Cultural Heritage should be documented and preserved in order to be inherited to future generations (Georgopoulos, A. and Agrafiotis, P. 2012). With climate change and sea level rise, archaeological monuments situated at the shoreline or submerged to various depths under the sea surface are gradually shifting with the risk of damage or even disappearance. Especially in the Mediterranean waters, where a wealth of monuments exists, the situation is even more crucial. Geometric documentation is therefore extremely critical in the case of submerge monuments.
Orthophotomaps are precise and georeferenced maps created by seamlessly mosaicked orthorectified densely overlapping aerial photos. The extended use of UAVs and drones along with software automations, facilitated their wide adoption as mapping tools for archaeological aerial surveys as they can provide much more visual information and if used as a backdrop to existing geo information, can help interpretation and understanding of the relationship among finds in a context rich map (Skarlatos et al. 2003; Skarlatos el at. 2004) Especially in coastal sites where there is the need to relate both overwater structures with underwater ones or finds, the use of seamless orthophotomaps, which reveal visual information of both sea bottom and ground structures, could be an invaluable tool.
Although photogrammetry has become a widely acceptable tool for the geometric documentation of inland archaeological sites and monuments inland with increased accuracy and minimal processing times, similar methods for documenting submerged archaeological sites, are insufficient, because the light rays bend when transmitted through water, hence the straight line rule does not apply. Orthophotomaps in submerged coastal zones, although extremely helpful for interpretation and survey, cannot be created because the water refraction affects both the 3D bathymetric model as well as final orthophotomosaic accuracy.
Nevertheless, since aerial imagery may penetrate more than 13m in clear Meditteranean water, coastal archaeological sites could also be benefited by mapping and monitoring using similar techniques and products. The need for such investigation has been raised during discussions with MARELab of the Archaeological Research Unit of the Cyprus University, regarding ways to survey coastal sites. During September 2012, in collaboration with Nautical Archaeological Society (NAS) and MARELab from University of Cyprus (UCy), sparse positions of some anchorages at an ancient anchorage coastal site were collected using a dual frequency GPS as handheld devices over buoys. This method proven to be not an easy nor a productive task, while the final output was a sparse point layer with find’s positions and depth over an otherwise blank map with main feature the coast line. The difficulties, inaccuracies and survey quality of coastal site recording, lead to the decision to further investigate the UAV possibilities of coastal mapping.
The aim of this project is to develop new methods for mapping coastal archaeological sites by overcoming the aforementioned refraction problem. Aerial mapping using technologies such as Autonomous Unmanned Aerial Vehicles (AUAV or drones), that have recently become affordable are widely adopted, hence by providing means for orthophotomap production is coastal areas which was previously impossible, will greatly enhance archaeological tools for coastal surveys. Image Based Methods (IBM) such as Structure from Motion (SfM) and Multi View Stereo (MVS) for 3D recording and documentation are also available tools for creating a seamless 3D model of coastal sites with correct depth and height information. Using such procedures and techniques, an impressive level of detail (down to 15mm per pixel) and accuracy (down to 30mm) is attained. The proposed methodology integrates both land and shallow water underwater orthophotos in a seamless and precise orthophotomosaic, thus providing means to gather reliable information both for documentation and analysis, bypassing the tedious task of single point GPS measurements. These tools will assist local archaeological research in Cyprus, for sites such as ancient Paphos, Amathounta ancient harbor and Agia Napa coastal site.