1 Introduction
Archaeological excavations are often irreversibly destructive, so it is important to accompany them with detailed documentation reflecting the accumulated knowledge of the excavation site.1 Graphical representations of archaeological sites such as drawings, sketches, photographs, topography, and photogrammetry are indispensable for such documentation and are an essential part of an archaeological survey. Photogrammetry has been extensively used in heritage documentation in many countries; it offers a rapid, accurate method of acquiring three-dimensional information, especially for large complex sites or objects, with relatively little time required in the field for data acquisition. With photogrammetry, we can obtain accurate 3D metric and descriptive object information from multiple digital images.2 These photos contain information about surface details of the site and can provide information on the condition of a monument, before, during, and after excavation and restoration, which is difficult to achieve by graphic documentation alone.
In campaigns in Alexandria, Egypt, undertaken from 2014–2016 we used several techniques for the first time to document archaeological sites or objects using photogrammetry either on land or underwater in collaboration with Centre d’Études Alexandrines (CEALex) and the Egyptian Ministry of Antiquities. In this paper we present our results in underwater and terrestrial archaeological photogrammetry, based on practical experience and using low-cost materials. We shall explain some examples of this documentation, beginning with a study in which underwater photogrammetry methods and digital modeling processing techniques were used to obtain a three-dimensional georeferenced digital surface model (DSM) for the submerged site of the lighthouse of Alexandria. Secondly, we shall see how photogrammetry with the aid of computer graphics was an important tool to perform the virtual restoration of a broken colossal female statue. Finally, we demonstrate the use of 3D scanning data sets to reveal an invisible inscription on a deteriorated archaeological object.
2 The Submerged Site of the Lighthouse of Alexandria
The underwater site of the lighthouse of Alexandria, lies to the east of Qaitbay Fort in the open sea. It is affected by environmental factors, mainly waves, current, and sewage discharge which changes the water quality. In addition, the prevailing wind is N-NW, but is variable from December to May.3 The rough sea, current, and visibility rarely exceeding three meters provide substantial challenges to, but also valuable experience in, site documentation by photogrammetry.4 In 1994, CEAlex launched the first scientific excavation in the field of underwater archaeology in Egypt on the submerged site of the lighthouse of Alexandria, well known as one of the Seven Wonders of the Ancient World. This is situated on the eastern extremity of the ancient island of Pharos at the foot of Qaitbay Fort, which was constructed at the end of 15th century by the Mamluke Sultan Qaitbay. The submerged site was discovered in 1960 thanks to the pioneering work of Kamel Abou Elsaadat.5
Figure 2.1
Fragments of a female colossal statue discovered by Kamel Abou Elsaadat and raised from the sea in 1962 at the foot of the Qaitbay Fort
Mohamed Elsayed
In 1968, Honor Frost undertook a UNESCO mission (Figure 2.2) on the site which led to the publication of a preliminary report revealing the importance of the site.6
Figure 2.2
Sketches produced by Honor Frost, during the UNESCO mission in 1968 (after Frost 1975, 129)
The underwater site of Qaitbay Fort holds the ruins of the lighthouse of Alexandria, which stood for almost seventeen centuries. It was built towards the beginning of the 3rd century BCE and was accessible until the end of the 14th century. The last mention of the visible presence of its ruins dates to 1420, almost 60 years before the construction of Qaitbay Fort.7
3 Excavations and Documentation
Underwater excavations began in 1994 under the direction of Jean Yves Empereur,8 director of CEAlex, in the hope of shedding new light on the question of the appearance of Alexandria’s lighthouse. These excavations led to the reconstruction of certain parts of the lighthouse and to an understanding of its design process. Nevertheless, the study of the site ran up against the limitations of traditional data recording methods. The extent and unique nature of this sunken site encouraged innovation in data gathering, both as regards the ancient material of more than 3,525 blocks (Figure 2.3) and the overall site itself, whose size and uneven contours make any analysis complicated to say the least.9
Figure 2.3
Plan of the underwater archaeological site of Qaitbay Fort (CEAlex)
In 2009/2010, a 3D photogrammetry data-gathering program was launched, particularly focusing on the broken statues that surrounded the lighthouse.10 The research and development department of EDF (Electricité De France) had begun this work in 1998, and it was continued between 2009 and 2012 as part of the AR-Search program (Agence National de la Recherche). In 2012–2013 the aim of the campaign was to continue the virtual restoration work on the pair of colossal royal statues that once stood next to the lighthouse. This work demonstrated that photogrammetry was the preferred solution for rapidly providing quality “digital duplicates” of either sunken or lifted objects.11 With around 30 blocks removed from the sea and more than 3,000 pieces still underwater—some weighing almost 40 tons—this was an important step forward.
From 2013, with the support of the Honor Frost Foundation (HFF), the gathering and processing of photographic data in this research were improved, which led to substantial development in data gathering techniques and the consequent improvement in the quality of results.12 Fully manual photographic data acquisition methods were applied, using a digital single-lens reflex camera and low-cost materials (Figure 2.4) to develop a digital surface model (DSM) of the entirety of Qaitbay underwater site, which exceeds 13,000 square meters. One of the objectives of this project was to use photogrammetry to create a digital duplicate of the submerged site.
Figure 2.4
Equipment used for photogrammetry on the Pharos site
Mohamed Elsayed and Yasser Galal
4 Methods and Techniques of Data Acquisition for the Digital Surface Model
Any underwater archaeological surveying technique must satisfy two competing requirements: speed and accuracy. While many different photogrammetric techniques have been used on underwater archaeological sites around the Mediterranean, it is often difficult to select one that will be suitable for a particular site. In our case the underwater archaeological site of Qaitbay was particularly difficult to record, not only because of the conditions of the sea and the weather, but also because of its uneven, rocky bottom and the variation of the depth in the site, from two to nine meters. Therefore, using heavy materials such as a metal photo tower or frames installed upon the site was impossible. Additionally, use of more modern equipment such as side-scan sonar, autonomous underwater vehicles (AUV s) or remotely-operated vehicles (ROV s) was not promising, due to both cost and the rocky bottom and shallow waters in some areas of the site. Therefore, methods and techniques of data acquisition had to be adapted to the weather condition and the topography of the site. Our technique for preparing the site for photography and topography was completely manual, using simple and cheap materials including a large-scale bar-pole, measuring tapes, buoys, ropes, tags and a compass (Figure 2.4).
The first step towards creating an accurate three-dimensional model of the site was to take a series of good photos using a high-quality camera. However, shadows can cause errors, with picture information lost, especially underwater. Therefore, the most important thing for the precision and the accuracy of the work when using manual methods was the diver’s competence in orientating himself underwater and in respecting the level of the flight plan, which could not exceed three meters above the site. The data capture method was generally inspired by aerial photogrammetry. The photographic work on the site was carried out by Mohamed Elsayed (underwater archaeologist—Ministry of Antiquities) using a Nikon D700 DSLR camera (full frame, 12.9 megapixel resolution) with a fixed 24 mm lens, in a hemispheric dome housing (Figure 2.5). This allowed for a wide coverage area and close proximity to the subject, especially in low visibility.
All photos were taken in manual mode without flash whatever the sea conditions. F-stop and shutter speed varied according to underwater visibility and illumination, either f/7.6 with a shutter-speed of 1/60, or f/5.6 and a shutter-speed 1/40, depending on the condition of the water itself. From 2014 until 2016, about 26 weeks were dedicated to the photographic survey, and eventually 50,152 photos (Figure 2.11) were used to create an orthophoto representing a part of a DSM covering an area of 7,200 square meters of the total 13,000 square meters of the site (Figure 2.10).
Figure 2.5
Underwater photogrammetry data acquisition by Mohamed Elsayed
photo by Philippe Soubias, processing by Mohamed Abdelaziz
The flight plan method developed for the Qaitbay site required a period of experimentation to manage the constraints of the underwater context, including the troublesome problems of visibility on the site. Further difficulty was occasioned by the large surface area (1.3 hectares) and the size of certain blocks, requiring the combining of several methods of shooting. For these reasons, it was impossible to gather data in order to process the entire site at one time. The data capture method was adapted to respond to these constraints, and so several methods were applied during the missions of 2014, 2015 and 2016. In order to increase the area covered by photography in previous missions we reduced the materials used in the operation to save preparation time for each zone. The flight plans of the initial experiences in 2014 served as the basis of future work, with the diver moving along lines attached to a rod, doubling back and forth after 20 meters of continuous shooting to capture the images that were to be processed into the surface model. At the end of the lane, the photographer would turn around and cover another swath while maintaining the overlap.
The diver maintained his path by visual orientation, checking his trajectory against the features of the seabed, the many ancient blocks, the uneven surface (Figures 2.6 and 2.7) and scales placed on the bottom. Measuring tapes and buoys marked the boundaries of the area to be covered during the flight plan.
The major difficulty encountered in the topography of the underwater site lay in the significant variation in the elevation of the seabed, and in the varying weather conditions: small or large swell, sunny weather, and sometimes overcast, but always with sufficient brightness. In some areas, there is a change in depth from eight meters to four meters moving northwards, and from nine meters to five meters while following a south-east direction. The photos taken during these flights were always close to an angle of 45 degrees (Figure 2.5), thus capturing the rugged topography of the site surface. The diver had to move in as straight a line as possible, and at a constant speed in order to ensure sharp images; he was instructed not to exceed a height of three meters from the bottom, and was authorized to approach as close as 1.65 meters above the seabed in conditions of bad visibility. The flight plans required a forward overlap of 70–80 percent and lateral or side overlap of the same order (Figure 2.6), which is superior to conventional aerial photography. This overlap rate reduced mistaken pairing by increasing the number of matching images within the software. The shots were taken in different weather conditions. In 2015 and 2016, the area covered was approximately 7,200 square meters.
Figure 2.6
Orthophoto and the flight plan
photos by Ashraf Husein and Mohamed Elsayed, processing by Mohamed Abdelaziz
Figure 2.7
Diver’s flight-paths above the seabed, Alexandria Harbor
processing by Mohamed Abdelaziz
5 Data Processing
At the beginning of the project in 2014 the available computer for data processing had limited capability: RAM—16G/GPU—GT630, 2G/CPU—Intel core i3. The final aim of photo processing with photogrammetry software was to build an orthophoto and a textured 3D model. The first step was the preprocessing of the images, which were dominated by blue/green and yellow color because of the turbidity of the water, low visibility and lack of light. These photos could not be used without prior enhancement; image processing was necessary for color correction, contrast, removing shadows and reducing highlights in order to get more matching points between pairs of images (Figure 2.8).
For this we used Adobe Lightroom and Photoshop for editing and for batch processing. All photos were captured in a RAW format and then converted into JPG files to be accepted by the photogrammetry software Agisoft Metashape 1.5 (formerly Photoscan). Metashape generates an accurate 3D model from images which are converted into a textured 3D model in four straightforward processing steps, namely: 1) Aligning Photos, 2) Building Dense Cloud, 3) Building Mesh, 4) Building Texture.13 In order to improve the results, some manual interventions were necessary, including image masking and deletion of erroneous points. Beginning by loading uncalibrated overlapping images into the software, the camera auto-calibration runs automatically through mathematical algorithms. The software detects and then tracks how many points move throughout the series of images. Even the images that are not calibrated will be automatically computed inside the photogrammetry software.14
Figure 2.8
Images processing
processing by Mohamed Abdelaziz
6 Orthophoto and Digital Elevation Model (DEM)
After finishing all the processing, we produced the orthophoto (Figure 2.9) and digital elevation model (DEM) (Figure 2.12), which can be obtained with Metashape in any projection required.15 From this, we can extract a plan, sections, volumes and elevation.16 The orthophoto, which was assembled from raw images, had an average pixel size varying between 0.64 mm and 0.50 mm/pixel because of the height variation of the camera and the surface of the seabed (Figure 2.8).
Between the years 2014 and 2016 a massive number of photos, about 50,152, were used to generate the final 3D model/orthophoto covering 7,200 square meters of the submerged site of Qaitbay (Figures 2.9 and 2.10).
Figure 2.9
Orthophoto created from 50,152 images
processing by Mohamed Abdelaziz
Figure 2.10
3D model/orthophoto covering 7,200 square meters of the submerged site of Qaitbay, with additional areas sketched in
processing by Mohamed Abdelaziz
Because the characteristics of the PC used in processing, as mentioned above, Metashape could not handle all of the photos at once. The best solution was to process a “chunk” each day (Figure 2.11), so we had in total 39 areas of the site representing the different daily captures on the underwater site during the different campaigns from 2014 until 2016.
Between each chunk there was an overlap of 25 percent which helped to merge the processed areas (Figure 2.7) through the use of manually-positioned digital marker features on each of the paired chunks.
In conclusion, creating a 3D model using photogrammetry was a good tool for documenting such a huge underwater archaeological site. We were also able to study the natural changes of the site. One remarkable observation after three years of work, between the years 2014 and 2016, was the constant movement of sand on the sea bottom. This resulted in the covering and uncovering of dozens of archaeological blocks (Figure 2.11), which appeared on earlier orthophotos and models only to disappear later on.
The DSM of the submerged site of the lighthouse near Qaitbay is not yet completed, but our initial experience in underwater photogrammetry to create such a DSM has provided good and efficient results, using the technical application of low cost materials for data acquisition and data processing.17 More than 7,200 square meters have been documented, representing about 60 percent of the total surface area of the submerged site. Nonetheless, we still need to make improvements in our work in the coming seasons in areas such as:
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Improved photographic equipment. Current DSLR or digital mirrorless cameras achieve far higher resolution than the Nikon D700 (e.g., 46.9 megapixels for the Nikon D850; or 62.5 megapixels for the Sony Alpha 7r).
-
Improved georeferencing of the model by increasing the number of GCP s (Ground Control Points) in the future zones before photographing, in order to increase the accuracy of the model. The DSM, once completed, will make it possible to generate lengthwise and transverse profiles, and to create a digital terrain model (DTM) of the site. It already allows for the production of orthophoto plans with a pixel size adapted to the power of our computers, which can, in absolute terms, reach a pixel resolution equaling between 0.64 mm to 0.50 mm on the seabed (Figure 2.8).
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We can improve and extend our digital elevation model (Figure 2.12) from which we can obtain ‘z’ points or the elevation for any block in the site without using a GPS, because the site is already georeferenced. In addition, we can also extract a section to see the uneven surface levels and compute the volume of any block in the model.
Figure 2.11
Orthophotos cover the area of work (7,200 square meters) during the seasons of 2014, 2015, 2016
processing by Mohamed Abdelaziz
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Producing a 2D drawing from the orthophoto using Auto-CAD, which can be a faster means of drafting a plan (within a few days), rather than spending extensive time on the submerged site.
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With the open-source 3D processing software MeshLab, we can apply an ambient occlusion rendering to the 3D model, thus improving visualization, which can help in counting the archaeological objects (Figure 2.13). MeshLab provides tools for editing, cleaning, healing, inspecting, rendering, texturing and converting meshes. It offers features for processing raw data produced by 3D digitization tools/devices and for preparing models for 3D printing.
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Since the Qaitbay Fort (Figure 2.14) site is not accessible to people who do not dive, and underwater conditions and visibility are not stable, we have created a virtual diving tour from the model of the site which could be developed in the future, after finishing the entire model. Preliminarily, it was possible to create a complete an animated video of the 3D model corresponding to an area of 7,200 square meters.
Figure 2.12
Digital elevation model
processing by Mohamed Abdelaziz
Figure 2.13
2D sketch (left) and an ambient occlusion rendering (right) of the 3D model from 2014, showing parts of the monumental door of the ancient lighthouse of Alexandria
processing by Mohamed Abdelaziz
7 Virtual Anastylosis
Restoration of physical monuments requires extreme caution and careful study. Archaeologists and conservation experts are very reluctant to proceed to restoration and or reconstruction projects without detailed consultation and planning. Currently, anastylosis (re-erection) executed on a real object is highly challenging. Contemporary technologies have provided archaeologists and other conservation experts with the tools to embark on virtual restorations or anastylosis, thus testing various alternatives without physical intervention on the monument itself, respecting international norms and conventions.18
Throughout our archaeological work we often find sculptures that require reassembly of their fragments and the restoration of their lost parts. Traditional restoration work has certain disadvantages: it is expensive because it involves the use of real materials, and it is often difficult or impossible to accurately predict and visualize the final result of the restoration before it is completed. Although virtual restoration cannot replace real restoration for a piece that is in danger of deterioration, it can be a very useful tool to plan interventions and to illustrate the hypothetical original state of an object without having to intervene directly in it, thus respecting the principle of minimum intervention.
Through virtual restoration we are able to think about the reassembly of the fragments of our sculpture in an unhurried and more systematic way, observing more easily the correspondences between the different pieces that compose an object. With a 3D model of a proposed restoration, we can also contemplate the visual impact of different materials that we may choose when carrying out a subsequent real-world restoration.
We have two important examples from the seafloor near Alexandria. First we discuss the first virtual anastylosis of a colossal statue of the goddess Isis of red granite (Figure 2.17). The sculpture is now composed of three separated parts. Two major fragments, the legs, and the torso and head (Figure 2.15) were raised from the sea in 1962 in front of Qaitbay Fort. The third fragment, the crown of the statue, was recovered in 1995. Then, we discuss the virtual reassembly of a group of three fragments of a sphinx of granodiorite stone, representing the sphinx’s base (N. 3002), neck and upper body (N. 1325) and the head (N. 1324) (Figure 2.18).
Figure 2.15
The head and torso of the colossal statue; legs in the background. Inset: Ashraf Hussein (CEAlex photographer) takes photos of the statue at the Maritime Museum of Alexandria
Mohamed Elsayed
8 A Virtual Anastylosis of a Colossal Statue of Isis
In 2011, CEAlex launched a project using photogrammetry to carry out virtual anastylosis of the colossal statue of Isis recovered in large fragments from the sea floor of Alexandria Harbor. As the statue fragments weighs more than 20 tons in total (calculated from our 3D analysis of the models), we used a crane to photograph the inaccessible surfaces.
For the virtual anastylosis, a photogrammetric survey of our piece was performed. We created the 3D model of the fragments of the statue using Agisoft Metashape. This operation consisted of the placement of the different pieces in a three-dimensional space as they should have been before its fragmentation (Figure 2.16). For this last operation we used the Autodesk 3ds Max and MeshLab. We used 445 photos to create the 3D models, with the results as follows (Figure 2.16):
Figure 2.16
Camera positions during the photogrammetry of the colossal statue of Isis
Figure 2.17
Virtual Anastylosis of the three main fragment of the colossal statue of Isis
processing by Mohamed Abdelaziz
9 Virtual Anastylosis of the Sphinx
In 2014, we undertook the restoration of three parts of a sphinx (total weight: 1.5 tons) which had been raised from the sea in 1995. However, as it was difficult to use lifting equipment to be sure that the head and the upper body fit together with the base of the sphinx, we proposed a virtual restoration of the three parts before the intervention of the conservator, to be sure of proper fit of the fragments (Figure 2.18).
Figure 2.18
Virtual restoration of the three parts before restoration, created using 315 images
photos and processing: Mohamed Abdelaziz
A photogrammetric plan was realized for the following parts: (1) the base of the sphinx with hind legs and human arms, of which the hands were holding a removable vase which has disappeared. This fragment fits with the (2) upper body and neck (3) the head (Figure 2.19). A royal cartouche (evidently Ramesses II) is partly inscribed on the base of the sphinx, and partly on the upper body.
10 Unrolling Cylinder Shapes to Facilitate Reading Near-Invisible Inscriptions
Our third example is very important because we were able to recover an all-but-invisible inscription on a piece of a granite column originally from the Roman theatre in Alexandria. With this example, we present the unrolling the cylinder shape, which allows us to show it in an undistorted 2D plane.
A useful way to document archaeological finds is the representation of so-called rollouts, the analysis of rotation-symmetric objects with paintings or inscriptions. The advantage of a rollout is that it can give an overall view of the object’s inscribed or decorated content. Typically, rollouts are created either by manual drawing or from photographs. Generating 2.5D rollouts from color and geometry acquisition represents a more reliable method, which assists scholars in the task of interpreting iconography. In this example, we addressed this problem by proposing an automatic method to unroll decorations or inscriptions on 3D cylinder shapes into 2D and 2.5D planar space, using MeshLab 2018.04 and Agisoft Metashape software.19
Figure 2.19
Reassembling and restoration of the three parts of the sphinx by CEALex—Roman theatre, Alexandria
Mohamed Elsayed
Running the radiance scaling shader in MeshLab also returned quite interesting results. This rendering technique permitted the depiction of object shape through shading via the modification of light intensities around specific features like concavities and convexities. The radiance scaling rendering technique works in real-time on modern graphics hardware, making it feasible for an interactive inspection in MeshLab. By using this filter or renderer on a photogrammetric model, it becomes possible to greatly clarify certain inscriptions, particularly after the color and texture information have been stripped from the model (Figure 2.21). This technique is particularly effective on granite, the surface mottling of which can make even well-preserved surface inscriptions difficult or impossible to detect with naked-eye inspection in natural light. Using a model of our column piece processed with Agisoft Metashape 1.5 and MeshLab 2018.04, we were able to reveal seven lines of a Latin inscription which were hardly visible on the object itself or in daylight photographs.
Figure 2.20
Unrolled cylinder decoration, showing a 3D surface projected onto a 2D plane
processing by Mohamed Abdelaziz
Figure 2.21
Unrolling cylinder shape, MeshLab software
processing by Mohamed Abdelaziz
Acknowledgments
The authors would like to thank the Centre d’Études Alexandrines for supporting us during these projects and welcoming us as part of the team.
Drap 2012.
Al-Ruzouq 2012.
El-Gindy 2000, 144.
Abdelaziz, Elsayed 2019.
These discoveries were due to the curiosity of Kamel Abou Elsaadat, who enjoyed fishing in Silsileh. He had located some of the sunken monuments in the eastern harbor of Alexandria in 1962, with the help of the Egyptian Navy under the supervision of Dr. Henry Riad, at that time director of the Greco-Roman Museum (Riad 1964). Abou Elsaadat recovered an anthropoid sarcophagus lid near Silsileh and a female colossal statue of Isis near Qaitbay Fort in Alexandria (El sayed 2012).
Frost 1975; Empereur 2000.
Hairy 2007.
Thanks to the alert given by the Egyptian filmmaker Asmaa Elbakry in 1993, the Egyptian authorities stopped the construction of a breakwater of concrete blocks to protect the Qaitbay Fort built in 1477 from the storms and waves of the Mediterranean Sea. A few hundred concrete blocks had already been thrown at the foot of the building before the alert (Corteggiani 1998, 25). In the fall of 1994, the Supreme Council of Antiquities of Egypt asked Jean-Yves Empereur, director of CEAlex, to undertake a salvage underwater excavation on the submerged site near Qaitbay Fort in collaboration with the SCA, the Arab Maritime Academy and the Egyptian Navy, but the project was interrupted by bad weather. One year later, in 1995, the Centre d’Études Alexandrines undertook, with the support of the Institut français d’archéologie orientale, the first scientific mission on the submerged site.
Hairy 2009.
Hairy 2011.
Reuter et al. 2011.
Hairy, Abdelaziz, Elsayed and Soubias 2016.
Van Damme 2015; Yamafune, Torres and Castro 2016.
Balletti et al. 2015.
Yamafune, Torres, Castro, 2016.
Abdelaziz and Elsayed 2019.
The first results were presented at the International Society for Photogrammetry and Remote Sensing annual meeting in 2019; see Abdelaziz and Elsayed 2019.
Stampouloglou et al. 2020.
Vergne et al. 2018.
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