**3. Drones in the service of heritage**

#### **3.1 Applications and methods**

A survey of the latest applications of the use of mandrels for cultural heritage purposes reveals two aspects. First, there are various subdomains or particular purposes with concrete tasks where drones, as providers of digital content, prove their usefulness. Specific applications can be classified as follows:


Second, the review of recent literature reporting various applications of drones in the service of cultural heritage reveals the complementarity of several digitization technologies with that of drones, as well as strengths, weaknesses, and limitations of these technologies [11–14]. As the main technique for capturing images, traditional aerial photogrammetry has now become accessible through drones at

#### *Heritage - New Paradigm*

a very good performance-cost ratio. Photogrammetry and laser scanning are the basic techniques applicable by various methods with distinct equipment, but for the production of digital content of cultural heritage objectives, several scanning techniques are available. The main concepts frequently used in the digitization of the material cultural heritage are based on the following methods:


In specific applications for the material cultural heritage, there are some peculiarities that influence the scanning techniques used, as follows:


Interesting studies addressing the combined use of air and ground scanning technologies for cultural heritage objectives are reported in [13, 15].

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

## **3.2 Outdoor and indoor missions**

We have seen that drones can be used successfully for both outdoor and indoor photogrammetry and laser scanning operations. Most applications are outdoor missions for HBIM tasks but some indoor missions are suitable for drones, in concrete situations these being the only means that can make data acquisition at reasonable cost-effectiveness. In [16], it is presented a comparative study of digitization of land surfaces, photogrammetry versus laser scanning, conducted for four types of drones. These results are interesting and useful for professionals in the field of cultural heritage. A project reported in [17] focused on HBIM for Byzantine churches in Cyprus using exclusively low-altitude outdoor photogrammetry, provides methodological details, and results obtained with a drone equipped with a 20 MP camera. In Romania, there are some important cultural heritage objectives that are being investigated by photogrammetry with the help of a drone. One of them is the large architectural monument—the medieval castle named Corvin Castle, also known as Hunyadi Castle, in Hunedoara (**Figure 2**). The other is the Adamclisi Fortress in Dobrogea, which is an ancient Roman architectural complex, today in ruins (**Figure 3**).

These applications require the planning of particular flight missions with predefined itineraries for photogrammetric capture with different viewing angles on ground objectives. Usually, two gimbal angles are used for the camera: −90°, that is, vertical downward direction, called nadiral view, and oblique direction at −45°. Practically, a methodology and planning of photography are established for each objective. The goal is to best capture the elevation of objects.

The indoor missions in the field of cultural heritage are to complete the HBIM from inside when the TLS and other MLS methods are not applicable. Recent case studies with the use of drones for visual inspection in enclosed spaces such as mine galleries, cisterns, or sewers are reported in [18]. In the case of indoor scanning missions, the drone does not benefit from GNNS services, that is, GPS signal for positioning; however, piloting the drone is done in P (positioning) mode when the vision systems to locate and stabilize itself and obstacle sensing function is

**Figure 2.** *The Corvin castle in Hunedoara, Romania.*

**Figure 3.** *The Adamclisi Fortess ruins in Dobrogea, Romania.*

enabled. Other indoor scanning purposes require drones hovering over the artifacts in order to capture the best image possible. In these conditions, hovering accuracy is the feature that counts, and the best results are obtained by piloting the drone in T-mode (Tripod), which makes the aircraft more stable during the shooting. An example for this use case is the inspection of the roman mosaic arts in Constanta during the expertise for restoration. This is the subject of nondestructive analysis by evaluation of the morphological and chromatic characteristics that represent suitable metrics for making decisions based on image processing [19]. **Figure 4** presents this artifact in the present condition of conservation. For a reliable analysis, quality imaging data obtained by correct photogrammetry techniques are required. Thus, for correct analysis, the images of the mosaic, as a primary source of data, must meet certain conditions from the acquisition phase, as follows: (i) to be taken orthographic shots, (ii) to be captured under uniform lighting conditions, without shadows, reflections, etc., (iii) to be taken from the same height (constant distance) for the entire interest surface, and (iv) the resolution must be as high as possible. In general, the photogrammetric method is sufficient for the inspection of artifacts such as flat decorative surfaces, so that 2D orthogonal images obtained by single shots provide all the planimetry and color information necessary for morphological and chromatic analysis. Using CRP with SfM techniques, it is possible to obtain details for DTM by estimating the deformations of the mosaic surface, the degree of degradation by erosion, and the lack of mosaic elements or the degree of intervention by adding material. ALS is not an option for scanning the decorative mosaic because an acceptable value of the GSD parameter cannot be achieved. Also, due to

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

**Figure 5.** *Different images of the artifact.*

the restriction of access on the surface of the mosaic, scanning by terrestrial means is not possible in this case. In **Figure 5**, it can be seen two shots taken manually at the arbitrary angle but also the effect of non-uniform environmental lighting.

#### **4. Conclusions**

Professional drones are actually considered UAS (Unmanned Aerial Systems), which means more than an unmanned aerial vehicle. They are equipped with specific scanning systems that define their role and operational functions. The drone is a sufficiently stable platform for close-range photogrammetry (CRP) missions and is an excellent indoor scanning device due to its small size, good maneuverability, and flight qualities. We see great potential for the use of drones for interior photogrammetry on decorative artifacts where the information of interest concerns their planimetry and chromatics. The ease of use of airborne cameras in terms of gimbal stabilizer-controlled mobility, controlled focusing, and exposure function combined with the drone's ability to hover at a short distance from the artifact gives drones high versatility for digital image acquisition. By using the auto exposure bracketing (AEB) function, for example, the camera can take several successive photos (usually three) with slightly different settings. Then, the images can be combined automatically, for example, in a single image with a high dynamic level or can be stored separately, so that the images with the most suitable appearance can be later taken from the batch.

Regarding the digitization of cultural heritage objectives, the main data are obtained through photogrammetric techniques, which in most cases cannot be exceeded in terms of accuracy and amount of data provided by LiDAR techniques. Moreover, the chromatic analysis of images can be performed exclusively by photographic techniques. Laser scanning techniques have several strengths that make them rather useful as complementary methods in digitizing cultural heritage objectives. Thus, the ALS technique generally helps in the case of infrastructures covered with vegetation and in the case of noisy photographic images when the estimation of the 3D model would be deficient.

Professional drones are becoming increasingly affordable handy tools for use in the field of material cultural heritage.
