New insights into urban planning of Caričin Grad: The application of modern sensing and detection methods

Caričin Grad, Justiniana Prima, urban planning, fortification, settlement, aerial photography, geophysical surveys, LiDAR, photogrammetry, excavations, GIS. Thanks to the application of modern non-destructive sensing and detection methods, in recent years a series of new data on urban planning in Caričin Grad was obtained. For the most part, the current research programme studies the Upper Town’s northern plateau, wooded until recently and hence the only previously unexplored unit of the city. In the course of this programme, the classical research method - the excavations started in 2009 - is for the first time combined with the systematic application of airborne and terrestrial sensing and detection techniques. The analysis of historic aerial photographs and topographic plans proved to be very useful as well. Along with them, LiDAR-derived DTMs, photogrammetric DEMs, different geophysical and orthophotographic plans are stored in the GIS database for Caričin Grad and the Leskovac Basin. In this way almost 80 percent of the plateau area was defined, and the obtained plan is hypothetical only to a small extent, which particularly refers to the unexcavated northern rampart of the Upper Town. Each source provided relevant information for the reconstruction of both the rampart and the settlement, which points to the value of a holistic approach to documentation from various dates. The first source to be studied were archival aerial photographs of Caričin Grad from 1938 and 1947 (Figs. 1, 2.1). The latter one was originally processed by Aleksandar Deroko and Svetozar Radojči}, who drew the plan of the town after it, labelling the unexplored Upper Town’s northern plateau as “a probable habitation area”. The route of the northern rampart was aslo rather precisely determined by the authors (Fig. 2.2). Recently, these photographs were rectified and georeferenced in the GIS. The 1938 shot reveals the position of some towers as well, and it is also indicative of the way of construction of certain buildings. From the spatial layout of whitish zones, originating from mortar scattered along the slope, it can be deduced which buildings were constructed in opus mixtum - the horreum and the so-called Building with Pillars east of it. Traces of mortar can be observed along the route of the rampart too. These archival images are particularly important because they record the topography of the site before it was filled with heaps of earth from the excavations. The topographic mappings of this area were conducted in 1981 and 2006 (Fig. 3). The first plan was drawn after an airborne stereophotogrammetric survey of Caričin Grad, and in 2006, after the wood was cut down, this whole area was surveyed with the total station, with a density of nine points per square meter. This survey also resulted in a 3D terrain model (Fig. 3.2) indicating the layout of the buildings, which was to be proved by geophysical surveys and archaeological excavations. In the course of the Serbian-French reaearch programme, in 2007 geomagnetic surveys were carried out by Alain Kermorvan of the University of Tours. Thanks to the application of this method the remains of collapsed stone structures could be observed, and in 2015, in cooperation with the Roman-Germanic Central Museum, Mainz, and the Ludwig Boltzmann Institute from Vienna, the middle and eastern parts of the plateau were scanned with GPR (Fig. 4.2). Precise plans of the buildings were obtained in the areas in which LiDAR scanning and photogrammetric and geomagnetic surveys failed to produce clear images. Within the framework of the ArchaeoLandscapes Europe project, in 2011 we managed to organise an airborne LiDAR survey of the wider area of Caričin Grad. With its density of some 20 points per square meter, this scanning proved to be crucial for our comprehension of the town. The standard DTM provided numerous important data, especially its version calculated in the focal statistics function of the ArcGIS software package (Fig. 5. 1-2). These models show not only the route of the Upper Town’s northern rampart, the position of its towers and the layout of the buildings, but also the line of the Outer Town’s western rampart. Visible only in the DTM, this entirely new aspect of the Caričin Grad fortification has been attested by the excavations. Highly important plans of the town, and of the northern plateau of the Upper Town in particular, were obtained by UAV photogrammetric surveys. The first drone survey was conducted in 2014 within the scope of the same project. It resulted in a cloud with up to 1,600 points per square meter (Fig. 6.1-2). Unlike the LiDAR technology, photogrammetry cannot penetrate vegetation; therefore the preliminary clearing of the ground proved to be a most important step. After the 2015 campaign was finished, the excavation area in the Upper Town was documented again in the same manner. Regular photogrammetric surveys make possible the control of the works and reliable visual monitoring of the progress of exploration (Fig. 9). After the wood was cut down in 2006 and enormous heaps of earth from twentieth-century excavations and restoration works were carefully removed by machinery in 2008 and 2010, without disturbing the original layers of debris, wide excavations could begin. At first only the humus layer was removed from fifteen-meter squares, which was followed by technical drawing. In 2009 and 2010 we did not explore the debris or the cultural layers (Fig. 7.1-2). The additional two squares were opened and documented in the same fashion in 2011, when previously recorded buildings 11 and 15C were explored in detail, together with the part of the corridor between them where a bread oven was found. These buildings were oriented south-north, cascading along the mild slope towards the northern rampart of the Upper Town. Fragments of pithoi and carbonised fruits were found in the buildings, allowing for an economic interpretation. Judging by coinfinds, the buildings ended in fire after the year 602. Some of the buildings on the northern plateau were oriented differently, following the route of the northern rampart of the Acropolis in the east-west direction. In 2012 building 18 was excavated, leaning on the rampart. Rectangular in plan and some 12 by 7.5 meters large, it had a storey and a 7 by 5.5 meters spacious paved atrium in the west. Parallel to building 18 is building 20, the only one on the northern plateau constructed in opus mixtum. The two buildings are separated by a four-meter-wide street, running from east to west. This street, corridor 4, was partly cut in the rock. In some sections it had a substructure of fragmented debris. Building 20 has been carefully excavated for several years now. After the initial documenting, the surface layer of debris was removed, but not the collapsed structures with characteristic construction details; to the east of the building a collapsed wall was uncovered, containing as many as eight successive rows of stone and brick. Beneath these layers are the occupation ones, so far investigated only to a small extent. Building 20 is rectangular in plan, covering 25 by 12.5 meters. In its central axis there is a row of masonry pillars, dividing the building into two naves. On its western side there was a vestibule with a pair of doors matching the main entrances to the building. In the back of the vestibule, between these entrances and in axis with the pillars, there was a staircase. Adetailed analysis of these features led us to conclude that building 20 was a horreum, the first such edifice to be discovered in Caričin Grad. Taking into account the details of its ground plan, pillars, parts of collapsed walls and especially arches, it will be possible to reconstruct the original form of the horreum. Judging by the existing estimate, although somewhat rough, it was 13.5 meters high. It could be observed that in its later phases the horreum was partitioned into several rooms, and some of its entrances were walled up. In the vestibule only these later occupation phases were documented, as the original brick pavement was removed from its northern part. This was followed by a significant accumulation of cultural layers, which were sealed by the debris stratum. South of the horreum there is a spacious courtyard connected with the western street of the Upper Town. The Upper Town’s northern rampart has never been graphically reconstructed, despite the fact that Aleksandar Deroko and Svetozar Radojči} published its accurate (although schematic) ground plan as early as 1950 (Fig. 2.2). This part of the town has gradually been left out of the research focus, mainly due to the vegetation growth. Upon employing all the methods described above, however, it is possible to undertake such an effort. The ideal reconstruction suggested here includes the rampart route, the disposition and the form of the towers, and the possible locations of the posterns. The line of the rampart can be traced following the trenches left by the locals dismantling the walls. Only the section of the northwestern rampart in front of the western postern of the Acropolis cannot be presented, being still covered by massive earth deposits. On the other hand, the recently discovered western rampart of the Outer Town can be traced to its full length in the LiDAR-derived DTM. Its form can be easily reconstructed on the basis of the results of the 2012 excavations and the section of the same rampart uncovered east of the main fortifications in 1955-56 (Fig. 8). Having studied the microtopography of the terrain, we were able to determine the position of a number of towers. They were clearly indicated by bumps, regularly distributed along the northern and northeastern sections of the rampart. The position of the tower below the Acropolis’ western postern could be easily determined as well, unlike the position of the tower opposite to the horseshoe-shaped one of the Acropolis fortification. Yet, it is hard to imagine that a hundred-meter-long section of the rampart was left unprotected. The rectangular shape of the towers is suggested because almost all the towers of the town’s outer fortification were constructed in that way. On the other hand, at present we cannot exclude the possibility that some towers were different, horseshoe-shaped in plan, like the ones on the Acropolis rampart. The disposition of the towers along the northeastern rampart of the Upper Town, in the area where the northern street presumably met the fortification, is not clear. This part of the site still lies under massive heaps of earth, and even the 1938 and 1947 aerial photographs are not indicative enough in this regard. However, the tower(s) might have been erected there, not only because the eighty-meter-long stretch of the rampart would be left without protection in an opposite scenario, but because it is likely that the northern street ended in a gate, or at least a postern. It is already known that some of the posterns on the Carič in Grad fortifications were defended by towers. The average distance between the towers of the town’s main fortification extends from 20 meters on the southern to 40 meters on the western rampart of the Lower Town; in our reconstruction the average interval on the Upper Town’s northern rampart is 44 meters. Another argument is that this gate might have connected the Upper and the Outer Towns. The position of the second postern is determined thanks to a depression in the terrain following the axis of another communication route in the Upper Town, leading from corridor 4 and running towards the north along the rows of buildings. Finally, the 3.8 meter width of the rampart in the section adjoining the northern tower of the Upper Town’s eastern gate may only indicate a staircase, the last reconstructed fortification element. On the plateau stretching between the northern ramparts of the Acropolis and the Upper Town fortifications a settlement developed with its radially distributed rows of buildings cascading down the slope. In the eastern part of the plateau there is the horreum, adjoined from the east by another building - the storage called Building with Pillars. Larger than the other buildings and constructed in opus mixtum, the two buildings follow the route of the Upper Town’s northern street, all of which indicates that they belong to the initial construction phase. One should not exclude the possibility that this part of the town was originally conceived as an economic district with storages and similar edifices. By all appearances, the original concept was soon abandoned. Already at the time of Justinian a settlement of numerous smaller buildings was created. With their walls of stone and wattle and daub, the buildings were roofed with tiles. Yet one should underscore that this construction phase, although less sophisticated than the first one, was accomplished according to a previously prepared plan; the spread of the buildings speaks to that effect. Shortly afterwards, if not at the same time, buildings were erected along the outer face of the Acropolis rampart - a clear indication of abandoning urban planning (Fig. 9). Public space was turned into private, in spite of the legal proscriptions of that time. During the last phase of the town’s life the buildings described, whether public or private, were partitioned into small rooms, often with fireplaces and with some of their entrances walled up. Just like the edifices constructed in opus mixtum, some of the more modest buildings from the second construction phase were used to store food - namely buildings 11 and 15C. The plan of this part of the site points to an organised settlement, most probably inhabited by persons servicing a significant clergy and administration. On the other hand, except for some houses - such as building 18 - small buildings along the Acropolis fortification, facing the main street, corridor 4, might have served as shops and workshops. Traces of furnaces, slag and bone working were also encountered in this area. The parallel application of classical research methods and modern techniques of sensing and detection enabled the reconstruction of the northern rampart and the urban matrix of the Upper Town’s northern plateau. Until recently among the least known parts of the town, this unit can now be regarded as one of the best defined. This is important not only for our understanding of Caričin Grad (Justiniana Prima), but also for the study of Early Byzantine urban planning in general