UAS IR-Thermograms Processing and Photogrammetry of Thermal Images for the Inspection of Building Envelopes  
[PERSON]  
1 Agrofocrestry Engineering Department, University of Santiago de Compostela, 27002 Lugo, Spain; 1 [EMAIL_ADDRESS] (M.G.-D.); [EMAIL_ADDRESS] (J.O.S. )1  
[PERSON]  
2 Agrofocrestry Engineering Department, University of Madrid--UCM, 28040 Madrid, Spain; 2 [EMAIL_ADDRESS] ([PERSON]  
2 Agrofocrestry Engineering Department, University of Madrid--UCM, 28040 Madrid, Spain; 2 igraco. challenges, since environmental conditions may interfere with thermal flow, making them unsuitable for actual working conditions. Passive IRT is typically associated with the analysis and the detection of potential anomalies in thermograms, showing defects in the envelope. This procedure is commonly applied in different types of building analysis and is frequently used to perform envelope study using a qualitative or quantitative approach. The aim is to evaluate the magnitude or importance of the target defects, so the measurement accuracy takes on special relevance [3,9]. Thermal audit campaigns require an in-depth study of the specific parameters linked to the properties of materials, such as emissivity and reflected apparent temperature (RAT) [4,10]. With changing environmental conditions, like ambient air temperature (AAT) and relative humidity (RH), not only thermal conductivity, but also specific heat and density of materials, vary, and this also influences in a significant way the heat transfer through buildings envelopes. Wind can also appreciably influence the heat losses through the envelope and the thermal measurements over the facades; therefore, it must be considered, too [11,12]. The thermography sector has undergone significant advances in recent years. The newest revolution in IRT comes from the democratisation of UAS technology and the emergence of lightweight devices easily installed on board. The integration of thermal sensors with drones opens up new possibilities for real-time aerial inspection in many fields. The massive capture capability of the UAS allows their application in thermal inspection of large areas such as in the agricultural field [13], hydrology [14] or industrial inspections, such as of photovoltaic farms [15, 16, 17]. However, the study of building envelopes is perhaps where the fusion of IRT and UAS technologies presents the most promising potential [18, 19, 20]. UAS-mounted thermal cameras can provide low-altitude aerial images, from a flexible perspective, with information on both qualitative and quantitative scenarios. For this reason, the method could be particularly well suited to inspecting difficult-to-access areas of the facade or the roof, where the thermal conductivity is higher than those areas with thermal bridges, air leakages or the presence of moisture in the walls. Three-dimensional thermography (3D-IRT) has been a growing field in recent years, with a particular focus on its application in building science. This growth is due to the fact that it allows the thermal behaviour to be analysed for use in thermal audits, providing complete information on the envelope as a whole, rather than a series of disconnected images. A three-dimensional reconstruction of the building envelope, although presenting some drawbacks, can provide both quantitative and qualitative information on thermal variations and, therefore, on the presence of defects. This approach, addressed in this study, still needs to be further developed since studies are scarce. In fact, ref. [21] identified a gap in the scientific literature related to the use of thermal imaging for building energy modelling and analysis. In the last decade, very few scientific papers have been published on this topic, highlighting the potential of this technology for identifying and analysing thermal anomalies in buildings [22]. One of the main areas of research in 3D-IRT has been the development of new and improved algorithms for data processing and analysis, even in real time [23].