# The Usage of Virtual and Augmented Reality in Underwater Archeology
## 3 Virtual and Augmented Reality Technologies
### Creating Virtual or Augmented Reality Applications for Underwater Archeology
Hydroacoustic devices used for research are usually mounted under the ship's hull using, e.g., mobile platforms. Less often, they are towed by a vessel or mounted to remotely  
Figure 3: The general division of data collection methods in underwater archeology. operated underwater vehicles--ROV (Remotely Operated Vehicle) or autonomous units AUV (Autonomous Underwater Vehicle). When using remote vehicles, detection devices are often used, i.e., side sonars or the so-called subbottom profiler--a low-frequency echo sounder [4; 45]. Among the visual methods, we can distinguish between active and passive methods. Active methods are most often based on devices with built-in optical sensor systems. An example of such a method is, among others, a 3D scanner. Scanners are currently primarily used for the 3D reconstruction of artifacts pulled from underwater and their documentation, and sometimes for reconstruction for museum purposes [4; 46]. Their main advantage is the lack of a need for direct interaction with the scanned object [47]. Another example is the LiDAR system, i.e., the use of laser measurements. LiDAR-type systems are used, in particular, for bathymetric mapping. They use a blue-green laser light wave [4]. These types of measurement systems are usually mounted on flying units (planes or drones) and provide continuous coverage of the entire imaging area between land and sea, which ensures the ability to observe the entire coastal zone and allows for its 3D modeling [4; 5; 48]. The group of passive methods is most commonly used for the purpose of documentation of sites; in addition to photographs and films, it also includes traditional forms of documentation, i.e., precise drawings, general sketches, or even a series of measurements taken at the site, etc. All these techniques require conversion from a 2D image, or a set of dimensions, to 3D models (solids, editable polygons, or meshes with unique coordinates for each point), i.e., processing into a 3D reconstruction [15; 49]. One of the specific methods used in this field is photogrammetry, which is widespread in underwater archeology [46]. Photogrammetry offers a realistic visualization of an object or place in its current form [26; 50; 51]. Its definite advantage is the fact that models can even be created with images that are not from professional cameras, but from data collected using sports cameras [46; 52]. However, the state of preservation of the underwater object is a separate problem. Photogrammetry brings the best results in the case of well-preserved objects not covered with a bottom layer. An example of this type of facility that has been reconstructed using photogrammetry is the wreck of the Swedish ship Mars from the 16 th century [26; 53]. Additionally, this reconstruction also shows the effectiveness of the method for objects of large dimensions, because the mentioned ship at the time of its sinking, i.e., in 1564, was considered to be one of the largest ships in the world [53]. However, it should be noted that such an object, especially in the aquatic environment, may be damaged, e.g., by bioerosion processes or even largely destroyed, which makes the documentation process difficult. In this case, it may be beneficial to recreate it using VR methods [51]. Visual data are collected depending on the underwater archaeological site--either using trained divers or autonomous vehicles. ROV and AUV systems are particularly used in hard-to-reach places, including those located at considerable depth. These vehicles can perform most of the tasks needed for photogrammetric purposes, reducing the need for the services of specialized technical divers, which proves to be a more cost-effective solution [4; 54]. It is worth noting that in extensive archaeological research, the possibility of combining visual and acoustic systems is increasingly used due to the advantages of both systems [4; 44; 48]. Other data collection methods include magnetic methods or Li-Fi (equivalent to terrestrial Wi-Fi).