3D geometric survey of cultural heritage by UAV in inaccessible coastal or shallow aquatic environments

RESEARCH ARTICLE 3D geometric survey of cultural heritage by UAV in inaccessible coastal or shallow aquatic environments Mariluz Gil-Docampo 1 | Sim on Peña-Villasenín 1,2 | Ana M. S. Bettencourt 2 | Juan Ortiz-Sanz 1 | Sara Peraleda-Vázquez 1 1 Agroforestry Engineering Department, University of Santiago de Compostela, Higher Polytechnic School of Engineering, Lugo, Spain 2 Landscapes, Heritage and Territory Laboratory (Lab2PT); History Department, University of Minho, Braga, Portugal Correspondence Sim on Peña-Villasenín, Agroforestry Engineering Department, University of Santiago de Compostela, Higher Polytechnic School of Engineering, Lugo, Spain. Email: [email protected] Funding information Xunta de Galicia Abstract Cultural heritage in coastal or shallow aquatic environments is often located in areas where access is difficult or where accurate survey and documentation may not always be possible with terrestrial or aquatic equipment. The combination of photogrammetry and unmanned aerial vehicles (UAVs) generates a range of possibilities across multiple sectors, including history, ethnography and cultural heritage studies. Additionally, these methods can be used to prospect new archaeological sites. This article presents three case studies that use UAV techniques and Structure from Motion and Multiview Stereo (SfM-MVS) photogrammetry to conduct topographic and geometric registrations of archaeological, historical and ethnographic sites (some of which are classified as cultural heritage sites). These examples are located in coastal or shallow aquatic environments that are difficult to survey with traditional methods. The results show that it is possible to carry out detailed geometric registration and heritage prospection over large coastal or shallow aquatic environments using a low-cost UAV. Furthermore, the results of this work show great advantages in terms of cost and quality, even in cases where the seabed is below a shallow water column. Other particularities of SfM-MVS application in aquatic environments are discussed. From an interdisciplinary perspective, this methodology will offer new possibilities for the study, restoration and conservation of archaeological, historical and ethnographic monuments. KEYWORDS 3D modelling, aerial prospecting, close-range photogrammetry, cultural heritage 1|INTRODUCTION A thorough examination of the visual, spatial and topographic records is critical for ensuring the accurate study, conservation and dissemination of cultural heritage archived within coastal or shallow aquatic environments. The traditional methodologies used for these examinations have been partially replaced in recent decades by large data acquisition techniques such as terrestrial laser scanning (TLS) (Brumana et al., 2014; Guarnieri et al., 2013) or Structure from Motion and Multiview Stereo (SfM-MVS) photogrammetry (Ortiz-Sanz et al., 2013; Santagati et al., 2013). These technologies allow for the generation of a high-resolution, 3D photorealistic depiction of a Received: 28 October 2022 Revised: 10 April 2023 Accepted: 22 April 2023 DOI: 10.1002/arp.1901 This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. © 2023 The Authors. Archaeological Prospection published by John Wiley & Sons Ltd. Archaeological Prospection. 2023;1–16. wileyonlinelibrary.com/journal/arp 1 location or feature and provide more complete and rigorous geometric documentation of heritage remains than can otherwise be obtained by traditional techniques. Additionally, these methods favour greater objectivity in the scientific-technical study and are a more attractive form of disseminating cultural heritage to the general public while also providing for more agile and efficient surveying (Aicardi et al., 2018; Dong et al., 2020). Although both TLS and SfM-MVS photogrammetry are the primary techniques used for 3D heritage registration, SfMMVS has become more commonly used than TLS due to its low cost and feasibility and shows great potential in a variety of tasks related to the geometric registration of cultural heritage sites (Peña-Villasenín et al., 2017,2020). Photogrammetry has a long history and has undergone important advancements with the introduction of SfM-MVS approaches (Remondino & El-Hakim, 2006; Snavely et al., 2006; Westoby et al., 2012). These methodologies are even more relevant in studies of rock art due to their noninvasive nature and the possibility of applying 3D rendering posttreatments, which allow documenting these heritage elements in an objective way (Gil-Docampo et al., 2019; Peña-Villasenín et al., 2019; Santos-Estevez et al., 2020). 1.1 |SfM-MVS photogrammetry The SfM-MVS process is divided into three main phases. First, an internal and external orientation of the images is performed. During this step, camera position and scene geometry are reconstructed simultaneously through the automatic identification of matching features by the scale invariant feature transform (SIFT) method (Chandran et al., 1997; Lowe, 1999; Lowe, 2004). These features enable initial estimations of camera positions and object coordinates, which are then iteratively refined using nonlinear least-squares minimization. The sparse bundle-adjustment system ‘Bundler’(Snavely et al., 2008) is used for this task. Second, an enhanced-density point cloud and a 3D mesh can be derived using the clustering view for multiview stereo (CMVS) algorithm and the patch-based multiview stereo (PMVS2) algorithm (Furukawa et al., 2010; Furukawa & Ponce, 2010). Once the densified point cloud is obtained, it can be used to generate 3D models, digital elevation models (DEMs), nadiral and side view orthomosaics and planimetric vectorizations of the study elements. Regarding photographic capture, numerous studies and documentation from photogrammetric software developers provide standardized guidance on the spatial distributions of photographic capture for SfM-MVS photogrammetry (Agüera-Vega et al., 2017; Martínez et al., 2013; Pepe & Costantino, 2020; Remondino et al., 2011; Remondino & El-Hakim, 2006). 1.2 |Documenting aquatic cultural heritage with UAVs and SfM-MVS It is often difficult to access heritage sites in coastal or shallow aquatic environments. Photogrammetry has shown great potential for registering a variety of cultural heritage elements. However, in inaccessible coastal or shallow aquatic environments, its application is difficult, and the archaeological prospection and geometric and topographic registration may be complicated by issues of accessibility. Therefore, geometric image registration is not always possible despite access to terrestrial or aquatic equipment, which results in an incomplete study. Simultaneous with the evolution of photogrammetry, there was an expansion in the civil applications of unmanned aerial vehicles (UAVs). Its ability to carry a wide variety of sensors allows multiple applications, such as acquiring images at low altitudes for 3D modelling of heritage sites or obtaining a geometric or topographic terrain record (Heincke et al., 2019; Themistocleous, 2020). The ease of use and quality of the results obtained currently make UAVs a common tool in cultural heritage research. SfM-MVS photogrammetry and UAVs generate a range of possibilities that, in many cases, remain to be explored. UAVs can be used for taking photographs in inaccessible areas or for aerial prospection over large terrains. The combination of SfM-MVS photogrammetry and UAVs can reduce the problems associated with prospecting and registering cultural features archived in inaccessible coastal or shallow aquatic environments. The development and optimization of work protocols with SfMMVS photogrammetry can have a considerable social and scientific impact because these techniques improve the quantity and quality of heritage records in inaccessible areas. The main objective of this study is to show how the integration of UAVs and SfM-MVS photogrammetry is capable of prospecting and geometrically documenting various examples of historic, ethnographic and rock art sites located in places that are difficult to access, such as shallow aquatic environments, intertidal zones or coastal areas. The case studies involve three areas of interest: (1) the remains of salt flats and associated tide mills; (2) the archaeological remains of a Roman bridge with evidence of fishery activities and weirs; and (3) coastal rock art. 2|MATERIALS AND METHODS This section describes the case studies and the main methodologies and equipment used for the field survey, processing work, and the exporting and publishing of the results. 2.1 |Study cases All case studies are located in the northwestern Iberian Peninsula (Figure 1). The chosen locations encompass several common heritage typologies located in coastal, intertidal or shallow aquatic environments. The main characteristics of the three case studies are presented below. 2.1.1 | Case 1. Tide mill and Ull o saline, Vilaboa, Spain Case 1 (Figure 2a) was a salt mining area with a tide mill and a long history of occupation. It is located in the innermost area of the Vigo 2GIL-DOCAMPO ET AL. 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Ría (a type of fiord) in Pontevedra, Spain. Located in the intertidal zone of the Ull o marshes, a location favourable for this type of structure, the remains of a tide mill are preserved, as well as the currently named Ull o saline. The salt flats were built in 1637 during the reign of Felipe IV (Méndez et al., 2000) and were abandoned in the 18th century when some of them were converted for agrarian production. Remains of this complex, including a dike with a north–south orientation that isolates the salt flats from the sea (Calo-Lourido, 1997), are still present on the landscape. The tide mill was built in the 19th century to take advantage of the tidal forces caused by the salt dam. Currently, only the foundations of the dike on which the complex was built remain, but these vestiges are important due to the few available examples of these features. The cultural complex, together with the marsh, which is a natural habitat for birds, constitutes a region of incalculable ecological and cultural value. The area was recovered as a walking area in 2006. The works are included in the ‘Salinas do Ull o’project, which promotes cooperation for the transfer of scientific knowledge regarding cultural landscapes. The intent is to enhance the value of the Vilaboa Salt landscapes as a heritage asset, although the project itself was conceived as a way to leverage cultural resources to promote sustainable and quality tourism. Furthermore, the project is intended to reinforce the maritime identity and social cohesion of the coastal communities. Among the most important actions taken to preserve the value of this heritage site is to develop a graphic and geometric record of the feature and monitor its conservation status. For this purpose, UAVs conducted surveys of the intertidal zone and the salt flats, and a general 3D record of the tide mill was developed. 2.1.2 | Case 2. Portomarín Fisheries, Portomarín, Spain Fisheries may be small dikes or dams built across the course of a river. They may be Vor W-shaped and are positioned where people fish or channel water to fish. Fisheries are a traditional hydraulic construction, are sometimes complex with weirs, and are generally located in a shallow place or a rocky outcrop where it is possible to fish for lamprey, salmon and other fish species. Minho fisheries were documented in the 12th century, but some of them probably date back to Roman times (Suárez, 2017). They are found in all Galician rivers, especially the Minho River, and have multiple architectural variants. In 1908, there were approximately 700 of these fisheries throughout only 25 km of the Low Minho River basin (Castro Fernández & L opez Facal, 2019; Domínguez, 2011; Suárez, 2017). The architectural details of this type of construction are an essential element of the history of this river. An ethnographic study was carried out to document these features, which included a highresolution geometric registration of 3.2 km of the Minho River near the old town of Portomarín, which was flooded after the construction of the Belesar Reservoir in 1963 (Figure 2b). In addition to multiple fisheries in the area, there are different hydraulic constructions (a mill and power plant) and the remains of an ancient Roman bridge (García et al., 2013; Suárez, 2017). In this case, UAV surveys of the course of the river will be performed during the time of the lowest flow to record the largest possible area. This is in addition to obtaining 3D records and detailed plans of the state of conservation of the existing heritage structures along a 3.2-km stretch that contains fisheries, weirs or archaeological remains. 2.1.3 | Case 3. Fornelos engraving. Viana do Castelo, Portugal Case 3 was a rock art engraving located in the parish of Carreço, Viana do Castelo, Portugal, in an area along the Atlantic coastline. The engraving was found on the pocket beach of Fornelos, on a coastal cliff that is difficult to access (Figure 2c). The first ‘sketch’of the carvings was performed by Lanhas (1969), but the first tracing FIGURE 1 (a) Iberian Peninsula. (b) Galicia and NW Portugal. [Colour figure can be viewed at wileyonlinelibrary.com] GIL-DOCAMPO ET AL.3 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License of the carvings was not made until the 1980s, which were considered to have an affinity with the schematic and semi-naturalistic paintings (Baptista, 1986; Baptista & Magalh˜aes, 1985); interpretations were also made by Bradley and Fábregas-Valcarce (1998) and Bradley and Valcarce (1999). New records were subsequently made (Bettencourt, Abad-Vidal, & Rodrigues, 2017; Bettencourt, Silva, et al., 2017), which divided the motifs into three different panels. These panels described various equines, horsemen, a dog and an anthropomorph. Considering these motifs and the interaction between the engraving and the features of the outcrop, it is likely that the images tell a real or mythical narrative. Previous researchers also defended the interpretation that the space chosen for the carving of these images, the orientation of the motifs, the topography of the outcrop and its colouration create an impressive scenic effect for the audience (Bettencourt, Santos Estevez, et al., 2017; Bettencourt, Silva, et al., 2017). Based on this new interpretation, and because the engravings are increasingly eroded by wind and wave action and are in danger of disappearing, it became necessary to conduct a 3D survey with UAVs and SfM-MVS photogrammetry of the entire south face of the outcrop to better understand the relationship between the motifs and their physical characteristics. The place was classified as Property of Public Interest in the Official Gazette of Portugal by Decree No. 26-A/92, DR, 1st series-B, No. 126 on 1 June 1992, and currently, the location is part of a hiking route, PR7, and is the responsibility of the Municipality of Viana do Castelo. FIGURE 2 (a) Case 1. Ull o saline. (b) Case 2. Portomarín fisheries. (c) Case 3. Fornelos engraving. [Colour figure can be viewed at wileyonlinelibrary.com] 4GIL-DOCAMPO ET AL. 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License 2.2 |Field work The standard field work for 3D modelling of heritage elements using both aerial and terrestrial photogrammetry consists of two basic tasks. First, an orientation and/or scaling system is established; and second, the photography is planned and executed. In regard to setting up a scaling system, there are two main methods. Scaling of the scene can be accomplished through reference measurements or by establishing a network of ground control points (GCPs) with known coordinates, which also allows the model to be oriented along the XYZ axes. In the case of scaling by reference, this orientation must also be done from a reference such as a plumb line or level for the Z axis to be properly oriented, and a compass is needed to orient the XY axes (Ortiz-Sanz et al., 2010). In inaccessible coastal or river areas, it is not always possible to establish these necessary references, which entails one of the main difficulties in the geometric registration of cultural heritage located in this type of area. In all cases, a DJI Phantom IV real-time kinematic (RTK) UAV with a maximum take-off mass (MTOM) of 1.39 kg, a 20-Mpx RGB sensor and a precise positioning module, which ensures centimetric accuracy in image positioning, were used. For flight planning, the GS RTK software that DJI provides with the Phantom IV RTK has been used. The recommended longitudinal and vertical overlaps of 80% and 70%, respectively, were applied. For the measurement of the GCPs, a Stonex S900T GPS RTK was used. Some of the measured points were used as check points (CPs) for independent geometric control (Table 1). Although the techniques and equipment used are common in all the study cases, some particulars are presented below. 2.2.1 | Field work in Case 1 This study area covers an area of 28 ha. For the cartographic and topographic survey, a standard photogrammetric flight was carried out at a height of 100 m, which ensures a ground sampling distance (GSD) of 2.75 cm. Figure 3a,b shows photos from these flights. On the other hand, for the survey of linear cultural features, such as the walls of the tide mill dam, oblique flights were made along both sides TABLE 1 Field equipment costs. Field equipment Cost ($) Case Phantom 4 RTK 5000 All cases Stonex GPS S900T 5000 Cases 1 and 2 Stonex TS R2 W PLUS 3500 Case 2 FIGURE 3 (a) Saline nadiral photo. (b) Tide mill nadiral photo. (c) Tide mill oblique photo. [Colour figure can be viewed at wileyonlinelibrary. com] GIL-DOCAMPO ET AL.5 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License of the element to model in detail the vertical walls and construction elements. For this, the camera was configured in automatic shooting mode every 3 s with manual piloting and maintained a constant speed of between 2 and 3 m/s, thus obtaining sufficient overlap. In Figure 3c, an example of an oblique image is shown. Regarding the measurement of GCPs, note that both the sides of the study area and the salt mine dam are easily accessible on foot, so it was possible to establish a network of GCPs with sufficient coverage. 2.2.2 | Field work in Case 2 The study occupies an area of 170 ha. As in the previous case, a standard photogrammetric flight at a height of 120 m, which ensures a GSD of 3.3 cm, was carried out for cartographic and topographic surveys (Figure 4a). For the survey of linear cultural features, as in the previous case, manual oblique flights were conducted with an imaging interval of 3 s. Because of the amplitude of the study area and the large number of survey elements that were often not accessible, it was necessary to carry out flights beyond visual line of sight (BVLOS). First, to locate the buildings (Figure 4d), and second, to obtain enough images to allow a detailed geometric record of specific buildings such as fisheries or mills (Figure 4b,c,e). To do this, several passes were planned at different heights around the elements of interest. In this case, it was difficult to establish the GCP network due to the presence of large, inaccessible areas. To reinforce the georeferencing of the construction elements, a Stonex R2W Plus total station (TS) was used to collect laser measurements, which FIGURE 4 (a) Nadiral photo. (b–e) Oblique photos. [Colour figure can be viewed at wileyonlinelibrary.com] 6GIL-DOCAMPO ET AL. 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License made it possible to measure the coordinates of various points along these inaccessible features. The rest of the GCPs were measured with the Stonex S900T GPS. 2.2.3 | Field work in Case 3 The third case study relates to the use of a UAV to photograph a recorded outcrop to elaborate its photogrammetry. Figure 5a shows a general photo of the outcrop, and Figure 5b shows a detailed photo of the engraved panels. The flight was carried out in manual mode without planning for autonomous flight, and a photographic arrangement was applied that was similar to those used with terrestrial cameras often used in this type of work (Gil-Docampo et al., 2019). 2.3 |Processing, export and publication For 3D model processing, Agisoft Metashape (Agisoft LLC, Russia) photogrammetric software was used, whereas the enhancement and rendering of the engraving were made possible with MeshLab (Visual Computing Lab, ISTI - CNR, Pisa, Italy). In all cases, the three main steps of the SfM-MVS process were carried out. First, the internal and external orientations of the images were determined. Second, an enhanced-density point cloud was obtained. Both processing phases were carried out at high quality so that the software could use the original image resolution for the process. This technique guarantees the best precision for the photogrammetric adjustment of the cameras and the reconstruction of the point cloud. In both cases, the zenithal and oblique images were processed in individual chunks. Then, 3D models, DEMs, nadiral and side-view orthomosaics, and planimetric vectorizations were merged. The latest versions of Agisoft Metashape incorporate a new mesh generation mode based on depth maps instead of processing from the point cloud (Verhoeven et al., 2021). This mode was used FIGURE 5 (a) General photo of the outcrop. (b) Detailed photo of the engraved panels. [Colour figure can be viewed at wileyonlinelibrary.com] TABLE 2 Processing costs. Software Cost ($) Agisoft Metashape 4000 MeshLab Free Workstation Intel(R) Core (TM) i9-9960X CPU @ 3.10 GHz NVIDIA GeForce RTX 2080 Ti 4000 GIL-DOCAMPO ET AL.7 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License FIGURE 6 Case 1 (a) Location of the photographic shots. (b) Location of ground control points (GCPs). [Colour figure can be viewed at wileyonlinelibrary.com] FIGURE 7 Orthophoto of the salt water inlet and outlet area. [Colour figure can be viewed at wileyonlinelibrary.com] 8GIL-DOCAMPO ET AL. 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License to process oblique images and minimize the noise in areas with water. The same approach was used in Case 3, which consists only of oblique photos, to obtain a better definition from the engraving. Table 2shows the cost of the software and hardware used to process the three cases. To publish the results on the SketchFab platform (which actually limits model uploads to 100 MB with the free subscription), the size of the mesh must be reduced. Subsequently, and in all cases, the mesh was exported in Wavefront (obj) format, and the texture file was exported in JPG format and uploaded to the online platform in compressed ZIP format. On the SketchFab platform, an enhancement render was configured to improve the visualization of the results. 3|RESULTS The results generated in the three case studies are shown below. 3.1 |Case 1. Tide mill and Ull o saline. In Case 1, a total of 3440 images were obtained, including 846 nadiral and 2594 oblique photos. In this case, 76 points were measured, of which 67 were used as GCPs and 9 as CPs. The work lasted 8 h and was conducted by two people. Figure 6shows the location of the photographic shots (Figure 6a) and the GCPs (Figure 6b). FIGURE 8 Contours (20 cm) in areas with the presence of water on orthophotos and digital elevation models (DEMs). (a) Area with good results. (b) Area with rectifiable errors. (c) Area with invalid results. [Colour figure can be viewed at wileyonlinelibrary.com] GIL-DOCAMPO ET AL.9 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. 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Archaeological Prospection,1–16. https://doi.org/10.1002/arp. 1901 16 GIL-DOCAMPO ET AL. 10990763, 0, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/arp.1901 by Universidade de Santiago de Compostela, Wiley Online Library on [20/06/2023]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License