## 1 Introduction  
This paper is an extended version of a paper published in the Contributions to International Conferences on Engineering Surveying, INGEO & SIG2020, Online Conference, Croatia, 22-23 October 2020 [1]. Structural health monitoring (SHM) of engineering structures can, besides physical sensors (accelerometers, LVDT, encoders), be performed by geodetic instruments and is usually done by GNSS in combination with robotic total stations (RTS). GNSS offers great coverage and RTS offers high-precision measurements. Today, modern total stations come with different integrated sensors. With the integration of image sensors into total stations (TS), so-called image assisted total stations (IATS), and due to rapid technological development, the different sensor classes, each with their specific advantages, can be unified, utilized, and fused as one single (nearly) universal instrument [2]. This integration offers wide coverage of different geodetic tasks to be resolved more quickly, easily, and more precisely in comparison with classical geodetic methods and instruments. It even offers possibilities to solve some tasks that were not possible in the past with the usage of classic TS. With an appropriate system calibration provided, these images and video frames are accurately geo-referenced at any time. They are particularly suitable for deformation monitoring of civil engineering structures, i.e., structural monitoring and geo-monitoring of hazardous areas, which are very hard to approach. Today, state-of-the-art IATS comes with a scanning function (image-assisted scanning total station--IASTS). 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 [3]. IATS have possible applications in semiautomated object reconstruction systems [4], fully automated deformation monitoring systems [5], industrial measurement systems [6], measurements of vibration amplitudes by means of high-frequency image measurements [7], and the capture of additional information such as high-frequency motions or intensity fluctuations of patterns using image sensor to derive the temperature gradient of the atmosphere as a decisive influence parameter for angular refraction effects and monitoring of cracks [8; 9]. Leica MS60, Trimble SX12 and Topcon GTL-1000 combine imaging with scanning on a level that is not rudimentary, so we can call them image-assisted scanning total stations (IASTS). In [10], Leica MS60, Trimble SX12, and Topcon GTL-1000 are compared with their main specifications and characteristics. All instruments have a scanning function, and they are classified as multi stations (MS), so-called IASTS. These three instruments, at the moment, represent the state-of-the-art geodetic instruments in the world. For the purpose of this study, we used Trimble SX10 predecessor of Trimble SX12. All instruments and sensors have their own advantages and shortcomings. The biggest shortcomings of TS, RTS, IATS and GNSS is the fact that we can only monitor one discrete point at time. We cannot monitor the whole surface of the engineering structure. In some ways, IATS overcome this deficiency using image sensor and obtaining images of one part of the structure, but can still only measure displacements of multi points but parallel to the image sensor in two axes. So, new methodologies and instruments need to be developed [11]. A fundamental change in geodetic deformation monitoring is currently taking place. Nowadays, areal measurements are increasingly used instead of pointwise observations (of manual selected discrete points) as done in the past. Frequently, laser scanners are used, as these allow a fast, high resolution and dense acquisition of 3D information. However, it is challenging to uncover displacements and deformations and changes in multi-temporal point clouds as no discrete points are measured, and only changes in line of sight (of non-signalized areas) can be clearly detected automatically as distance variations.