*3.2. The Tachymetry Technique*

The tachymetry technique is related to creation of the spatial polar coordinates, i.e., vertical and horizontal angles, and the distances from the given reference points. Currently, total stations are theoretically available for measurements with a precision up to 1 mm and for measuring directions of 0.15 mgon (0.5'). However, considering practical experiences, the received accuracy is commonly significantly smaller, i.e., 1–5 mm for the distance measuring and 0.9–3 mgon (3–10') for the direction measuring. In the analyzed example, to decrease the length of sight (the distance between the

measuring points and the total station position), the positions of the total stations were located on both banks of the river (Figure 2). Thus, the length of sight did not exceed 100 m. Notwithstanding the significant discrepancy between the accuracy in the field and lab conditions, the tachymetry technique still seems to be the most precise way to determine the point location. This technique is distinguished by the discretion of measuring. A Leica TC2002 (Heerbrugg, Switzerland) total station was used for both tests. Several measuring points were selected (targets were placed on the transverse beams) for observing the bridge deflections (Figure 3). The bridge deflections were computed using the method proposed by Anigacz and Kokocinska-Pakiet [1].

**Figure 2.** Top view on locations of total stations, laser scanner, and digital camera.

**Figure 3.** The examined bridge structure seen through the total station: (**a**) distribution of the measurement shields on the transport gallery, (**b**) view of the measurement shield through the telescope of the total station.

The tachymetric measurements, as the most accurate measurements, were taken as the reference results for the other testing techniques. The received bridge deflection results have been confirmed by measuring from the second position of the total station. To distinguish the observed bridge deflections, the two-sided measuring shields were applied, which allowed measuring the distance from both sides of the bridge. The measuring shields were installed on the transverse beams of the bridge deck (Figure 3a). Thanks to this location of measurement points, it was possible to determine the deflection at 1/4, 1/2, and 3/4 of the span. A view through the telescope of the total station is presented in Figure 3b. The hole in the center of the shield has a 5 mm diameter. Based on multiple measurements of the total station-shield set used, a 1–2 mm real accuracy was obtained.
