## 3 Data Acquisition
### Cortes de Pallas Test Site  
Cortes de Pallas is a Cretaceous limestone area with historical geotechnical issues. On 6 April 2015, a cliff called La Muela partially collapsed, and some facilities of the electricity power plant and the main access road to the village were seriously damaged. At the end of 2017, once the consolidation works were finished, the Infrastructures Area of the Diputacio de Valencia commissioned a deformation monitoring plan to detect possible displacements of huge boulders or potential malfunction of the installed anchoring systems [2]. However, the detection of possible displacements of some centimeters with the required level of significance in a short period, e.g., two or three years, is a quite challenging task that can only be approached by means of high-precision geodetic techniques due to the peculiar topography of the zone. The whole area involves distances from 500 to 2000 m with height differences reaching 500 m. Moreover, 10 geodetic pillars mounted on presumably stable locations at 15 target points (referred to as check points) were installed in the rock wall by professional climbers using abseiling techniques, because they are not directly accessible. Furthermore, the measurements have to be undertaken by necessity from the opposite shoreline which is about 600 m away because the cliff of interest is facing a water reservoir (Figure 1). Therefore, the Diputacio de Valencia opted for periodical geodetic surveys along with image-based techniques like CRP and long-range TLS (since 3D models derived from CRP and TLS techniques proved compatible, only the former will be use for the experiment) as the most feasible join solution for the deformation monitoring plan. The periodical geodetic surveys, which are based on sub-millimetric EDM techniques and performed annually, have a triple objective: (i) to establish and monitor a high-precision 3D reference frame realized by 10 pillars, coded 8000+ (Figure 2a), (ii) to determine the 3D coordinates of 15 additional check points, represented by 360\({}^{\circ}\) prisms coded 1000+ (Figures 2b and 3) placed in specific points of interest on the monitored rock wall, and (iii) to provide a ground control network for image-based techniques. As a demonstration of the potentiality of the high-precision geodetic techniques used [23; 24], the displacements found for the reference frame pillars between the years 2018 and 2019 are shown in Table 1. Except for pillar number 8005 (Figure 2a), where a significative vertical displacement of \(-\)6.12 mm (see Table 1) was detected, the reference frame can be considered stable and well-controlled. The resulting reference frame has an overall accuracy of 1 mm and its scale is metrologically consistent with the unit of length of the International System (SI). Further information about the geodetic surveys is given in Section 3.2. This high-precision geodetic method, which is very demanding and time-consuming, can be only applied to a limited number of relevant points of interest, which are 15 targets in the case at hand. Figure 1: Perspective view of Cortes de Pallás site area. An additional problem with both CRP and TLS techniques, especially when they are performed statically, like in the case at Cortes de Pallas, is the presence of occlusions. Since the CRP images can easily miss some shadowed areas, the 3D model obtained in each campaign usually has small patches with no information. A possible and efficient solution could be the use of mobile mapping solutions like the Kaarta Stencil 2 ([[https://www.kaarta.com/products/stencil-2-for-rapid-long-range-mobile-mapping/](https://www.kaarta.com/products/stencil-2-for-rapid-long-range-mobile-mapping/)]([https://www.kaarta.com/products/stencil-2-for-rapid-long-range-mobile-mapping/](https://www.kaarta.com/products/stencil-2-for-rapid-long-range-mobile-mapping/))). Being dynamic, this system can provide a continuous 3D survey of the problematic area in Cortes de Pallas in less than one hour.