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Article Locating and Mapping the Traces of the Covered Ancient Theater of Amphipolis (Eastern Macedonia, Greece)

Dimitris Kaimaris 1,*, George Karadedos 2, Charalampos Georgiadis 3 and Petros Patias 4

1 School of Spatial Planning and Development (Eng.), Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece 2 School of Architecture, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece; [email protected] 3 School of Civil Engineering, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece; [email protected] 4 School of Rural & Surveying Engineering, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece; [email protected] * Correspondence: [email protected]

Received: 16 September 2018; Accepted: 23 October 2018; Published: 1 November 2018

Abstract: Ancient Amphipolis (Eastern Macedonia, Greece) is one of the most important archaeological sites of Greece. Despite the restricted excavation studies, important monuments have been discovered, such as the city walls and the Macedonian burial site at the Kasta hill, etc. Currently, the location of the ancient theater is unknown and only assumptions can be made regarding its location. In the current study, we aim to detect the accurate location of the ancient theater using archaeological prospection tools, data collected from the excavated sites, and testimonies of people of the modern city. For the ﬁrst step of the approach, the approximate location of the ancient theater was determined using information derived by archived geospatial data (multi-temporal aerial photographs, satellite image, and Digital Terrain Model (DTM) of the area) as well as information regarding the neighboring excavated sites. For the in-depth study of the area of interest, a drone was used for the acquisition of high-resolution geospatial data. The generated orthorectiﬁed image (3 cm spatial resolution), DTM, and Digital Surface Model (DSM) allowed the determination of the potential location of the buried orchestra’s center using geometric rules for the design of ancient theaters. Furthermore, using the produced DSM and DTM, terrain cross-sections were generated.

Keywords: Amphipolis; ancient theater; drone; orthorectified image; DSM; DTM; terrain cross-sections

1. Introduction The ancient city of Amphipolis is built in the place of the Thracian tribe of Idonon’s older settlement (“Eννέα Oδoί” = “Nine Roads”), close to the delta of the Strimona River [1]. The settlement is reported by Thucydides [2], Stravonas [3], and Herodotus [4]. Recent archeological studies revealed the existence of buildings older than Amphipolis, upon which the north wall (constructed during the classical era) of the city was build [5]. This study veriﬁes the Thucydides writings, which mention: “residents of the city of Amphipolis with the previous name Nine Roads” [2]. Amphipolis’ geographic location caught the attention of Parians, Thasians, Andreions, Chakideons, Milisions, and later on the attention of the Athenians and Spartans. Its location was a strategic and economic node because it gave the opportunity to control the land access to the Thracian mainland—which was rich in resources (ship construction timber, ore deposits)—from the coastal area,

Heritage 2018, 1, 306–319; doi:10.3390/heritage1020020 www.mdpi.com/journal/heritage Heritage 2018, 1 307 as well as the maritime transport originating from Thasos (an island 70 km to the east, southeast of Amphipolis), Chalkidiki, the Aegean Sea islands, and Hellespont. The Parians were the first that realized the strategic and economic importance of Thasos and the Thracian coasts in the beginning of the seventh century B.C. They colonized Thasos Island, lead by Telesikli. Around 660–650 B.C. they started colonizing the coastal area of Thrace, along with the Thasians, and established the city of “Peraia” or “Ipeiros Thasion”. After fierce battles with the Thracians, they started building fortified cities in the vicinity of “Nine Roads”, such as Galipsos, Oistimi, Neapolis (Kavala), Strimi, and later on Krinides, which was renamed Philippoi by Philippos II. The income of Thasos due to mining operations reached 200 to 300 talanta, giving the island the ability to build a powerful military naval force and establish trading routes [5]. The Athenians started to get interested in the area from the time of the Peisistratus’ exile (at the end of the sixth/beginning of the fifth century B.C.), who were amassing resources (funds and mercenaries). Thrace and Eastern Macedonia were temporarily conquered by Persians during the expedition of King Darius to South Greece. Following the defeat of the Persians, Thasos became a member of the first Athenian Alliance in 477 B.C. The Athenians’ attempt to conquer the Peraia of Thasos and the Paggaio Mountains forced the Thasians to leave the alliance. Kimon forced the Thasians to surrender after a three-year siege of the city, enforcing the abandonment of the Peraia mines [5]. The expansion of the Athenians in the area (focusing on the control of the silver and gold mines of the Paggaio Mountains) started with the conquest of Hionas in 476 B.C. In 465 B.C., an expeditionary force of approximately 10,000 colonists lead by Leagro and Sofani conquered “Nine Roads”. Unfortunately, when they attempted to enter the Thracian mainland, they were decimated by the Thracian tribe of Idonon at Dravisko. In 437 B.C., the Athenians storming from Hionas repelled the Thracians and, with new colonists lead by Agnona, set foot in the area and began to build the city of Amphipolis in the location of “Nine Roads” [2]. The Spartan general Vrasidas occupied the city of Amphipolis in 424 B.C., provoking an attitude of awe toward the Athenians mainly because of the strategic and economic importance of Amphipolis [2]. Despite three attempts of the Athenians to regain Amphipolis, the city remained independent until its occupation by the Macedonians. In 358–357 B.C., Philippos II occupied Amphipolis after a sudden attack [5] and used it as a base to conquer the wider area east of the Strimona river. During the Hellenistic era—up to the battle of Pidna in 168 B.C., when the Romans occupied the state of Macedonia—Amphipolis was the center of exploitation of all the mines in the area, an important commercial center, one of the most powerful royal mints of Macedonians, a powerful fortress of great military importance, and a naval base of Alexander the Great, from which his fleet started the expedition to the East. Amphipolis, in which the three more important admirals (Nearchos, Androstehis, and Laodemon) under the command of Alexander the Great lived, was one of the six cities where a magnificent temple costing 1500 talanta was going to be built under the command of Alexander the Great (the Macedonian burial site at the Kasta hill) [6]. Following the conquest of Macedonia by the Romans, Macedonia was divided into four administrative regions. Amphipolis (conquered by Aemilius Paullus) was the capital city of the first region [3]. Aimilios-Paylos established athletic games that took place yearly with the participation of Greeks and foreigners from Greek and Asian cities. The crossing of Via Egnatia through Amphipolis helped to maintain its strategic and economic importance in the Roman and early Christian periods. Taking into account the abovementioned facts and the quality of the structures found at the excavated sites, one can assume that the theater of Amphipolis, which reflected the social life of this important city, would be as important as Amphipolis. An indication related to the importance of the theater is the discovery of a clay tile demonstrating the faces of actors in a tomb at a cemetery dating from the Hellenistic period [5]. The Amphipolis’ ancient theater is expected to be of equal importance to that located in the city of Philippoi, since it had the same historical evolution and importance. In addition, the political and Heritage 2018, 1 308 economic importance of Amphipolis was greater through the Hellenistic and Roman periods [5]. Following the information acquired from the other ancient theaters of Thrace, such as those in Maroneia [7], Thasos [8], and Philippoi [9], it is expected that this theater will also have later Classic, Hellenistic, and Roman phases. Furthermore, the discovery of an elaborate Roman lion-foot, which has not been discovered in the other theaters, reveals that during its Roman phase the Amphipolis’ theater was at its peak. In addition, during the Roman period, gladiator duels and beast-related spectacles were very popular in the wider area, suggesting that the Amphipolis’ Theater might have been transformed into an arena, as was the fate of other existing theaters in the area of Thrace [5]. The use of aerial images (aerial archaeology) in archaeological studies has its roots in the beginning of the 20th century, while satellite images (remote sensing archaeology) have been used for the same reason since the 1970s. Currently, state-of-the-art platforms equipped with digital sensors such as autonomous Unmanned Aerial Vehicles (UAVs) and satellites are employed. A variety of digital sensors, for instance panchromatic, multispectral, hyperspectral, thermal, lidar, and radar, are used for the qualitative and quantitative documentation of known archeological sites or for the detection of unknown ones. All the above are employed in conjunction with traditional field work to help archaeologists [10–18]. Using a drone equipped with a modern digital 18 Mp resolution camera allows the three-dimensional (3D) mapping of an archaeological site with a spatial resolution of 1–2 cm [19]. The appearance of marks on the ground or on the vegetation are caused by the interaction of a buried monument (a “concrete construction”, e.g., the boundaries of a building, or an “open construction”, e.g., an ancient trench) with the ground or vegetation [20,21]. In aerial and remote sensing archaeology, significant efforts are made for automatic detection, digital visual enhancement of marks in images, and interpretation of the phenomenon of interaction between buried monuments and the ground or vegetation, using panchromatic, multispectral, and hyperspectral sensors [11–13,21,22]. This study aims to pinpoint the location of the ancient theater of Amphipolis. In order to achieve this aim, the authors utilize tools of archaeological prospection and data from existing excavations that are combined with testimonies of the residents of Amphipolis. For the presentation of the ancient city of Amphipolis and the understanding of its geomorphology, various geospatial archived data—such as the Digital Terrain Model (DTM) of the wider area—are employed to discover the potential location of the ancient theater. In addition, recently acquired drone images are used to perform an in-depth study of the potential location.

2. Methodology and Data Collection

2.1. Methodology The use of archived multi-temporal geospatial data of the study area can allow the detection of temporal marks of buried infrastructure/buildings, the topology of the specific site as well as geomorphological changes, the detection of changes in the ground coverage (e.g., bare ground to vegetation), and the spatial correlation of known monuments, etc. [20–28]. As a result, various multi-temporal geospatial data (historic and modern aerial photos, satellite images, etc.) covering the study area were gathered and studied. Combining the abovementioned information with testimonies of the residents of Amphipolis, it is possible to minimize the spatial extent of the search area for the detection of the buried structure. Following the study area definition, the data acquisition phase can be realized. The data acquisition involved a UAV for image acquisition and a Global Positioning System (GPS) for Ground Control Points (GCP) measurement. The data were processed using digital photogrammetric processing software. As a result, the area of interest can be quantitatively and qualitatively documented and studied in depth using the produced Digital Terrain Model (DTM), Digital Surface Model (DSM), and orthorectified image. Heritage 2018, 1 309

In this study, a hexacopter drone was used for image acquisition, a TopCon Hiper SR GPS receiver was used for GCP measurements, and Agisoft PhotoScan® software was used for the photogrammetric processing of the acquired data. Agisoft PhotoScan® is a standalone software product that performs photogrammetric processing of digital images and generates 3D spatial data. The processing in Agisoft PhotoScan® consists of the following steps: image alignment, dense point cloud generation, mesh generation (Triangulated Irregular Network (TIN) creation), DSM production, and ortho image generation. During the image alignment step, which is focused on reconstructing a three-dimensional scene, the relative orientation between the images is reconstructed through the matching of tie points between them. If control points or other metric information is used, such as distances between points, the absolute orientation is also computed at this stage. The following processing step generates a dense point cloud of the area, thus creating a large number of points that complete the 3D model. The next step generates a mesh or a wire frame model by creating triangles between the points. Using the generated mesh, the DTM of the area is generated in the subsequent step. An optional step can generate a textured model using an image overlay [29,30]. Finally, by exploiting the high-spatial-resolution DSM, one can determine the potential center of the buried orchestra using the geometry rules for designing an ancient theater on one hand, and create cross-sections in various directions on the other hand. It must be noted that the ﬁndings of this study have the purpose of providing additional information to archaeologists and they can only be conﬁrmed/documented by excavating the site.

2.2. Archived Data During an older aerial and remote sensing archaeology study of the wider area between Amphipolis and Philippoi [20–28,31], various geospatial data of the study area were collected and processed. More specifically, black and white (B/W) orthorectified aerial images were obtained in 1945 (1:42,000 scale), 1983 (1:7000 scale), and 1998 (1:40,000 scale) (Table1). In addition, an orthorectified QuickBird-2 image (4 bands: red, green, blue, near infrared) with a spatial resolution of 0.6 m acquired on 5 February 2005 was also used. Furthermore, the panchromatic orthorectified images and the DTM (5 × 5 m grid) obtained in 2007, provided by the National Cadastre and Mapping Agency SA (Société anonyme), were also utilized.

Table 1. Multi-temporal data.

Spatial Data Type Scale Provider Year Product Resolution (m) Aerial Hellenic Military Orthorectified 1:42,000 1945 1.0 Photographs Geographical service image (B/W) Aerial Hellenic Cartographic and Orthorectified 1:7000 1983 0.2 Photographs Cadastral Organization image (B/W) Directorate of Topography, Aerial Orthorectified 1:40,000 Ministry of Rural 1998 1.0 Photographs image (B/W) Development and Food Satellite image Orthorectified - DigitalGlobe 2005 0.6 QuickBird-2 image (B/W) Aerial National Cadastre and Orthorectified 1:20,000 2007 0.5 Photographs Mapping Agency (NCMA) SA image (B/W) Digital Terrain Raster format - NCMA SA 2007 5.0 Model (B/W)

2.3. Unmanned Aerial Vehicles The drone (Figure1) is able to ﬂy completely automated or controlled manually via remote control. It consists of the spindle, where the batteries and the digital camera are housed, and the ground station. Heritage 2018, 1 310

TheHeritage movement 2018, 2, x FOR of the PEER Unmanned REVIEW Aerial Vehicle (UAV) is realized through six propellers. The camera5 of 14 gimbal (where the digital camera (DSLR Canon 1200D, with CMOS sensor (22.3 mm × 14.9 mm), 18 MP resolution(22.3 mm × using 14.9 themm), Canon 18 MP lens resolution (EF-S 17–85 using mm the f/4–5.6 Canon IS USM lens lens,(EF-S focal 17–85 length mm minimum-maximum:f/4–5.6 IS USM lens, 17–85focal length mm, aperture: minimum-maximum: 4–5.6) is placed 17–85 to allow mm, itsaperture: vertical 4– rotation5.6) is placed by 180 to degrees allow its (± 90vertical degrees rotation from theby 180 nadir). degrees The (±90 digital degrees camera from self-calibration the nadir). The is di performedgital camera during self-calibration the image is processingperformed during (in the ® correspondingthe image processing photogrammetric (in the corresponding image processing phot software,ogrammetric e.g., Agisoft image PhotoScan processing). software, The UAV usese.g., theAgisoft autopilot PhotoScan Wookong®). The M of UAV DJI, whichuses the incorporates autopilot a Wookong Controller, M an IMU,of DJI, and which a GPS. incorporates Its horizontal a accuracyController, is an approximately IMU, and a GPS.±2 m, Its its horizontal vertical accuracy accuracy reaches is approximately±0.5 m, and ±2 them, angleits vertical measurement accuracy ◦ ◦ accuracyreaches ±0.5 in them, threeand the axes angle ranges measurement from approximately accuracy in 1 theto three 2 (according axes ranges to the from speciﬁcations approximately of the 1° autopilot).to 2° (according The liftingto the specifications capacity of the of dronethe autopilo is 2.5t). kg, The and lifting the ﬂightcapacity time of rangesthe drone between is 2.5 kg, 10 and 15the min flight [32 time,33]. ranges between 10 and 15 min [32,33].

Figure 1. Drone.

3. Results and Discussion

3.1. The Potential Location of Amphipolis’s Ancient Theater 3.1. The Potential Location of Amphipolis’s Ancient Theater Amphipolis is surrounded by the Strimonas River (south, north, and west), while in the east lies Amphipolis is surrounded by the Strimonas River (south, north, and west), while in the east lies the Paggaio Mountains. Figure2 presents the location of ancient Amphipolis, depicting the known the Paggaio Mountains. Figure 2 presents the location of ancient Amphipolis, depicting the known archaeological excavation sites. The contour lines (6-m intervals) in Figure2 were generated by the archaeological excavation sites. The contour lines (6-m intervals) in Figure 2 were generated by the DTM provided by the NCMA (Table1). The outer wall of the ancient city (Classical/Hellenistic periods; DTM provided by the NCMA (Table 1). The outer wall of the ancient city (Classical/Hellenistic periods; fifth to first centuries B.C.)) is drawn in black. The inner wall of the late Roman period (fourth to sixth fifth to first centuries B.C.)) is drawn in black. The inner wall of the late Roman period (fourth to sixth centuries A.D.) is drawn in red. Within the inner wall, four basilicas, a central building, a reservoir, and centuries A.D.) is drawn in red. Within the inner wall, four basilicas, a central building, a reservoir, and parts of residential buildings (presented in red) of the same period (late Roman) exist. Southwest of parts of residential buildings (presented in red) of the same period (late Roman) exist. Southwest of the the abovementioned constructions, Hellenistic period buildings can be found (presented in black), and abovementioned constructions, Hellenistic period buildings can be found (presented in black), and to to their south lie the Temple and the Gymnasium of the city. East of the outer wall, one can observe their south lie the Temple and the Gymnasium of the city. East of the outer wall, one can observe two two locations of Macedonian burial sites (presented in blue) and the cemetery of the Hellenistic period locations of Macedonian burial sites (presented in blue) and the cemetery of the Hellenistic period (presented in black) [34–36]. (presented in black) [34–36]. According to information collected from the current residents of Amphipolis, seating stones were According to information collected from the current residents of Amphipolis, seating stones removed for the construction of modern-era buildings from the location north of the excavated site of were removed for the construction of modern-era buildings from the location north of the excavated Gymnasium, called “skalakia”. This location is near to the city Temple. site of Gymnasium, called “skalakia”. This location is near to the city Temple. Additional information regarding the location of the ancient theater of Amphipolis can be drawn Additional information regarding the location of the ancient theater of Amphipolis can be drawn from the study of the terrain relief of the ancient ci cityty (Figure (Figure2 2)) combinedcombined withwith thethe locationslocations ofof alreadyalready known structures,structures, specially specially the the locations locations of of the the Temple Temple and and the the Gymnasium. Gymnasium. More More specifically, specifically, the only the potentialonly potential location location of the of ancient the ancient theater theater is located is located within thewithin boundaries the boundaries of the dashed of the circle dashed in Figures circle in2 andFigures3, between 2 and 3, the between Gymnasium the Gymnasium and the south and part the of south the outer part wall.of the The outer contour wall. linesThe contour at this position lines at (Figuresthis position2 and 3(Figures) resemble 2 theand spatial 3) resemble morphological the spatial characteristics morphological of an characteristics ancient theater’s of structure.an ancient theater’s structure. Figure 4 presents the available multi-temporal data of the potential location of the ancient theater. Figure 4a presents the contour lines of the potential ancient theater location with intervals of 2 m. In the center of Figure 4c (detail of the 1983 orthorectified image) lies the potential location of the ancient theater. Northwest of the location, above the southern inner wall, the excavated basilica is shown. To

Heritage 2018, 2, x FOR PEER REVIEW 6 of 14 Heritage 2018, 2, x FOR PEER REVIEW 6 of 14 the south, the Gymnasium (323–31 B.C.) excavation site is presented. Finally, west of the location, the coverthe south, of a theHellenistic Gymnasium period (323–31 building B.C.) can excavation be observed, site iswhile presented. between Finally, the po westtential of theposition location, of thethe theatercover of and a Hellenisticthe Hellenistic period period building building can one be canobse observerved, while negative between vegetation the po markstential [20], position suggesting of the thetheater existence and the of Hellenisticburied constructi periodons building (Figure one 4d, can shown observe in yellow). negative Figure vegetation 4d shows marks the [20], contour suggesting lines the existence of buried constructions (Figure 4d, shown in yellow). Figure 4d shows the contour lines usingHeritage the2018 orthorectified, 1 image as a base map. 311 using the orthorectified image as a base map.

Figure 2. Map of ancient Amphipolis. The contour lines derived from the NCMA DTM were used as Figure 2. Map of ancient Amphipolis. The contour lines derived from the NCMA DTM were used theFigure base 2. map. Map Theof ancient absolute Amphipolis. heights of The the contour inner wall lines area derived range from from the 18 NCMA m to 152DTM m were(latitude used Ν as: as the base map. The absolute heights of the inner wall area range from 18 m to 152 m (latitude N: 40.81699881the base map. and The longitude absolute Ε :heights 23.8430717 of the in innedecimalr wall degrees area range at the from center 18 ofm theto 152 image, m (latitude Reference Ν: 40.81699881 and longitude E: 23.8430717 in decimal degrees at the center of the image, Reference System:40.81699881 WGS84). and longitude Ε: 23.8430717 in decimal degrees at the center of the image, Reference System: WGS84). WGS84).

FigureFigure 3. 3. DetailDetail of of Figure Figure 2 showing the the area area of the po potentialtential location of the ancient theater. Contour linesFigurelines derived derived 3. Detail by by theof the Figure NCMA NCMA 2 showingDTM DTM were were the used used area as asof the thethe base basepotential map. map. location of the ancient theater. Contour lines derived by the NCMA DTM were used as the base map. Figure4 presents the available multi-temporal data of the potential location of the ancient theater. The observation of Figure 4b (detail of the 1945 orthorectified image) reveals that these locations have Figure4a presents the contour lines of the potential ancient theater location with intervals of 2 m. In the not beenThe excavated observation and of that Figure one 4b cannot (detail distinguish of the 1945 theorth loorectifiedcation of image)the ancient reveals theater. that theseIn Figure locations 4e (detail have center of Figure4c (detail of the 1983 orthorectiﬁed image) lies the potential location of the ancient ofnot the been 1998 excavated orthorectified and that image), one cannot one distinguishcan clearly the see lo thecation expansion of the ancient of the theater. Gymnasium In Figure excavation 4e (detail theater. Northwest of the location, above the southern inner wall, the excavated basilica is shown. andof the for 1998 the firstorthorectified time the emergence image), one of canthe Sanctuclearlyary see (north the expansion of and in of contact the Gymnasium with the Gymnasium excavation andTo the for south, the first the time Gymnasium the emergence (323–31 of B.C.) the Sanctu excavationary (north site is of presented. and in contact Finally, with west the of Gymnasium the location, the cover of a Hellenistic period building can be observed, while between the potential position of the

theater and the Hellenistic period building one can observe negative vegetation marks [20], suggesting the existence of buried constructions (Figure4d, shown in yellow). Figure4d shows the contour lines using the orthorectiﬁed image as a base map. Heritage 2018, 2, x FOR PEER REVIEW 7 of 14 site). The same applies for Figures 4f (detail of the 2005 orthorectified QuickBird-2 satellite image)

Heritageand 3h2018 (detail, 1 of the 2007 orthorectified images). Figure 4g presents the vegetation (in red) using the312 infrared band of the satellite image; the names of the mentioned locations are also presented.

Figure 4. a Figure 4. ((a)) The The terrain terrain relief relief at at the the location location of ofthe the ancient ancient theater theater using using the the2-m 2-minterval interval contour contour line line of NMCA 2007; (b) detail of the 1945 orth rectified image; (c) detail of the 1983 otho rectified of NMCA 2007; (b) detail of the 1945 orth rectified image; (c) detail of the 1983 otho rectified image; (d) image; (d) 1983 orthorectified image overlaying linear negative vegetation marks and contour lines 1983 orthorectified image overlaying linear negative vegetation marks and contour lines (a); (e) detail (a); (e) detail of the 1998 orthorectified image; (f) detail of the 2005 orthorectified QuickBird (QB) of the 1998 orthorectified image; (f) detail of the 2005 orthorectified QuickBird (QB) 2 image (bands: 2 image (bands: red, green, blue); (g) orthorectified QB2 image displaying the names of buildings red, green, blue); (g) orthorectified QB2 image displaying the names of buildings and presenting and presenting vegetation in red by exploiting the infrared band (bands: near infrared, green, blue); vegetation in red by exploiting the infrared band (bands: near infrared, green, blue); (h) detail of 2007 (h) detail of 2007 orthorectified image. orthorectified image.

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The observation of Figure4b (detail of the 1945 orthorectified image) reveals that these locations have not been excavated and that one cannot distinguish the location of the ancient theater. In Figure4e (detail of the 1998 orthorectified image), one can clearly see the expansion of the Gymnasium excavation and for the first time the emergence of the Sanctuary (north of and in contact with the Gymnasium site). The same applies for Figure4f (detail of the 2005 orthorectified QuickBird-2 satellite image) and 3h (detailHeritage 2018 of the, 2, 2007x FOR orthorectifiedPEER REVIEW images). Figure4g presents the vegetation (in red) using the infrared8 of 14 band of the satellite image; the names of the mentioned locations are also presented. It is obvious obvious that that at at the the potential potential location location of of th thee ancient ancient theater, theater, low low plants plants were were cultivated cultivated up up to to1983, 1983, while while from from 1998 1998 and and onward onward trees trees were were mainly mainly cultivated cultivated (for this (for reason this reason no new no newimages images after after2007 2007were wereused, used, as it asis itimpossible is impossible for formarks marks of ofthe the possible possible position position of of the the theater theater to to be be found). found). Another important feature is the underground drainagedrainage pipeline passing through the Gymnasium, which is directed northeast to the potential location of the ancient theater. This pipeline might have been used to drain rainwater from the ancient theater.theater. Combined with the above structures (Temple andand Gymnasium), the possible location of the ancient theatertheater reveals reveals a uniquea unique architectural architectural ensemble ensemble of the of ancient the ancient city, thus city, providing thus providing an indication an ofindication its existence. of its existence. For thethe acquisitionacquisition of of high-spatial-resolution high-spatial-resolution images images covering covering the potential the potential location location of the ancientof the theater,ancient theater, a drone a was drone used. was used.

3.2. Flight Plan-Data Processing The flight flight height was set to 85 m above the terrain surface (the highest point of the study area), area), allowing the acquisition of high-spatial-resolutionhigh-spatial-resolution images with a Ground Sampling Distance (GSD) varying from 2 to 3 cm [32,37]. [32,37]. The The flight flight plan plan was was re realizedalized by four four flight flight strips strips with with a a forward forward overlap of 80% and a side overlap of 40% (Figure5 5).). InIn total,total, 5656 imagesimages werewere acquired.acquired.

Figure 5. Drone flight ﬂight plan.

® Agisoft PhotoScan® softwaresoftware was was used used for the photogrammetric processing of the acquired images. TheThe processingprocessing was was realized realized using using the the Greek Gr Geodeticeek Geodetic Reference Reference System System 87 coordinate 87 coordinate system (GGRS87system (GGRS87 (EPSG:2100)). (EPSG:2100)). The first The step first was step the was automated the automated alignment alignment of the imagesof the images using theusing “high the ® accuracy”“high accuracy” option option in Agisoft in Agisoft PhotoScan PhotoScansoftware.® software. For the generation of the point cloud, the “high“high quality” option was selected (to specify the desired reconstruction quality, and higher quality settings can be used to obtain more detailed and ® accurate geometry). At At the the stage of densedense pointpoint cloudcloud generation,generation, AgisoftAgisoft PhotoScanPhotoScan® calculates depth maps for every image. To To sort out the outliersoutliers generated during the point cloud generation, ® Agisoft PhotoScan® hashas several built-in filtering filtering algorithms. Since Since the the area to be reconstructed does not contain meaningful small details, the “aggressive” depth filtering filtering mode was selected to sort out most of the outliers. For the aerial triangulation, eight Ground Control Points measured with GPS in the GGRS87 coordinate system were used. The image overlap was 80%, the image sidelap was 40%, and the camera was well calibrated. Therefore, the internal geometry of the bundle was quite rigid and the eight GCPs were adequate for the exterior orientation. The GCPs were measured with a planimetric accuracy of 7–10 mm and a height accuracy of 9–17 mm. The aerial triangulation was computed with an error of 0.048 m. For the mesh generation, the “height field” option was selected in the surface type option (The height field surface type is optimized for modeling planar surfaces, such as terrains or bas-reliefs).

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For the aerial triangulation, eight Ground Control Points measured with GPS in the GGRS87 coordinate system were used. The image overlap was 80%, the image sidelap was 40%, and the camera was well calibrated. Therefore, the internal geometry of the bundle was quite rigid and the eight GCPs were adequate for the exterior orientation. The GCPs were measured with a planimetric accuracy of 7–10 mm and a height accuracy of 9–17 mm. The aerial triangulation was computed with an error of 0.048 m.

HeritageFor 2018 the, 2, x mesh FOR PEER generation, REVIEW the “height field” option was selected in the surface type option9 of 14 (The height field surface type is optimized for modeling planar surfaces, such as terrains or bas-reliefs). TheThe dense dense point point cloud cloud was was selected selected as as source source data data (to (to specify specify the the source source for for the mesh generation procedure),procedure), which which thus thus generated generated a a high-quality high-quality output output based based on on the the previously generated dense dense pointpoint cloud. cloud. Moreover, Moreover, for for the the creation creation of ofthe the text texture,ure, the the“orthophoto” “orthophoto” option option in mapping in mapping mode mode was selectedwas selected (in this (in mode this mode the whole the whole surface surface of the of object the object is textured is textured in the in orthographic the orthographic projection). projection). The “mosaic”The “mosaic” option option was wasselected selected in blending in blending mode mode (w (whichhich selects selects the the way way pixel pixel values values from from different different photosphotos will will be be combined combined in in the the final final texture), texture), givi givingng higher- higher- qualit qualityy orthophoto orthophoto and and texture texture atlases, atlases, sincesince it it does does not not mix mix image image details details of of overlapping overlapping ph photosotos but but rather rather uses uses the the most most appropriate appropriate photo. photo. Finally,Finally, the Digital Surface Surface Model Model (DSM), (DSM), Digital Digital Terrain Terrain Model (DTM), and ortho ortho image image of of the the study study areaarea were were generated generated (Figure 66)) withwith aa spatialspatial resolutionresolution ofof 33 cmcm [[32].32].

Figure 6. Photogrammetric processing results. (a) Orthorectiﬁed image; (b) Digital Surface Model Figure 6. Photogrammetric processing results. (a) Orthorectified image; (b) Digital Surface Model (DSM); and (c) Digital Terrain Model (DTM). Height variation: 79.3 m (black)–123.5 m (white). Spatial (DSM); and (c) Digital Terrain Model (DTM). Height variation: 79.3 m (black)–123.5 m (white). Spatial resolution: 3 cm. resolution: 3 cm. 3.3. Determining the Probable Position of the Buried Theater’s Orchestra Center 3.3. Determining the Probable Position of the Buried Theater’s Orchestra Center Based on the assumption that the terrain relief follows the inclinations of the buried ancient theater’sBased koilon, on the itassumption is possible that to determine the terrain the relief potential follows center the inclinations of the buried of theater’sthe buried orchestra. ancient theater’sThe study koilon, of the it contouris possible lines, to determine created using the pote thential DSM, center provides of the an buried additional theater’s tool orchestra. to be used The in studyachieving of the this contour goal (Figure lines, created7a). In theusing area the of DSM, the contour provides lines an thatadditional resembles tool theto be koilon used morphologyin achieving this(Figure goal7 a,b),(Figure chords 7a). wereIn the placed area of randomly the contour on each lines contour that resembles line. The the vertical koilon lines morphology originating (Figure from 7a,b),the middle chords of were the chordsplaced willrandomly theoretically on each intersect contour at line. the centerThe vertical of the lines buried originating theater’s orchestra.from the middleIn our study,of the thechords lines will intersected theoretically at more intersect than oneat th point.e center As of a result,the buried the mean theater’s of all orchestra. the intersection In our study,points the was lines selected intersected as the potentialat more than center one of point. the orchestra. As a result, The the absolute mean of height all the of intersection the selected points point waswas selected 90.1 m. as the potential center of the orchestra. The absolute height of the selected point was 90.1 m. 3.4. Terrain Cross-Sections 3.4. Terrain Cross-Sections The nearest theater to Amphipolis is the theater of Philippoi, (approximately 55 km away), which is stillThe standing nearest today. theater Philippoi to Amphipolis was an ancient is the city theate of Easternr of Philippoi, Macedonia (approximately (Greece), and 55 its locationkm away), has whichbeen populated is still standing since the today. Neolithic Philippoi era. Itswas history an ancient began city in 360/359 of Eastern B.C., Macedonia when colonists (Greece), from and Thasos its location has been populated since the Neolithic era. Its history began in 360/359 B.C., when colonists from Thasos established the first city at that location, “Krinides”. In 356 B.C., when the city was threatened by the Thracians, they asked for the help of Philippos II. Philippos II, realizing the economic and strategic importance of the city, occupied and fortified the city, renaming it Philippoi. The city flourished during both the Macedonian and Roman periods. Via Egnatia passed through the city [20]. Apostle Paul founded the first Christian church on European soil in this city. The ancient theater of the city goes back to times of the Macedonian king (in the middle of the fourth century B.C.) and it has the addition of an epitheater. It was surveyed in 2012 during test flights of an RC (Remote Control) Balloon [22]. Besides the acquisition of images (Figure 8a), Ground Control Points were also measured using a GPS receiver. The objective was the generation of a high-spatial-

Heritage 2018, 1 315 established the first city at that location, “Krinides”. In 356 B.C., when the city was threatened by the Thracians, they asked for the help of Philippos II. Philippos II, realizing the economic and strategic importance of the city, occupied and fortified the city, renaming it Philippoi. The city flourished during both the Macedonian and Roman periods. Via Egnatia passed through the city [20]. Apostle Paul founded the first Christian church on European soil in this city. The ancient theater of the city goes back to times of the Macedonian king (in the middle of the fourth century B.C.) and it has the addition of an epitheater. It was surveyed in 2012 during test flights of an RC (Remote Control) Balloon [22]. Heritage 2018, 2, x FOR PEER REVIEW 10 of 14 HeritageBesides 2018 the, 2 acquisition, x FOR PEER ofREVIEW images (Figure8a), Ground Control Points were also measured using10 a GPSof 14 receiver. The objective was the generation of a high-spatial-resolution orthorectified image (Figure8b) resolution orthorectified image (Figure 8b) and a DSM of the theater. The inclination of the theater’s resolutionand a DSM orthorectified of the theater. image The (F inclinationigure 8b) and of thea DSM theater’s of the koilontheater. is The 42% inclination based on of the the produced theater’s koilon is 42% based on the produced 3D model. koilon3D model. is 42% based on the produced 3D model.

Figure 7. (a) The orthorectified image and the contour lines (colored yellow) with 2-m intervals. The Figure 7. 7. (a()a )The The orthorectified orthorectiﬁed image image and and the the contour contour lines lines (colored (colored yellow) yellow) with with 2-m 2-mintervals. intervals. The blue rectangle denotes the area of the terrain relief that was used (b) for the determination of the blueThe bluerectangle rectangle denotes denotes the thearea area of the of the terrain terrain relief relief that that was was used used (b (b) )for for the the determination determination of of the potential center of the orchestra. (b) Contour lines (green), with the placement of a random number potential center ofof thethe orchestra.orchestra. ((bb)) Contour Contour lines lines (green), (green), with with the the placement placement of of a randoma random number number of of chords (red) on each contour line. The vertical lines originating from the middle of the chords (blue) ofchords chords (red) (red) on on each each contour contour line. line. The The vertical vertical lines lines originating originating fromfrom thethe middlemiddle of the chords (blue) and the potential location of the center of the orchestra (black circle) are also shown. The potential and the potential location of the center of the orchestraorchestra (black circle) are alsoalso shown.shown. The potential center is also presented in (a,c). The potential location of the koilon and the orchestra are presented center is alsoalso presentedpresented inin ((aa,c,c).). TheThe potential potential location location of of the the koilon koilon and and the the orchestra orchestra are are presented presented in in white. The location of the cross-sections is presented in yellow. white.in white. The The location location of of the the cross-sections cross-sections is presentedis presented in yellow.in yellow.

Figure 8. (a) The overlap of the 2012 images (spatial resolution:resolution: 1.5 cm) acquired with an RC balloon Figure 8. (a) The overlap of the 2012 images (spatial resolution: 1.5 cm) acquired with an RC balloon at the location of Philippoi’s ancient theater (personal archive). ( b) The orthorectiﬁedorthorectified image (spatial at the location of Philippoi’s ancient theater (personal archive). (b) The orthorectified image (spatial resolution: 3 cm) of Philipoi’s an ancientcient theater (personal archive). resolution: 3 cm) of Philipoi’s ancient theater (personal archive). Assuming that both theaters (the Amphipolis and Philippoi Theaters) have similar planimetric Assuming that both theaters (the Amphipolis and Philippoi Theaters) have similar planimetric dimensions and height differences, the documentation of the dimensions (top view and height dimensions and height differences, the documentation of the dimensions (top view and height differences of its architectural parts) of the theater of Philippoi are used to study the potential location differences of its architectural parts) of the theater of Philippoi are used to study the potential location of the buried ancient theater of Amphipolis. Using the center (dimensions X and Y) of the orchestra, of the buried ancient theater of Amphipolis. Using the center (dimensions X and Y) of the orchestra, as identified above, the orientation and the absolute altitude of the orchestra (dimension Z) as identified above, the orientation and the absolute altitude of the orchestra (dimension Z) underground were approximately chosen (Figures 7c and 9). The above shows that, through the underground were approximately chosen (Figures 7c and 9). The above shows that, through the following cross-sections, the ancient theater of Amphipolis could be located under the positions following cross-sections, the ancient theater of Amphipolis could be located under the positions noted in study. noted in study. The AA’ cross-section (Figures 7c and 9) bisects the buried theater’s koilon. The highest point of The AA’ cross-section (Figures 7c and 9) bisects the buried theater’s koilon. The highest point of the natural ground is 121.41 m (left, absolute height) and the lowest height of the natural ground is the natural ground is 121.41 m (left, absolute height) and the lowest height of the natural ground is 81.5 m (right, in the vicinity of the Gymnasium and the Temple). The buried orchestra has an absolute 81.5 m (right, in the vicinity of the Gymnasium and the Temple). The buried orchestra has an absolute

Heritage 2018, 1 316

Assuming that both theaters (the Amphipolis and Philippoi Theaters) have similar planimetric dimensions and height differences, the documentation of the dimensions (top view and height differencesHeritage 2018, of 2, x its FOR architectural PEER REVIEW parts) of the theater of Philippoi are used to study the potential11 of 14 location of the buried ancient theater of Amphipolis. Using the center (dimensions X and Y) of theheight orchestra, of 82.0 as m. identiﬁed The surface above, model the orientation is represente andd theby a absolute black line, altitude which of describes the orchestra the cross-section (dimension Z)of underground both the vegetation were approximately (trees and bushes) chosen and (Figures the prot7ectivec and 9covers). The of above the archaeological shows that, through structures the of followingthe Gymnasium cross-sections, (right). the It is ancient obviou theaters that at of some Amphipolis sections could the surface be located model under is the the same positions as the notedterrain inmodel. study. The depth of the koilon and the orchestra ranges from 0.7 m to 8.1 m.

Figure 9. Cross-sections AA’, BB’, CC’, and DD’. The terrain model (Figure6c) is shown in blue, the Figure 9. Cross-sections ΑΑ’, ΒΒ’, CC’, and DD’. The terrain model (Figure 6c) is shown in blue, the surface model (Figure6b) is shown in black. The approximate absolute height of the buried orchestra surface model (Figure 6b) is shown in black. The approximate absolute height of the buried orchestra is 82 m (red circle). The intersection of the koilon and the orchestra is presented in red. is 82 m (red circle). The intersection of the koilon and the orchestra is presented in red. The AA’ cross-section (Figures7c and9) bisects the buried theater’s koilon. The highest point of the naturalThe BB’ ground cross-section is 121.41 is m perpendicular (left, absolute height)to the AA’ and cross-section the lowest height and intersects of the natural the two ground tiers is as 81.5well m as (right, the inends the of vicinity the koilon. of the The Gymnasium terrains andlocate thed Temple).to the left The and buried right orchestraof the cross-section has an absolute have heightabsolute of 82.0 heights m. The of 103.63 surface m model and 107.1 is represented m, respectively, by a black while line, its whichlower level describes lies at the 90.1 cross-section m. The terrain of bothrelief the follows vegetation the morphology (trees and bushes) of the andburied the theater protective across covers the oflength the archaeological of the cross-section. structures The ofburied the Gymnasiumtheater is placed (right). at a It depth is obvious ranging that from at some 1.7 m sections to 8.1 m. the surface model is the same as the terrain model.In The the depth CC’ cross-section, of the koilon the and theater the orchestra is placed ranges at a depth from 0.7 ranging m to 8.1 from m. 0.4 to 8.1 m, while in the DD’The cross-section BB’ cross-section it is placed is perpendicular in a depth ranging to the AA’from cross-section 1.8 m to 8.1 m. and In intersects both cross-sections, the two tiers the as terrain well asrelief the ends follows of the the koilon. geomorphol The terrainsogy of the located buried to thetheater. left and right of the cross-section have absolute heightsTherefore, of 103.63 if m the and ancient 107.1 theater m, respectively, was oriented while as itsshown lower in levelFigure lies 7c atand 90.1 the m. orchestra The terrain was located relief at an altitude of 82.0 m (absolute), it is possible that the position under study is identical to the position of the ancient theater, as the terrain follows its slope and shape.

4. Conclusions It is obvious that the use of geo information tools such as multi-temporal orthorectified images and DTM combined with existing studies and oral testimonies allow for the discovery of the potential location of buried archeological sites. In this study, the potential location of the ancient theater of

Heritage 2018, 1 317 follows the morphology of the buried theater across the length of the cross-section. The buried theater is placed at a depth ranging from 1.7 m to 8.1 m. In the CC’ cross-section, the theater is placed at a depth ranging from 0.4 to 8.1 m, while in the DD’ cross-section it is placed in a depth ranging from 1.8 m to 8.1 m. In both cross-sections, the terrain relief follows the geomorphology of the buried theater. Therefore, if the ancient theater was oriented as shown in Figure7c and the orchestra was located at an altitude of 82.0 m (absolute), it is possible that the position under study is identical to the position of the ancient theater, as the terrain follows its slope and shape.

4. Conclusions It is obvious that the use of geo information tools such as multi-temporal orthorectiﬁed images and DTM combined with existing studies and oral testimonies allow for the discovery of the potential location of buried archeological sites. In this study, the potential location of the ancient theater of Amphipolis was determined by using and combining the available geospatial data (terrain relief, aerial and satellite images) with existing studies and testimonies of the area’s residents. Drones are a useful digital image acquisition platform that, when coupled with high-accuracy GCPs, can generate various high-resolution geospatial outputs. To determine the potential location of the ancient theater, the DSM, DTM, and ortho image of the study area were generated with a spatial resolution of 3 cm. The generated outputs led to the determination of the orchestra’s center by exploiting geometric design rules of ancient theaters and studying multiple terrain cross-sections in various directions within the area encompassing the potential location of the ancient theater. As a result, alternative scenarios can be generated for the dimensions, the orientation, and the accurate position of the ancient theater. The methodological procedure that was followed in this paper could be used for other covered ancient theaters in Greece. However, this is not an easy task since we know very little regarding the location of covered ancient theaters based on existing texts. Furthermore, no testimonies or marks exists to point to a speciﬁc location. Finally, in addition it is very time-consuming to acquire permits from the Ministry of Culture and the Ephorate of Antiquities to study these areas. In this study, the potential location of the ancient theater of Amphipolis was successfully determined, but an archaeological excavation of the site can only verify the generated results.

Author Contributions: All the authors contributed equally in this research. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

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