## 1 Introduction  
Unmanned aerial vehicles (UAVs) are commonly known as UAS (Unmanned aerial systems) and RPAS (Remotely piloted aircraft systems). There are small distinctions between their meanings, although they are terms that are usually used interchangeably. While an UAV can fly autonomously without human intervention, a RPAS must be controlled by someone from a remote station even if the flight is in automatic mode. A UAS is a much broader term and describes all the systems that allow the operation of an UAV. Generally, all these systems are denominated drones. In recent years, the use of UAVs has proliferated in the civilian sector, although originally these systems were designed for military applications. UAV technology in the military sector is more advanced than in others. Its development began to be explored during the wars of the 20 th century. From the 1990s -when global positioning systems (GPS) became available to all users- a new market started to emerge in the civilian sector for UAVs. GPS development opened up the possibility of maintaining the position of an UAV in a global reference system in almost all parts of the world and in real time. In the early 2000s, UAVs became more popular because of the cost reductions and their technological development in different and functional platforms [1]. UAVs can be classified according to their design (fixed wing, helicopter and multi-rotor) and their maximum mass at take-off (up to 25 kg, up to 150 kg and more than 150 kg). The most commonly used UAVs to carry out tasks in the civil sector -particularly in urban environments- are the multi-rotor and low weight types due to the restrictive regulations of the countries for safety reasons. The legislation of each country establishes a maximum weight of the UAV to be able to operate it in cities. These systems are propelled by multiple rotating engines attached to their fuselage. Inside the fuselage are the electronic equipment and under it the payload (sensors) and the landing gear. The weight of all these components is crucial to perform civil tasks. In recent years, researchers have shown great interest in the use of UAVs for civilian purposes such as search and rescue [2], remote sensing [3], construction and infrastructure inspection [4], agricultural precision [5], delivery of goods [6], real-time monitoring of road traffic [7], surveillance [8] and providing wireless coverage [9]. Their characteristics of multi-rotors such as hover, stability, manoeuvrability, vertical take-off and landing (VTOL) and low maintenance cost, make them the optimal system to carry out tasks that require low speeds, low heights and stable hover in the air such as taking photographs or taking measurements. UAVs are an ideal technology for architecture, engineering and construction fields (AEC) as they can access spaces that are unsafe and inaccessible to human workers. They have better features in terms of time, accuracy, safety and costs compared to conventional methods [10]. They also allow the reception of real-time telemetry data during a flight thanks to the connection of the UAV station with any smartphone device. It is possible to know instantly the percentage of battery charge, the height, the distance to the pilot, the distance to the landing point, the ascent speed, the forward speed, the GPS coverage status, the reception of the camera images, etc. The most commonly used sensors for telemetry in this field are commercial cameras as the basis for image and video acquisition, followed by laser scanners, thermal cameras, radio frequency and ultrasonic beacon system [11]. The most important constraints on UAV operation are the lack of autonomy due to short battery life, the loss or presence of GPS signal interference (as occurs indoors), climatic conditions and the restrictive regulatory systems to operate in urban spaces [1,12,13].