Making Money With Drones, Drones in the Construction Industry. Second Edition.: Making money with drones, #1

1.- Introduction.

Since the conquest of the sky and the access to the air transport, the human being has given multiple utilities to the flying machines which were derived in the armament and transport industry mainly. With these airplanes, new industries were developed, independent from those that already existed, and special emphasis was made on this aspect. Aircraft designed with little or no support function obtained other classic industries such as the agricultural or construction sectors, since high capitals were needed for the acquisition of aircraft. In addition, the action of operating aircraft to support other industries required large operating spaces, which did not always coincide with the development spaces of classic industries. In the same way, operating costs with aircrafts were not feasible in small industries or companies, so the utility of the aeronautical sector in other industries was very limited.

The constant technological advances in electronics, telecommunications and aeronautics allowed the development of unmanned aircraft. In the 21st century these aircraft could be sized at will allowing their use in multiple industries, that is, a multipurpose and multifunctional aircraft could be created. Such aircraft were classified as unmanned aerial vehicles (UAV) or drones colloquially being their first use as target for Sidewinder AIM-9 experimental air-to-air missiles. It was later found useful for reconnaissance and later for espionage in heavily defended air zones, its small size making it invisible to thermal and electromagnetic radars. In addition, it was possible to add a new faculty to the UAV aircraft which is the vertical takeoff or VTOL, which gave the UAV a greater multifunctionality. 

In the civil aspect it was seen useful in recreational and advertising actions since the UAVs could carry acceptable payload to carry high quality cameras and allow a better registration of the photos and images in multiple angles of vision. Other recreational applications were speed and maneuverability competition. The advantages of this type of aircraft in the collaborative aspect with other technologies allowed the user to quickly adapt to the operation of this technology, which allowed its market to expand rapidly and with it its value.

In the same way, other ways to take advantage of the versatility and multifunctionality of UAVs have been sought as support in other industries such as agriculture and mining, allowing the emergence of precision agriculture and focused mining. The objectives of the integration of UAVs in other industries is to simplify the work done, improve the management perspective and reduce the costs of work processes. In the contemporary construction industry, multirotor drones are a tool that have the potential to facilitate all construction activities and transform them into safe activities, consequently saving time, costs and injuries, in the end all these new methods will contribute with a higher quality construction. (Li and Liu, 2018, p.2) That is why we are looking for linking technologies that relate the results obtained with UAVs and construction management systems. The UAV, apart from flying intelligently, must collect and transmit all kinds of information from the construction area. Increasingly, efforts in UAV design are focused on minimizing the dimensions of the drones without impairing their performance and functionality. Finally, the aim is to automate the entire drone process as the first step towards automation in the construction process. "Multi-rotor drones have not yet been widely used in the construction industry, as this field has been slow to adopt emerging technologies. (Li and Liu, 2018, p.2)

Figure 1: Comparison between the usefulness of conventional aircraft and UAVs as supports for other industries

Planning and monitoring of construction activities is one of the key areas where drones and UAVs can significantly improve performance and speed. In fact, the construction industry can take advantage of these technologies in almost every practical aspect. For example, drones and UAVs can potentially be used at several stages in a construction project, including pre-planning, detailed survey and site mapping, construction process monitoring, post-construction controls, and sales and marketing. (Anwar, Amir and Ahmed, 2018, p.1) That is, we can equip aircraft with control devices which are technologically related to construction process management technologies. These link technologies between the aeronautical and civil construction industries must be developed according to the requirements and possible areas of use of UAVs in this industry. UAS are also useful for monitoring large construction sites. A larger construction site becomes more difficult to monitor by human personnel and this is where UAS can play a role. (Mosly, 2017, p.237) The flight and parking of the drone at specific heights allows for extensive, real-time observation of the construction site. These and many other applications are currently being researched and respond to current construction problems.

Figure 2: Relationship between the drone industry and the construction industry.

2.- UAS applications in the pre-construction stage.

We must know how to differentiate and classify the existing processes in construction, in the same way we will classify the possible areas of development of UAVs in this industry. In the pre-construction stage is where we delimit the states and conditions of the context where the construction will be carried out to decide the strategies and procedures of construction to be developed. We found two processes where UAVs can be useful.

2.1.- Project evaluation.

The first objective in a construction project is to identify the context where the work is going to be carried out, it is here where photography and video recording are necessary to identify the possible risks and natural obstacles present. "Unlike the information obtained through traditional aerial or satellite platforms, the fixed images or videos obtained with UAS have a better resolution in both temporal and spatial dimensions. In the project initiation process, surveyors or sponsors can adopt UAS to implement aerial surveys to evaluate and determine the feasibility of a project. This application is particularly practical in urban areas, where space is often limited and surrounded by buildings. Aerial photographs can be used to capture and measure the context of the construction, to estimate site dimensions, height restrictions and access routes. Using UAS for this task will save time, costs and other resources for organizations or project managers. (Zhou, Irizarry and Lu, 2018, p.3) The UAV allows the first approach of the personnel to the future construction zone, from the information obtained from the drones the altitude contours, ground conditions, type of surrounding constructions and other important initial factors can be obtained.

2.2.- Site planning.

After the evaluation of the project, its validation and knowledge of the context, the work environment can be planned. It is necessary to know what type of constructions and facilities surround the space where the work is going to be developed. "It is known that the combination of site activities with the detailed planning and programming of a project requires clear spatial analyses as references. Since regular site interview methods and design design are not good for demonstrating a visualized experience, a UAS allows project managers to see physically or virtually, offering an opportunity to visualize the design, planning and organization of the site prior to construction. For example, an ideal logistical plan can be developed and proposed based on aerial information about surface features and artificial features on or around the site. In addition, a UAS is appropriate for obtaining accurate information about the water source, electricity source and related piping, which contributes to effective planning for water and electricity on the site. (Zhou, Irizarry and Lu, 2018, p.3) This requires thermal and multifunctional sensors with small dimensions for UAVs to carry. Similarly, information transmitters are needed so that the data received in the aircraft can be sent to the information management devices. In general, site planning with drones is based on obtaining information from the context of the construction that is not visible to the naked eye and that cannot be obtained with simple instruments. Soil composition, energy sources, volumes of surrounding material and moisture are some examples.

The study of soils is one of the starting points for any construction work, with them we decide the type of materials and procedures for construction. Rapid UAV sampling of the ground can speed up decision-making, especially if ground penetrating radar (GPR) is incorporated. Unfortunately, there are many limitations to the use of GPR due to radar emission frequencies and possible interference with other devices. One solution is through terahertz scanning, this type of identification is very different from conventional X-ray because it is completely harmless and non-invasive. "This system is also known as submillimeter radiation and operates between 300 GHz and 10 THz depending on the energy input level of the device and is capable of detecting signatures of materials such as asphalt and concrete. In the spectrum, THz scanning is between microwaves and infrared light waves. The THz beams transmitted through barrier materials can be used for material characterization, layer inspection, finding buried explosives. An unpublished report by Mott MacDonald demonstrated the viability of THz scanning as an alternative to ground penetrating radar for establishing soil layers and identifying subway objects at soil depths of 6 m, even under paved surfaces. (Laefer, 2020, p.13) This system can be very advantageous in urban areas where municipal plans sometimes do not guarantee actual ground conditions. It is always necessary to update the location of water and drainage pipes, gas pipelines and electrical lines. This would prevent possible pipe breaks, electricity shortages and the suspension of vehicle traffic. At the same time, occupational safety problems would be avoided and only one pilot, one co-pilot, the engineer in charge and a specialist in reading the terahertz scanner would be required. These registration systems are increasingly smaller and are made with carbon nanotubules, the current models are 45 centimeters long. 

Another alternative system for soil analysis is hyperspectral imaging. All of these reading systems require a universal processor and display called building information modeling (BIM). "The availability of accurate spatial and geometric information of the facilities is critical to the success of any construction, remodeling project and maintenance strategies. Due to these needs, the main benefits can be accumulated by applying modern technologies such as laser scanning and digital imaging to finally be analyzed and integrated by BIM as a general reading format for pre-construction inspection. One of the key requirements for the success of BIM is the automation of the information channeling from data acquisition and analysis to data storage. In particular, the accurate identification and classification of building materials is an essential requirement within the automation process. 3D point clouds can be created through the use of hyperspectral imaging obtained by laser scanning. (Amano, Lou and Edwards, 2018, p.4) The idea of hyperspectral recording is specially configured to identify the types of materials present both in the subsoil and in existing structures.

"Laser scanning assigns three-dimensional spatial coordinates and intensities at different points in a scene, which construct the data as a point cloud. This non-destructive data acquisition is useful in obtaining images of historical and vulnerable installations. The remote capture of cultural heritage buildings is valuable as a documentation and conservation management application. One of the unique features of the point cloud is the ability to view the entire facility with complete freedom from the BIM format. Laser scanning has also been used for construction quality control, condition assessment, component tracking and project progress monitoring. (Amano, Lou and Edwards, 2018, p.5) This feature gives more versatility to the BIM system for the visualization of the construction context, seen from 3D geometric, thermal, electromagnetic, hyperspectral and terahertz reconstruction, consequently misinterpretations and subjectivities would be eliminated.

"The characteristics of the materials can be represented by a spectral identity of the surfaces. The spectra are estimated by the reflectance values which are physical properties of the material itself and are independent of the scene illumination. Therefore, it would be useful if the spectral information could be integrated into the 3D geometric information so that the point cloud can be significantly recorded in BIM. The spectral characteristics of urban building materials, such as concrete and clay tiles that exhibit the effects of aging, have been examined to establish a spectral library for implementation in BIM. (Amano, Lou and Edwards, 2018, p.10) Increasingly there are records of different types of materials under different conditions. With this aspect it is possible to link the percentage of materials involved with the different geometries of the structures. The classification of materials would be related by their spectral bands.

"Hyperspectral 3D models provide a large amount of high dimensional data that requires advanced data analysis methods. The complexity and computational cost of the analysis is much higher than for a single image property. It is possible to reduce such demands by subsampling the data. However, the requirements for the fidelity of the final results in terms of spectral and spatial resolution must be considered beforehand. The difference in the spatial resolution of imaging systems can cause inaccuracies in the recording of images. (Amano, Lou, and Edwards, 2018, p. 16). The ability to achieve high-quality image integration, with identification and classification of building materials will be more automatic. This will greatly enhance the employability of BIM in identification, construction, inspection, and remodeling projects involving existing facilities, resulting in increased efficiency, reliability, security, and reduced costs.

Figure 3: Data collection by terahertz scanner and hyperspectral cameras.

The construction area is always a variable in relation to time due to climatic and environmental factors and human interactions. An example of this is landslides and accumulations of earth as a function of natural phenomena. The great problem of this phenomenon is to know how environmental changes affect the construction zone. That is why it is necessary to measure the changes with great speed both in the evaluation stage, construction and monitoring. All kinds of phenomena must be recorded such as snowfalls, landslides, water accumulation, floods, increase in vegetation and geographical changes. To monitor active landslide hazards and understand the processes involved, spatial and temporal measurements are required, such as displacement rates, extent and changes in surface topography. For them, remote sensing has been an integral method of landslide research for many decades, with several different techniques being used. (Niethammer et al., 2012, p.2) Remote sensing can be further harnessed through the use of a UAV. Aerial imaging can provide important data on surface texture, but photogrammetric DTMs are often not as accurate and precise as airborne digital terrain models (DTMs) based on LIDAR. (Niethammer et al., 2012, p.2) An example where LIDAR technology is applied occurred in France in 2012.

The study was conducted on the landslide in Super-Sauze, the southern French Alps. The landslide has occurred in a torrential basin located at the top of the Sauze torrent on the left side downstream of the Ubaye valley and is one of several that have been persistently active since the 1970s. The slide extends over a horizontal distance of 850 m between elevations of 2105 m at the crown and 1740 m at the foot, with an average slope of 25°. Thanks to geometric reconstructions, the total estimated volume of the slide was 750000 m³. The displacement velocities of the unstable slope vary from 0.01 m to 0.4 m per day. (Niethammer et al., 2012, p.3) With this system one can quickly observe the changes made by nature and make quick decisions to reverse them. The final analyzed data can be sent digitally online to the engineers and decide the changes to be made in the area. This is true for all construction processes, this process is also valid for the calculation of volume of materials or river measurements involved in the project. The generation of DTM was carried out using the VMS short range photogrammetry software and a GOTCHA image matching algorithm developed by University College London. (Niethammer et al., 2012, p.4) This European system is very similar to Agisoft Photoscan's image processing.

The other purpose of site planning is to identify potential geological hazards in the construction zone which can affect both the construction of the building and its useful life. One of the scenarios most affected by geography is the construction of rural properties, roads, bridges and tunnels. During the last decades, geomatics techniques have been widely used in the monitoring of natural hazards. In these, geological hazards such as landslides, debris flow, rockfall, etc. are established. Terrestrial and aerial laser scanning in conjunction with photogrammetry are the most widely used systems for monitoring these phenomena (Buill et al., 2016, p.1) It is precisely these landslides that cause accidents on roads and mining areas. They need to be identified, their risk potential assessed and safety protocols for both construction and operation planned. Photogrammetry-based data can be used to estimate data on landslide volume and frequency. Similarly, another potential risk is landslides and alluviums that can completely destroy an urban area located on the slopes of hills, hills and mountains. Another scenario is quarries for the extraction of construction materials, in this context it is never known how the extraction of materials can affect the stability of the terrain. The data collection method focuses on image capture with UAVs considering two different format sources such as images and video. The former provides more resolution, but the latter works faster and is more dynamic for interpretation. In the study of a rockslide, the objective is to evaluate the potential volume of the rupture scarp. An unstable rock mass detached from a slope can be a massive block or a set of intact blocks bounded by pre-existing discontinuities depending on the fracture pattern. A characterization of discontinuities can be performed with photogrammetric techniques in order to obtain a range of block sizes using In-Situ Block Size Distribution (IBSD). (Buill et al., 2016, p.2)

More conventional methods for evaluating originating sources of landslides required the physical presence of technicians to evaluate cracks and debris. Many times these procedures increased the instability of the ground affecting the viability of the construction. For this type of inspections, non-invasive procedures are necessary, which must be performed with sensors. The most widely used methodologies for slope monitoring are currently based on the massive capture of 3D information. Among the most commonly used methods are airborne and ground-based LIDAR systems, ground-based radar and aerial photogrammetry from unmanned platforms (UAVs). These methods should replace or complement other classical methods based on visual analysis of photographs, slope sampling grids or line scanning. Dense digital terrain model (DTM) (about 1 point per square meter) were obtained using airborne LiDAR systems (ALS). (Buill et al., 2016, p.2) As it is required to monitor small cracks it is necessary to establish the LIDAR system as the most accurate and back it up with imagery. In the case of discontinuity characterization, photogrammetric survey has been performed by capturing photos and videos with UAV, and from the ground (photo). According to this research the working area has been one of the facades of a quarry located in the Garraf region in Spain. The rock wall of the quarry is about 100 m long and 75 m high, currently presents several scars and cracks of geological interest, since rockfall occurs frequently. (Buill et al., 2016, p.2-3) Human inspection with cameras is ineffective in walls and gullies as all points are never reached. Even with 20-fold zoom approach it is not possible to obtain a good resolution of cracks located in high parts. This is why the UAV is ideal for monitoring at any height and from any distance from the target, and is less dangerous. 

"Photographic image or 4K video capture from the UAV platform proves advantageous compared to other geomatics techniques and can be used to track unstable slopes and assess damage after events along with monitoring the affected infrastructure. For the determination of accurate and high quality DTMs that allow the determination of crack families in rock massifs, it is necessary to use UAVs. These systems equipment allow high stability in all types of terrain and obtaining high resolution photographs,