Coastal cities that are built on the remains of ancient ones are facing a double challenge. On the one hand, they are required to design a public space according to the current urban planning approaches and policies, and on the other to rescue and, consequently, to enhance cultural heritage (CH). The ancient structures and building stock operate as tangible witnesses of the city’s historical development through centuries [1
]. Given that the coastal zone of a city is subject to constant changes, the excavation works (usually in the framework of large-scale public work projects) reveal ancient structures designed to facilitate commercial maritime transport operations (harbors, quays and jetties) or defense structures (walls, onshore towers, coastal defense walls, etc.) [2
]. In addition, the coastal zone accepts more pressure for economic exploitation. During the second half of the 20th century, in many coastal cities, additional public space was created with embankments, and port installations were covering the basins of the ancient harbors and changing the ancient coastline.
In many cases, the combination of natural and anthropogenic structures on the coastal zone, the rising sea level and the partial destruction or embankment of the ancient infrastructures, have dramatically changed the old boundaries of coastal cities. Ancient coastal defense walls, artificial breakwaters, and quays “have moved” in relation to the current land, or they are preserved below the actual sea level [5
]. Although this displacement is not always clearly defined, there are structural elements on the ancient structures, like lead clamps between the stone plinths (now submerged), which provide an excellent reference for the ancient sea level [6
Excavations in coastal cities call for a multi-disciplinary approach by archaeologists, geologists, geographers, surveyors, and other scientific groups. These scholars need access to accurate spatial data (like high-resolution digital surface models and orthophoto maps), captured with fast procedures and low-cost tools to map or to monitor ancient port installations or other CH coastal areas. These data should be available in a convenient way through web-based information systems, combining also old findings from previous excavations in the same area.
Traditionally, a variety of remote-sensing data including low-, medium- and high-resolution images (e.g., MODIS; Landsat; QuickBird), as well as ready satellite products (e.g., ASTER Global Digital Elevation Model, ASTER GDEM) were used to extract valuable information regarding natural and anthropogenic hazards as well as to assess the overall risk for CH sites and monuments [8
]. Although the spatial resolution of satellite imagery has significantly improved in the last decade (tens of centimeters), the data collected is still not sufficient to map medium to small characteristics in CH, as centimeter-level accuracy is needed [9
]. Furthermore, traditional remote-sensing data are not capable of monitoring small-scale changes due to inherent limitations such as spatial resolution and data volume. Other limitations of the satellite imagery include the way of data acquisition (e.g., stereo or tri-stereo image mode), the difficulty in data acquisition due to satellite installation trajectories and revisiting times in a specific time frame as well as their high cost.
By contrast, unmanned aerial systems (UAS) are able to provide extremely high-resolution images at low cost, but in a limited geographic area [10
]. UAS (or drones), consisting of an unmanned aerial vehicle (UAV) and a sensor (e.g., optical sensor, Lidar), provide digital images with spatial and temporal resolutions that can overcome some of the limitations of spatial data acquisition using satellites and manned aerial photography. The use of drones has exploded, because of their agility and quality to map an area with high end-products. They deliver digital images with spatial (1–5 cm) and temporal resolution superior of satellite imagery, on demand, and most importantly, economically and more reliable than the high-resolution satellite images [11
]. Currently, a UAS is a viable option for collecting remote sensing data for a wide range of applications, including scientific, agricultural and environmental ones [10
]. In combination with new image-processing algorithms, i.e., structure from motion (SfM), UAS can provide high-resolution geoinformation at a low cost for areas of a small geographical scale [11
]. UAS data acquisition in combination with SfM pipeline consists an efficient, quick and accurate methodology which are used in a plethora of sciences and applications for the creation of geospatial information. The UAS-SfM methodology is used for 2D and 3D visualizations since it enables the development of high-resolution orthophoto maps, accurate digital surface models (DSM) and digital terrain models (DTM), and finally very detailed 3D models of the study area [19
]. Furthermore, scientists are using UAS-SfM methodology for mapping and detecting; cultural sites, coastline changes, coastal erosion and the human pressure in the coastal zone [9
The large volume of images acquired for UAS surveys, introduces new needs, leading to the creation of new methods and tools for effective data organization, management and retrieval [28
]. A key issue regarding information systems for such purposes is the definition and incorporation of metadata profiles in accordance with the application domain [29
]. Since all data are geo-referenced, metadata standards for geospatial information could be adopted [32
]. In addition, certain extensions are needed in order to explicitly introduce metadata elements (required for retrieval operations) that are usually “hidden” in other elements in the geospatial metadata standards. For example, UAS mission information should explicitly accompany the photographs taken, instead of being included in other metadata elements, like abstract or description.
The contribution of the work presented in this paper is twofold. Firstly, we describe the production of high-resolution 3D visualizations for two CH areas on Lesvos Island, Greece: the ancient harbors of Mytilene and Eresos cities. The SfM methodology was applied to photographs captured from UAS. Examination and analysis of the output images produced by the UAS-SfM methodology, revealed for the first-time unknown aspects, such as the detailed geometry of the ancient installations, their real dimensions, the state of preservation (destruction, repairs, structural phases, etc.) and their relation with the city’s coastal monuments (streets, defense walls, agora, sanctuaries, cemeteries, etc.).
Secondly, we describe the development of a metadata cataloging system dedicated to UAS-acquired photographs and secondary products. The system incorporates an extension of the INSPIRE Metadata Regulation [33
] and is based on the open source ESRI Geoportal Server platform. In this way, all datasets related to a CH area are available for discovery, view, download or re-use, instead of being stored in isolated silos after project completion [35
2. State of the Art
Several recent publications have described methods and techniques that measure CH areas with the use of UAS. In [36
] the use of quadcopters and methods for creating image mosaics, 3D point clouds and image models are described, in order to monitor natural and cultural resources. Also, a number of future directions in improving UAS technology are proposed. In [37
] UAS platforms and photogrammetric methodologies were used to analyze and map indigenous settlements in the Caribbean. In Zheng Sun and Yingying Zhang [39
], drones and 3D modeling are used to survey Tibetan architectural heritage. The paper evaluates the accuracy of the UAV-SfM method for surveying a Tibetan stupa and illustrates how the results could be elaborated in a next-step analysis and used for management purposes.
Over the last decade, there have been some promising works regarding information systems for storing, searching and retrieving data acquired from UAS. More specifically, in [40
] a server-based software tool is illustrated, supporting an automated workflow for raster data pre-processing, georeferencing, cataloging and dissemination in near real time. The ISO 19115 metadata standard by the International Organization for Standardization (ISO) was used for image documentation, recording both geometric and keyword metadata elements. Also, in [41
], a similar tool is presented, emphasizing on the generation of precision metadata needed for geo-pointing at a particular target coordinate on the ground. Both studies are based on proprietary products and focus on near real-time provision of UAS-acquired images to ground stations. Each image is accompanied with a set of metadata elements that supports the functionality of the application domain.
As previously stated in [25
], the UAV-SfM derived products enable change detection, the discovery of new hidden structures, pattern analysis and documentation of coastal zones, in various spatial and temporal scales. The miniaturization of sensors, the increase in the flight capabilities and the agility of a UAV fulfill researchers need in spatial data acquisition for CH mapping [13
]. UAS can be seen as a new tool for mapping CH areas in the coastal zone. Furthermore, the high level of automation, the ease of deployment, the ease of survey repeatability, and the low running costs of UAVs allow frequent missions that provide spatial datasets with a resolution of less than 5 cm and a high temporal repetition due to the ease of survey deployment [12
]. In previous studies geo-referenced orthophotos and DSMs are used to measure and depict the morphology of CH areas in 2D and 3D, allowing archaeologist to assess the condition of the ancient building stock, especially after extreme environmental phenomena [9
], e.g., heavy rain, earthquake etc.
The application of the UAV-SfM pipeline for the two ancient harbors of Lesvos Island offers to archeologists and local authorities high-resolution geoinformation (orthophotos, DSM, etc.) that can be used to support their objective. Furthermore, it provides a valuable snapshot of the current state of the harbor building stock in high-resolution visualizations. The UAS-SfM methodology derivatives are valuable tools for rapid mapping of the harbor installations and may be used to automate the detection procedures, where conventional mapping techniques are difficult and time-consuming. It is worth noting that for the Mytilene and Eresos ancient harbors there were not detailed topographical maps previously available for the archaeologists and scientists to study these installations.
During the applied surveying and representation procedures, a large number of different files were produced by various users in different periods of time, requiring considerable amounts of time for cataloging and disk space for storage. To overcome problems often encountered in such cases (like storing data in silos without discovery mechanisms, duplication of files, undefined version control, lack of the overall UAS flight history for an area, or making unnecessary duplicate flights), all files were documented in our metadata cataloging system. The metadata profile used, extends the INSPIRE geospatial metadata regulation with elements about CH mapping projects and their accompanying UAS flights, in order to be convenient to end-users. Keeping files in different local servers and the metadata records in another, offer flexibility in space utilization and access rights management, and uniformity in cataloguing and searching operations.
Contemporary research activities of the spatial sciences require flexibility in terms of data re-usability, mainly due to limitation of funds and resources [62
]. In this work, we have shown that the utilization of UAS for capturing rapidly, with agility and at low-cost aerial photographs of high resolution and accuracy, in conjunction with the SfM and MVS techniques, provide quality 2D visualizations of CH areas for archeologists and local authorities. The quality of the orthophoto maps produced from UAV-SfM methodology enables the identification and measurement of hidden details in the archaeological sites over the total extent of the mapped area. Thus, this can help to measure details of ancient structures that were previously vague without the need to visit them, or to purchase expensive high-resolution satellite imagery. Furthermore, the detailed geoinformation produced from the data acquired using UAS can provide the researchers with valuable information for the current state of the harbor installations building stock. Moreover, the derived data products enable detailed 2D and 3D analysis thus change detection or landscape development analysis for archeological areas is possible. The high resolution and precision of the derived orthophoto maps and visualizations following UAS-SfM process cannot be achieved from satellite datasets.
In addition, in this study, we have presented a web-based information system for the documentation of all data (acquired or produced) with metadata elements customized for the UAS-SfM pipeline. The system supports searching operations with keyword-based or geographic criteria, enhancing data discovery and re-usability. The adaption and extension of the INSPIRE spatial metadata regulation of the European Union enable interoperability with other metadata catalogs through standard geographical web services and harvesting functionality. Keeping surveying and representation procedures for CH areas at low-cost makes it possible to carry them out more frequently, with obvious positive results in destruction prevention and preservation/protection decision-making. It is concluded that the UAV-based data acquisition provides a valuable alternative for mapping and monitoring of coastal zones with archaeological monuments and sites. Several important beach characteristics, as well as archaeological information, can be revealed using high-resolution orthophoto maps in contrast to high-resolution satellite images.
Potential future work includes the expansion of the study to other coastal CH areas of Lesvos Island, supporting authorities in the direction of the establishment of an overall protection policy. Also, a number of improvements of the cataloging system are planned, mainly in the field of searching functionality and flexible user privileges support. More UAS data acquisition surveys in different temporal periods should be implemented to monitor spatiotemporal changes in the CH areas to identify affected areas. Having detailed geoinformation in different time periods can reveal the dynamics of erosion and other dangers affecting cultural heritage. As more flights will be done and more data are created for the CH areas of interest, the archeological and cultural authorities will have a valuable metadata tool to define and prioritize areas that are in more immediate danger and to preserve them accordingly, as well as to manage coastal areas of cultural heritage.