METEORA

CASE STUDY:

The METEORA project aims at the documentation and digitization of a part of the Archeological Site of Meteora. Particularly, the main study area consists of two inaccessible rocks: the rock of St. Modestos (‘Modi’) and the rock of the Chain of Apostle Peter (‘Alyssos’). At the top of both rocks, there are ruins of old Monasteries. Within this project, the Monastery of the St. Modestos is being digitally reconstructed; it is visualized both via the visualization system of the platform and through the Augmented Reality app. The choice of these two rocks lies in the existence of important findings, in the unexplored – until today – establishment and evolution of their monastic community as well as in the special topography and geological morphology. Apart from Modi and Alyssos, the study area includes the majority of monuments in the Holy Site of Meteora, including the six active Monasteries, the Pyxari rock, as well as some hermitages and other cultural heritage sites in the wider area.

Meteora is located near the city of Kalambaka, in the northwestern part of Thessaly, at an altitude of about 600 meters above sea level. It has been declared as an Archeological Site, and more specifically as a “Historic Preserved Monument” with the decision no. 10977/16.5.1967 of the Minister of the Presidency of the Greek Government. This declaration extends the protection of Meteora to the wider area beyond the building complexes of the monasteries. Meteora includes two Protection Zones. It has been declared as a Holy Site (Law 2351/1995), as it includes one of the most important complexes of Orthodox monasteries in Greece, with an active monastic community of a long history. At the same time, it is a special geological phenomenon with giant rocks that were generated 23 million years ago. At the tops of some of these rocks, six Monasteries were built and are still operating, while throughout the area there are some smaller abandoned monasteries and churches of the 14th century. Meteora has been characterized as an UNESCO World Heritage Site (https://whc.unesco.org/en/list/455/), based on the cultural criteria of UNESCO (C 1-5) as well as on a criterion concerning the natural environment (N 7). Meteora has also been proposed as a monument of geological heritage thanks to its important geomorphology. The area of Meteora along with the Antihasia Mountains are included in the European network of protected areas NATURA 2000 with code GR1440003, which is the main European means for the conservation of natural habitats, wild fauna and flora.

SPATIAL DATA:

Flight planning: The geometric documentation of cultural heritage sites of such extent and morphological complexity as that of Meteora needs specialized techniques. The metric and imagery information has to be geometrically accurate and of high quality, resulting from economic, fast and automated methods. Techniques of aerial photogrammetry are recommended for mapping large and inaccessible areas with the above requirements. Within this project, conventional aerial images from manned or unmanned aerial vehicles are captured and airborne laser scanning is also implemented (airborne LiDAR – Light Detection And Ranging). 2220 vertical and oblique aerial images were captured by a manned aircraft, covering the entire site of Holy Meteora, i.e., an area of 10km². A camera of type NIKON D800E with a lens corresponding to a focal length of 50 mm was used. Each image has a size of 7360 × 4912 pixels. The images were taken from a low altitude and correspond to an average scale of 1: 20,000. They were collected in July 2017. About 50 ground control points (GCPs) were collected through RTK GPS measurements for the scope of georeferencing. In order to capture the rocks Modi and Alyssos in greater detail, a Phantom IV RTK unmanned aerial vehicle (UAV) with a 20 MP camera was used and a total of 5000 vertical and oblique images were acquired. Special targets were placed on the ground for georeferencing purposes and their coordinates were determined using a dual-frequency GNSS receiver. Also, the coordinates of specific points on the rocks were determined by topographic measurements .

Planning of scans: LiDAR systems, provided by Geosystems Hellas S.A., were used for scanning the Archaeological Site of Meteora, where the rocks Modi and Alyssos are located. LiDAR is a remote sensing method that uses pulsed laser to generate altitude data in the form of point clouds. LiDAR systems are suitable for mapping areas with vegetation and for applications that require quick response. Before performing the laser scanning stage, it is necessary to determine the parameters that will ensure the acquisition of a dense point cloud with satisfactory accuracy. Within this project, the dual-channel airborne mapping system RIEGL  VQ-1560i-DW was used. This system incorporates 2 band LiDAR sensors (one activated in near infrared and one in green) in the same cone, as well as two photogrammetric digital aerial cameras of type Phase One IXU-1000RS; one camera captures color aerial images and the other one captures infrared aerial images. The data collection was supported by a GNNS system and an inertial sensor of type Applanix AV-610.

PROCESSING:

Generation of high-resolution 3D models: The vertical and oblique aerial images as well as the terrestrial images were oriented through the Structure from Motion (SfM) technique and a sparse 3D point cloud was generated. The SfM method simultaneously determines the camera poses, i.e., the camera exterior orientation parameters and optionally the camera interior orientation, and sparse 3D point clouds. The camera poses and the object geometry are determined by automatically locating homologous feature points (tie points) in overlapping images. The SIFT algorithm or its variants are mainly used for feature extraction and description. For each feature point, a descriptor vector is calculated based on its “neighborhood”. A RANSAC-based algorithm is generally used to identify and eliminate erroneous matches and improve the success rate of the SfM method. Large overlaps among the images are generally required for the successful implementation of SfM. In the context of the METEORA project, GCPs were measured in the field for the scope of georeferencing the SfM results. The Agisoft Metashape software as well as the RealityCapture software were used for comparative evaluation of the results. Then, dense point clouds were computed through a dense image matching process. For the implementation of dense image matching, computer vision algorithms (e.g., semi-global matching) are used to calculate the 3D object coordinates for each pixel, and not only the coordinates of the feature points. This process results in very dense point clouds, covering all parts of the object depicted in at least three images. During the last stage of the 3D modeling process, the 3D model of the scene is produced as a continuous surface, in the form of triangles (TIN) or other more advanced forms (curves, nurbs). The 3D surface model can be further enriched with texture mapping, thus including the maximum possible information. The creation of 3D surface models for the Archaeological Site of Meteora as well as the Modi and Alyssos rocks was implemented using the Geomagic Studio software.

Photogrammetric processing of LiDAR data and aerial images taken by the system RIEGL VQ-1560i-DW: As far as the processing of aerial images is concerned, the RAW RGB images were initially converted to .tif images. Then, the Phase One IXU 1000RS camera parameters were imported into the Erdas Imagine Professional software and tie points were automatically generated, as required for the aerial triangulation process. The processing of LiDAR data includes the conversion of LiDAR cloud points into the .las format and the organization of data in blocks of 500m × 500m so that they can be easily processed. Two procedures were applied for strip alignment through the Terrasolid Terrascan software:

• 1st application: strip alignment per scanner
• 2nd application: strip alignment per flight line