Caste of Kruje, Albania, Watchtower, after earthquake survey, December 2019

Abstract

Earthquake 2019 On 26 November 2019, a strong earthquake struck the Albanian coast, with a seismic centre nearby Durres at 41.38° North and 19.45° East, with a main seismic hit at eighth Mercalli's scale. It was one of the strongest earthquakes that occurred in recent years in Albania. The event caused 51 deaths in population and produced heavy damage. 111 cultural monuments and heritage sites were assessed in the affected regions. Out of these, 53 were deemed unsafe or temporarily closed due to serious risk. Major landmarks such as the Castles of Durrës, Kruja, and Prezë sustained considerable structural damage. The seismic event exerted a pronounced impact on the built environment of Durrës and adjacent settlements, while considerable structural damage was also observed extending into the Tirana region. In the aftermath of the earthquake a long series of small and sometimes intense shakes keep on manifesting for weeks after the main event. The impact on the Skaderbeg's Castle in Kruja was well evident, some cracks in the main walls and in the nearby building appeared immediately, but most of all, part of masonry from the top part of the tower fell down, large cracks opened in the tower and in the vault of the Dollma Tekke. In the case of the tower, the previous intervention using concrete caused a hammer effect on masonry, intensifying the damage and the fall of blocks, leaving large holes from which the concrete work emerged. To help the built heritage recovery, the Albanian Ministry of Culture asked for help from the digital survey expert operating at the Catholic University in Tirana, which has a teaching agreement with the Dipartimento di Architettura at the University of Florence. This brought to the organization of a quick expedition to Kruja on 19th December 2019, with the intention of documenting the state of the castle after the seismic event and creating a proper base for supporting further interventions. Digital Survey intervention The digital survey operation was completely conducted by SfM/IM photogrammetry, mixing terrestrial and aerial shooting into a detailed and complete digital model. Before arriving in place the operating unit was informed about the situation of the buildings and had a detailed view of the scenario from a video taken using a drone by the emergency units operating in the area. The situation was not looking good, the top part of the tower was completely fallen, the main shake, very intense, caused the rapid horizontal movement of the concrete slab applied in the old restoration, it acted like a rigid element on the top of a more elastic structure, causing heavy stress on the old masonry and producing the opening of old and new cracks. The stones on which the tower stands were appearing even more concerning, with large cracks and the old bolts in the consolidation work showing their poor conditions and a visible lack in their function. The survey planning was made starting from the tower, where one of the first limits was the impossibility to access the inside because of fallen parts and effective risks for the operators. Then the photogrammetric survey was operated from a certain distance, with a generous round turn of terrestrial shooting and a consistent campaign of drone shooting. The terrestrial shots were taken using a Nikon D850 with a 47.3 megapixel full-frame sensor and mounting a Nikkor 24-120mm F4 lens set at 35mm focal length, producing 637 pictures all around the tower and along the masonry works from the base and up to the top. The aerial photogrammetry was operated using an UAV Anafi with a 21Mp Sony 1/2.4’’ CMOS sensor and with a wide-angle f/2.4 ASPH lens, 35mm equivalent focal length: 23-69 mm (sensor set in photo shooting mode), 26-78mm (sensor set in video shooting mode) from the French manufacturing company Parrot, producing 808 pictures.The photogrammetric survey was divided over two days, the first dedicated to the Tower and the Tekke, the second focusing on the surrounding terrain in order to carefully document the entire rock ridge on which the tower stands, highlighting the cracks and detachments that are actually present. In this way, during the data calculation phase, it was possible to develop large environmental sections that documented the entire site. Due to strong geomagnetic interference at the site, which made it difficult to fly the drone, it was not possible to operate with planned flights in sequence. Instead, it was necessary to operate exclusively with a long series of manual flights. This resulted in a longer survey time, but also made it possible to get very close to the top of the tower in order to document in greater detail all the information needed for a comprehensive assessment of the state of the tower's damage. In fact to partially compensate for the impossibility to access inside, the UAV unit was operated with specific attention on the top part of the tower, capturing even inner details and surfaces. The sequence of drone pictures counted at the end 809 shots The two photogrammetric series were planned to obtain a generous overlapping between the last shots taken by the terrestrial session and the first shots taken using the drone, so as to simplify the connection and continuity between the two series in the photogrammetric elaboration. A secondary, but very important sequence of shots was dedicated to the cliff, with a specific series of 160 pictures taken by the drone on the cliff, to document the general shape and the condition of the cracks. The further processing work was aimed at the creation of a full 3D model, operating all the classical steps in the Structure from Motion Image Matching procedures. The work was done using Epic Games Reality Capture 1.0, feeded with the whole set of 1606 pictures and producing a 744 million triangles polygonal surfaces then simplified to 241,3 million triangles and to 15 million triangles and 1.5 million triangles for multimedia uses, the reduced versions were textured and exported in OBJ format, while the textured full model was used to produce high resolution orthopictures (each at about 475 Megapixel overall resolution) of all the fronts and top view of the tower in two different versions, one with the tower and another with tower, near area and front of the cliff. See full paper in Formed 2026 proceedings, published on February 2026 www.fortmed.eu