## Methods
### Surveying Methods
#### 2.1.2 Digital Close-Range Photogrammetry  
For a high-resolution mapping of selected morphologic structures, we decided to use Structure-from-Motion (SfM) method of digital close-range photogrammetry (DCRP).  
The SfM photogrammetric method is one of the most advanced methods of processing photogrammetric images. Conventional photogrammetric methods require a set of identical points present on photographic images with their known spatial position. These identical points are then manually identified on individual images, and based on them, the camera position is subsequently determined for each image. In contrast, cameras position and scene geometry in the SfM method are reconstructed simultaneously based on the automatic identification of characteristic matching features--key points, using various algorithms, for example, Scale Invariant Feature Transform (SIFT). These key points allow an initial estimation of cameras position and object coordinates, which are subsequently iteratively refined, for example, by non-linear least-squares minimization. The number of key points identified on images depends primarily on the image resolution and texture of the imaged surface. Density, sharpness, and resolution of a set of images, in combination with a textural variety of the captured scene, will determine the quality of the resulting point cloud. Similarly, the shorter distance between the camera and the imaged surface will increase the spatial resolution of the image and hence increase the density and resolution of the resulting point cloud [34; 35].  
Appropriate illumination of the imaged surface is a prerequisite for photogrammetric imaging of selected objects. In the case of the Ochtina Aragonite Cave, it is a show cave accessible for the public, so most of its space is artificially illuminated. Spotlights along the tourist path provide sufficient illumination even for the parts of the cave located farther away from the path. Therefore, no additional illumination was needed. During photogrammetric imaging in such areas, it is necessary to ensure that individual camera stations do not partially or entirely cover the source of illumination and thus avoid undesirable shadows on the imaged surface. However, if illumination in the underground is not sufficient (or not present at all) and, therefore, there is a need for additional artificial illumination of the given spaces, it is necessary to ensure that this illumination is sufficiently strong, constant and stable to prevent variable lighting and the occurrence of undesirable shadows. Otherwise, it could lead to problematic image processing (image orientation, generation of the dense point cloud, lower accuracy, increased noise, etc.) [36].  
For photogrammetric survey, all images were captured by digital camera DSLR Pentax K-5 with the lens Pentax SMC DA 15 mm f/4 ED AL Limited, by using a stable tripod, 12-sec self-timer, ISO 100, aperture f/13, and RAW format.  
For image processing, the Agisoft PhotoScan(r) Professional Edition (APSC), Version 1.2 software (Agisoft LLC, St. Petersburg, Russia, 2016) for photogrammetric processing of digital images and creation of 3D spatial data was used. Subsequent high-resolution modelling of selected features into a 3D polygonal mesh model was done in the 3D point cloud and mesh processing software CloudCompare version 2.10 (GPL software, 2020).