## 3 The New Approach
### Dataset Extension  
In order to create a sufficiently detailed digital model of the area and to enable verification and simulation of the passability, low altitude aerial photogrammetry with a drone was used. The primary objective of the measurements was to obtain a digital three-dimensional spatial model with sufficient accuracy and detail. In contrast to classical measurement methods, the digital image correlation method was used, to facilitate the speed of data acquisition and processing [19]. The advantage of its application is that it does not require the explicit identification of individual points but uses image recognition algorithms to automatically identify the key features among the images, link them, and subsequently reconstruct the spatial information. Firstly, the Structure from Motion technique (SfM) was utilized. The SfM method uses a number of unordered images that depicts a static scene or an object from arbitrary viewpoints and attempts to recover camera parameters and a sparse point cloud that represents the 3D geometry of a scene [20]. The second stage consists of a further reconstruction of the geometric scene. This was carried by the Dense Multi-View 3D Reconstruction algorithm (DMVR), which was run on an already-aligned image set. The algorithm operates on a pixel base, thus enabling it to obtain a higher level of detail than the SfM [20]. The scale, transformation, and validation of the resulting model  
Figure 1: Diagram of the generalised procedure steps. were performed with signalised points measured with the GNSS station. The resulting accuracy was 2.5 cm in position (X,Y) and 3 cm in height (Z). In this way, the resulting model was aligned with cadastral maps and it was possible to clearly define individual parcels intended for potential remediation. Figure 2 show a part of the assessed area in which the pilot project was carried out. Figure 2 depicts positions of captured images (over 3000 photographs were taken) together with the necessary overlap for successful reconstruction. Blue areas that are represented with the highest frequency represent the highest overlap of each image. The number of overlapping images then decreases to lower values in the extreme areas of the site. At the same time, the figure below also shows the statistics of the performed measurements. The photogrammetric outputs are similar to laser scanning but with significantly lower time demands and corresponding costs, and only a slightly lower accuracy (RMS of 2.1 cm). For the purposes of this study, a textured triangular polygon mesh of the area was created together with a detailed orthophoto with a resolution of 2 cm/px [19; 20]. Such detail is essential, especially in places where road width ratios reach very small values that border on the minimal required passage profile of the FRS vehicles. The advantage of this approach is also the fact that the resulting model corresponds to the current situation at the time of measurement, e.g., shows parked vehicles, dustbin placements, fences, etc. Furthermore, it is complemented by the textures of the environment  
Figure 2: Camera locations and image overlap with parameters. for effective presentation and orientation; therefore, it tells about the actuality and related deficiencies much more accurately than could be obtained from a panorama view, satellite imagery, or classical aerial maps. In order to assess the passability, it is not necessary to work directly with a 3D model of the environment. The initial site analysis can be performed on a 2D basis (sufficiently detailed orthophoto is available). However, the use of a 3D model is necessary for evaluation of the clearance at locations where there are features that restrict the clearance profile of fire engines. These elements are, for example, unmaintained vegetation, road signs, streetlights, or bridge structures. An illustration of the digital model is presented in Figures 3 and 4. Figure 3 shows the resulting spatial model, mesh, together with the location of each captured image.