development from 1981-1990
The coaxial configuration of a camera (integrated into the telescope) using theodolite optics was better, but also technically more complicated than biaxial implementation onto the telescope leading to parallax, which is particularly harmful in situations where range-finders operate at close target distances. When using the coaxial configuration, complex beam splitter elements must be installed. The Leica TM3000V video theodolite is the best-performing example, widely used in academic research [11]. The image of the FOV of the telescope on the CCD array contains the image of the targeted point, as well as a reference frame built into the telescope to replace the reticle. This frame defines the target line of the telescope with the center of the lens system, and it is used at the same time to adjust the telescope and camera axis. Using the CCD camera, a wide-angle lens can initially be used to display an overview image of the object space with a size of \(9\)\({}^{\circ}\times 12\)\({}^{\circ}\). These wide-angle optics can be exchanged for the optics of a precision telescope via an optics coupler. Instead of the wide-angle FOV, however, a narrowly limited measuring FOV is available. Switching between the measuring and wide-angle FOV is controlled by the software. Switching to the measuring FOV enables high increases in accuracy, since details of objects that are far away are now displayed on the entire sensor surface. Figure 6: Video theodolites Kern E2-SE (**left**) [29] and Wild TM3000V (**right**) [30]. These instruments were only used for special applications and not for standard surveys due to their complex morphology, which required an image display unit and a control unit in addition to the instrument [32]. With the launch of laser tracker systems in the 1990s, video theodolites were replaced within a few years; this remained a trend until 2005, when Topcon introduced the first commercially known IATS model GPT-7000i, which was the forerunner of today's state-of-the-art IATS. However, technical innovations that were necessary for automation using video images and were, therefore, introduced on the market have been preserved to this day, for example, the motorized endless fine drives for instrument control, as well as reflector detection and tracking using integrated image sensors, which are common today, from the second generation of the Wild TM3000V. As in the TOPOMAT project, a target illuminator in the form of an infrared collimator was installed here for permanent deformation measurement on reflectors. This diode integrated in the telescope illuminated the roughly approached reflector. The reflected signal was captured on the infrared sensitive CCD array and used for precise targeting [30]. The concept involving the difference of two currents in a photoelectric cell, named \"Electronic Eye\", become commercially available in 1987 when Geodimeter presented the model 4000 \"One-Man-System\" with automated target aiming, being the first instrument that required only one expert to operate it (Figure 7) [22]. Since then, automated target aiming and tracking have become standard functions in all TS, i.e., RTS. Integrated cameras and photogrammetric methods became supporting tools for theodolites. This system worked using a theodolite-like active target point receiver, whereby the telescope of this unit, called the remote processing unit (RPU), had to be aligned with the total station, called the control and processing unit (CPU), in order to facilitate a coarse search.