**Abstract**

Digitizing is the way for a revolutionary approach in knowing, analyzing, continuous monitoring, and preserving the tangible immovable cultural heritage. The built cultural heritage requires the most performant means and techniques to acquire information indoor and outdoor. Drones are the best platforms for this purpose in terms of operating costs, data accuracy, and mission planning flexibility. In this chapter, we present a survey on the main applications of drones in the field of built cultural heritage analyzing the usability of this technology. Essential technical issues that are important for the operation and understanding of the use of drones in specific missions for the study of built heritage are also discussed.

**Keywords:** built cultural heritage, drones, digitization, aerial photogrammetry

### **1. Introduction**

Digitization has become an ongoing goal on the agenda of economic development and social transformation. It offers a broad perspective on the very near future of humanity embodied in current paradigms such as the web-driven economy, e-government, e-society, or e-communities that are based on digital democracy and promote it at the same time. Digitization is a priority all over the world and is seen as a strategy for the profound development of all sectors of human activity. One of the most relevant examples is the program promoted by the European Commission under the slogan "A Europe fit for the digital age", which guides how digital technology is changing people's lives by empowering people with a new generation of technologies [1]. Several concepts are now circulating such as big data and cloud computing and a number of technologies such as data mining, data analytics, data fusions, and deep learning are currently used and are constantly improving to keep up with the huge production of data in all fields.

Cultural heritage is a part of data production and has been contributing to the informational treasure of humanity for millennia. The digitization of cultural heritage is only a step in collecting and manipulating data with two major purposes: storage for information preservation, respectively data analysis for the study, and advanced research. A recent European Commission report on shaping Europe's digital future focuses on 3D digitization of cultural heritage [2]. This is a roadmap for the digitization of tangible cultural heritage that highlights that the integration of data obtained through different scanning techniques is the right approach for the future. Knowledge of technologies for transforming tangible heritage objectives into data by scanning across different spectral bands and dimensional measurements,

including software components for data analysis and presentation, is very important. The digitization of tangible cultural heritage is not only a fashionable technology but a tool that tends to become a standard for the collection, preservation, and dissemination efforts of arts and cultural heritage worldwide [3]. All in all, digitization is a necessity for a better knowledge and interpretation of things, so research becomes much more efficient using data instead of physical artifacts, especially in the case of tangible real estate. Sometimes access to the physical object is impossible, or very expensive, and then, a set of data captured with the right sensors is very useful. On the other hand, data become more democratic and thus can reach the general public through the media or virtual products in the service of knowledge of cultural heritage. In fact, through digitization, tangible cultural heritage becomes digital heritage, which is a subcategory of intangible cultural heritage.

In the last decade, drones have been used in many industries such as construction and infrastructure, agriculture, environmental monitoring, mining, GIS, and so on. For all these areas, drones provide imaging data of various types: single aerial pictures, thermal and multispectral images, stereoscopic images, video content, data from laser scanning, and remote sensing. A significant number of bibliographic sources report on drone technology and airborne sensors and their specific applications and services. Most case studies are presented even by professional drone manufacturers, and a wide collection of information can be found on their websites, for example, [4–6]. A recent report on the leading manufacturers of drone technology, including their applications, can be found in the reference [7].

At present, drones have begun to be part of the arsenal of means of investigating cultural heritage, offering the possibility to fly over and supervise heritage objectives from the air, with low operating costs. In principle, they offer photogrammetry services but the applications are open to possible remote exploration and sensing tasks in archeological sites, instead of humans. An extensive and recent synthesis of the use of drones in the service of cultural heritage, including examples of applications and case studies conducted around the world, can be found in [8].

#### **2. Drones and digitizing**

A drone is an unmanned aerial vehicle (UAV) that can be remotely controlled by a human operator in a specific area of action. This type of aircraft is an excellent platform for various scanning equipment, and sensors capable of transmitting acquired data in real time, as well as its current position. Drones can provide a wide range of services, but most applications include airborne surveillance and monitoring tasks. There are drones for military purposes and drones for civilian use, but we will discuss here drones with civilian applications.

The basic mission of drones in the service of cultural heritage is to scan various objects, artifacts, sets of objects, places built of cultural interest, and using different techniques for obtaining digital images.

#### **2.1 Photogrammetry**

Traditionally, aerial photogrammetry is the science and technology of obtaining reliable information about physical objects and the terrestrial environment through the process of recording, measuring, and interpreting photographic images captured from height. Currently, digitization has extended the field of photogrammetry to the analysis and processing of images based on mathematical and geometric models with softwareimplemented algorithms. Automatic image processing works with huge amounts of data that drones are able to provide by mobile scanning over areas of interest.

#### *Use of Drones for Digitization and Monitoring the Built Cultural Heritage: Indoor and Outdoor DOI: http://dx.doi.org/10.5772/intechopen.100346*

Aerial images can be processed and interpreted in different ways. One of the most used methods is a 3D reconstruction based on 2D images. This task defines particular uses of the drone in controlled overflight scenarios, which differ from one objective to another. Another method is orthophotography through which the objectives are mapped 2D, resulting in the digital map of the objective and the area flown over with the planimetry information. These methods include geometric models and algorithms for analytical geometry. Another category of methods aims at chromatics and image illumination, which involve extracting components and color ranges, estimating specular reflection, determining ambient lighting and its interaction with materials in order to render physical objects. This is where digital image analysis algorithms take place in the visible or multispectral domain. The combination of methods, for example, orthophotography with chromatic methods produces orthomosaic maps, and by the combination with multispectral data, various indexed maps are obtained based on normalized difference vegetation index (NDVI), optimized soil-adjusted vegetation index (OSAVI), chlorophile map, or processing CIR Composite (color infrared), or digital surface model (DSM).

#### **2.2 Laser scanning**

This is a technique for directly obtaining 3D images using laser radiation using LiDAR (laser imaging, detection, and ranging) devices. Unlike photogrammetry, which is a passive method of capturing images, LiDAR is an active method that involves laser emission in the NIR or UV spectrum. Mobile laser scanning is also beginning to be accessible to drones through aerial LiDAR equipment that has evolved to meet the requirements of weight, size, and performance. Laser scanning involves technical conditions and additional requirements to photogrammetry. Knowing the position of the drone as accurately as possible at all times is crucial for the quality of LiDAR data and therefore, these systems have integrated inertial navigation sensors with very high accuracy. Laser scanning has several definite advantages versus classical photogrammetry, but it cannot surpass resolution performance, image realism, data accuracy, and ultimately the cost of photogrammetry equipment. In the LiDAR technique for each scan radius (direction) only two parameters are obtained: flight time—which is directly proportional to the distance and intensity of the reflected radiation. With this information about each scanned point, a synthetic image of the objects is built respecting their geometry with a certain precision, while all the chromatic characteristics are conventionally chosen. However, some advantages prevail for laser scanning technology: It can operate at night, in an atmosphere with clouds and smoke, and can reconstruct more precisely the surfaces covered by vegetation. Also, the time required for post-processing LiDAR data is much shorter than when processing photo images. In various applications, LiDAR technology is used in addition to the classic photo-video technique.

The drone, as a system, is capable of providing raw imaging data for the abovementioned processing, while a suite of application software programs effectively performs the appropriate processing to extract the desired information. In fact, these are stand-alone software tools that perform advanced data processing including artificial intelligence techniques.

#### **2.3 Technical issues**

The configuration of drones for civilian use is of a VTOL (vertical take-off and landing)-type aircraft with fixed wings or the most popular with rotary wings. Here are the main component systems (subsystems) of a professional drone for civilian use:


Last but not the least, a special and vital component of drones is the software system that is distributed on both components: built-in drone, respectively on the portable remote control unit. The software component actually defines the drone's brain and its so-called intelligence, effectively ensuring all the processes for its proper functioning.

**Figure 1** shows an overview of the Mavic 2 Enterprise model, where the main subsystems can be identified. Full details can be found by accessing the official manufacturer's website available from: https://www.dji.com/mavic-2-enterprise/downloads

*Use of Drones for Digitization and Monitoring the Built Cultural Heritage: Indoor and Outdoor DOI: http://dx.doi.org/10.5772/intechopen.100346*

**Figure 1.** *Professional drone Mavic 2 Enterprise with native surveillance camera mounted in front of the gimbal joint.*

#### **2.4 Features and functional parameters**

Here, we will review the basic characteristics of drones and detail the functional parameters that are relevant to the tasks of digitizing the objectives of tangible cultural heritage.

We mainly distinguish between technical characteristics and operational characteristics, the latter depending largely on the former, and together, they determine the use class of the drone, its performance, and finally the purchase price on the market. First of all, we need to understand that drone performance is the result of a technical compromise that is reflected in their operational capabilities. Current technology manages to optimize this compromise by balancing power and speed requirements versus flight distance and height, weight and gauge versus air range (maximum flight time), data processing, and transmission capability versus sensor resolution.

In general, the mission of a drone is to acquire images with very good resolution from precisely defined and very well-controlled positions. In other words, drones must provide quality digital material for photogrammetry and image processing techniques. Thus, in addition to the general performance of maximum speed, maximum service ceiling above sea level, and maximum flight time, the following features are very important: hovering accuracy range, parameters of the camera, and gimbal of camera. In **Table 1** has given selectively these characteristics for a reference model—the Mavic 2 Enterprise drone.

Considerations related to the accuracy of data collected by drones are discussed in [9]. The quality of the images provided by a drone is described by three essential characteristics [10].


3.Spatial resolution or angular resolution describes the smallest details visible in the image. Unlike theoretical GSD, spatial resolution can be expressed in a different unit, which takes into account blur, image noise, contrast, and in general the effects of image processing: compression, denoising, edge clarity, etc. Spatial resolution is therefore a correct metric to quantify the ability to detect and characterize an object in the image. Spatial resolution is often expressed in "pairs of lines per millimeter." This unit is used to describe the spatial frequency of alternating black and white line patterns.


#### **Table 1.**

*Selected features of Mavic 2 Enterprise drone.*

*Use of Drones for Digitization and Monitoring the Built Cultural Heritage: Indoor and Outdoor DOI: http://dx.doi.org/10.5772/intechopen.100346*

Finally, another photometric parameter that influences image quality is the ISO exposure value at the image sensor. Under normal lighting conditions (daylight), the exposure value is set to the lower limit of the range values and vice versa, and at lower lighting levels, the exposure value is set above. However, a high ISO value of exposure produces image noise, and a long exposure time produces motion blur when the camera moves. This reduces the image quality, and eventually the ability to distinguish small details in the image.
