# Technological Development and Application of Photo and Video Theodolites
## 1 Introduction  
A Theodolite is a geodetic instrument for measuring horizontal and vertical directions, i.e., angles in the horizontal and vertical plane. There are optical and electronic theodolites [1, 2, 3]. To determine the point positions in the coordinate system, in addition to angles, we must measure the distance between the instrument and the target. We can measure the distances mechanically (with chains and measuring tapes), optically (with optical distance-meters and interferometers), and electronically (with electro-optical distance-meters and electronic distance-meters (EDMs)). The accuracy of mechanically and optically measured distances is low and not appropriate for terrain characteristic point coordinate calculations. With time, several technological developments have been incorporated into theodolites; thus, the accuracy of angle and distance determination has significantly increased. Perhaps the biggest development of theodolites was its integration with an electronic distance-meter (EDM), which emerged around 1940 and became commercially available in the late 1960s [4]. The type of EDM incorporated into modern total station instruments is commonly of the electro-optical type, using infrared and laser light as a carrier signal [3]. Instruments which can measure angles and distances simultaneously, as well as record the results and,to a certain extent, process them, are called electronic tachymeters or total stations (TS) [2]. TS instruments are electronic digital theodolites integrated with EDM instruments and electronic data collectors, which are aimed at replacing manual field data recording; they are capable of providing electronic angle readings, as well as distance measurements [3]. Today, many different sensors and measurement methods are combined in TS, such as highly accurate angle reading, electronic distance measurements (EDM) to reflectors and to any other surface (reflectorless EDM), tilt correction by two-axis inclinometers, different types of motorization to drive both the horizontal and the vertical motion of the instruments, servo, piezo, and magnetic motors (robotic total station--RTS), image CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor) sensors (CCD or CMOS) for autofocus, automated aiming (i.e., automatic target recognition (ATR) and tracking of signal points), integration with global navigation satellite system (GNSS) positioning, wireless communication and operation using a controller, additional cameras for documentation (image-assisted total station--IATS), and IATS with a scanning function (image-assisted scanning total station--IASTS). Due to the rapid hardware development, as can be seen in Figure 1, these different sensor classes, each with their specific advantages, can be unified, utilized, and are fused as one single (nearly) universal instrument [5]. Modern TS are multi-sensor systems which can determine the three-dimensional coordinates of target points by combining horizontal angle, vertical angle, and distance measurements [6]. Figure 1 shows the technological development of TS over the years, i.e., from the mid-20 th century until today. The main hardware and software, sensor integration, and their impact on instrument performances and operator efficiency for conducting the measurements in the field are shown. It is evident that, following the integration of EDM with the basic theodolite in the 1960s upon discovering tachymeters, the dimensions and weight of the instruments decreased, as well as the expert efficiency regarding the measuring techniques and time for conducting measurements in the field. Contrarily, instrument performance and the possibilities for surveying and monitoring projects have rapidly increased. Different types of motorization developed to drive instruments in horizontal and vertical motion have been specially designed and implemented in RTS to automatically search for reflectors and, later, for reflector tracking.