# Graphic Applications of Unmanned Aerial Vehicles (UAVs) in the Study of Industrial Heritage Assets
## Case Study and Results
### UAV Flight Operation Analysis  
The UAV flight operation was carried out in three sessions, due to the availability of three UAV batteries. The first flight operation was focused on the factory. Different orthophotographs (irrigation water pond, chimney, plot, etc.) and aerial photographs of the factory were taken. An additional video was recorded with factory landscape images. The first part of the second flight operation was focused on the factory and different photographs from the chimney conservation state were taken. The second part was focused in the aqueduct, taking several photographs in different positions focusing in the general structure of the aqueduct (Figure 6b), the weather vane (Figure 7a), the waterway (Figure 7b) and the arches' characteristics (Figure 7c). Finally, a video was recorded in a single shot around the aqueduct and the factory. The UAV was positioned in the mid-point between the aqueduct and the factory. The aqueduct was recorded initially, flying the UAV around it, in a south-north direction. Then, the land between the assets was recorded, paying special attention to the waterway. Finally, the factory was recorded, flying the UAV around it, in a north-south direction. This fact allows analysing the environment around the assets and following the waterway completely, from the aqueduct to the factory. Regarding the factory, a poor state of conservation is observed. The building roof has completely disappeared, except from the office building, allowing one to appreciate the construction of the factory. In addition, the floor and many internal partitions of the buildings have been destroyed. In spite of this, the building envelopes still remain standing, which would allow their reconstruction, maintaining the main construction characteristics of the factory, except for the workers houses, located in the north of the enclosure, of which there are hardly any remains. Furthermore, the main historical and cultural interest element, due to its singular decoration, is the chimney. It remains complete despite the observation of different cracks along its structure and the deterioration of its upper part (Figure 8). Two important cracks can be observed in the chimney structure. One of them is located in the central part of the structure (Figure 8a), oriented to the east. This crack runs vertical and horizontally, bordering each of the bricks and its unions. The other one (Figure 8b) is located at the upper chimney zone, oriented to the northeast, running vertically and bordering along the brick unions. Chimney zenith photographs (Figure 8c) show that some of the upper bricks have disappeared and others are loose, presenting a major fall risk. This inspection allows considering the chimney conservation need. Despite that an imminent detachment of the chimney structure cannot be ensures, the lack of maintenance may cause this to occur over time. The aerial photographs have allowed indirect measurements of different asset elements. Particularly, the chimney and the irrigation water pond measurements. The results are listed in Table 3.  
Figure 7: (**a**) Weather vane on the pinnacle; (**b**) aqueduct waterway; (**c**) arches. Possible errors in the dimensions measured from UAV photography depend on different variables: the distance between the UAV and the element to be measured, the visualization angle (cosine error) and the camera resolution. A higher error can be expected in the length of the irrigation water pond due to the longer camera distance from the photographed element. Taking into account the image size of the camera (\(4000\times 3000\), Table 2), the division between the length of the water irrigation pond in the photograph (\(38.20\) m) and the image size (\(4000\)) one can estimate the maximum error obtained in the indirect measurement, which is \(0.01\) m.  
To compare this with conventional camera errors, one should analyze how a certain dimension \(W_{h}\) is measured (i.e., width of a crack at a height \(h\)). In this case, the following variables could be used (Figure 9).