## 3 Contemporary 3D TLS and the Methodology of Their Use for Documenting Archaeological Artefacts
### TLS Scanners  
Contemporary 3D terrestrial laser scanners are universal and fully automatic measuring devices. They differ in their purpose and range of scanning. The general principle of operation of these devices is based on the measurement of the distance from the device to the object, on the basis of the analysis of the reflection of the laser beam from the surface of the object. Most devices use a near infrared laser. The maximum TLS scanning range can be divided into short-range--up to 70/150 m (e.g., Surphaser 100 HSX, FARO Focus S70), medium-range--up to 350 m (e.g., Trimble TX8, FARO Focus S350, Leica RTC360), and long-range, where the range is measured in kilometres (e.g., Teledyne Optech Polaris, Riegl VZ-6000). The basic data obtained is a cloud of 3D points of the places where the laser reflects from the objects around the scanner, recorded in the coordinate system relative to the position of the scanner. In addition to the coordinates of the reflection point, the scanner can measure the energy of the reflected signal, which allows for the determination of the brightness level of the surface at any given point (Figure 2a). Laser detection alone does not allow the assigning of a colour to the detected surfaces. Most of the modern TLS devices also have the ability to take a spherical photograph of the environment (Figure 2b), on the basis of which the scan is coloured. There are three groups of solutions:  
1. Use of an external camera/digital camera mounted in a special holder and synchronized with the scanner. 2. Use of a built-in spherical camera or set of cameras permanently installed in the scanner housing. 3. Use of a built-in camera using the same optical path as the laser measurement. The advantage of the first method is the possibility of using a popular (reasonably priced) digital camera with parameters matching the required effect; the disadvantage is the need to calibrate the camera after changing the parameters (e.g., focal length). Solutions two and three eliminate the need for calibration, as the cameras are already factory calibrated. The disadvantage is the rigid limitation of the resolution of the obtained spherical image, resulting from the physical parameters of the built-in devices. In the case of solutions one and two, additionally, errors in the assignment of colours may occur, due to the difference in the position of the laser emission point and the focal point of the spherical image (Figure 3). These errors are most significant in close proximity to the device. Solution three eliminates this problem, due to the positioning of the camera in the axis of the laser beam emission. The obtained coloured scan (3D point cloud with colours) is saved in coordinates relative to the position of the scanning device. In order to be able to determine the location of the scanned areas in the real world, it is necessary to obtain data on the geographical location of the scanner, along with its height above sea level, the rotation angle of the scanner's base position in relation to the north direction, and the direction and angle of the scanner from the horizontal position. With this information, it is possible to pre-position and align multiple scans of a given area. Figure 2: Exemplary data obtained with TLS scanning (excavations in Lublin, Poland): (**a**) a three-dimensional point cloud with a brightness level, (**b**) panoramic image of the surroundings to colour the scan. Source: authors’ own. Depending on the model of the device, TLSs are equipped with various sensors allowing the obtaining of the above-mentioned data. Determining the geographical position of the scanner is performed with the use of the global navigation satellite system (GNSS) sensor module. Currently, the GPS, BeiDou, and Galileo networks are available worldwide. Depending on the module used by the TLS, it can pinpoint its position using one or more networks, which results in an average outdoor positioning accuracy ranging from 2 to 10 m. Actual accuracy can be diminished by signal interference.