Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Title Geomorphological traces of conflict in high-resolution elevation models Author Ralf Hesse State Office for Cultural Heritage Berliner Strasse 12, 73728 Esslingen am Neckar, Germany e-mail: ralf.hesse@rps.bwl.de Abstract High-resolution digital elevation models, often derived from airborne lidar, are rapidly gaining importance in both archaeology and geomorphology, in particular where these two disciplines overlap in their interest in anthropogenic changes to the relief of the earth surface (“archaeogeomorphology”). Inter-group and inter-state conflict are one aspect of human behaviour which commonly causes such relief changes. Conflict archaeology and conflict geomorphology, which are both young sub-disciplines within their scientific fields, have until now only touched upon a small part of the wide range of issues which they can encompass. While conflict archaeology has for a long time been almost synonymous with battlefield archaeology, the few papers explicitly discussing conflict geomorphology are mainly concerned with the impact of bombing on soil geomorphology. The application of highresolution digital elevation models in investigating past conflicts can and should, however, encompass all geomorphological traces of conflict. These include defensive structures such as earthworks, primary and secondary traces of warfare itself (e.g. bomb craters and rubble mountains), conflict-related traces associated with military training and weapons testing facilities as well as, potentially, traces of conflict sustenance (e.g. conflict-related mining and infrastructure). Examples highlight the potential of high-resolution digital elevation models for the detection, mapping and quantification of conflict-related relief changes and thus for the understanding of conflicts. As suitable data are becoming increasingly available, the study of prehistoric and historic conflicts will benefit across the discipline boundaries between archaeology and geomorphology. In the field of heritage management, the detection, Accepted manuscript, published in Applied Geography 46 (2014) 11-20 visualisation and protection at landscape-scale of what is often seen as “dark” heritage is expected to gain importance. Keywords Lidar, conflict, warfare, fortification, bomb craters, geomorphology, earthworks Highlights 1. • Geomorphological traces of past conflicts are widespread features. • Their recognition is facilitated by high-resolution digital elevation models. • An overview of common conflict-related archaeogeomorphological features is given. Introduction Traces of past conflicts are common features in the present-day landscape. They are, however, frequently unrecognised, overlooked or regarded uninteresting and thus often underrepresented in registers of archaeological sites. For example, out of the presumably thousands of relicts of the Word War II “Siegfried Line” (“Westwall”) along Germany’s western border (not counting trenches, approx. 3500 structures had been built in today’s federal state Baden-Württemberg; Kieser, 2010) only 25 (all of them bunkers) have been recorded in the state-wide archaeological data base of Baden-Württemberg as of November 2012. Only in recent years, their historical and archaeological significance has been recognised (Fings and Möller, 2008b), and an effort is made to map and compile information on the remnants of the “Siegfried Line” (Kieser, 2010). Recognising traces of past conflict can help to improve our understanding of these conflicts, including their spatial extent and their temporal development or the strategies and technologies employed. In many cases, comprehensive mapping of conflict-related features within a given area is desirable to provide sufficient data for analysis. The study of geomorphological traces of past conflicts is per se an transdisciplinary endeavour as it combines elements of geomorphology and archaeology, but also military geography and peace and conflict studies. Conflict archaeology as a sub-discipline of archaeology has been rapidly evolving since the late 1990s, and since 2005 the Journal of Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Conflict Archaeology is dedicated to this field of research. Conflict archaeology has, however, to a large part been concerned with battlefield archaeology, and battlefield archaeology has been the primary interest of the founding editors of the Journal of Conflict Archaeology. As recognised by the founding editors, this focus on one aspect of conflict, battlefields, does not address the full breadth of the subject which is now becoming evident from the papers published in that journal (Pollard and Banks, 2005). Transdisciplinary links between conflict archaeology and geomorphology, however, are still uncommon. In contrast to conflict archaeology, conflict geomorphology as a sub-field of archaeogeomorphology (cf. Thornbush, 2012) is represented by only a few papers (e.g., Hupy and Schaetzl, 2006, 2008; Hupy and Koehler, 2012; Stal et al., 2010), and does not yet appear to be recognized as a scientific term or as a sub-discipline of geomorphology. As of September 03, 2012, a Google Scholar search resulted in 271 entries (excluding citations) containing the search term “conflict archaeology”, but none for “conflict geomorphology”. The combination of the search terms “conflict archaeology” and “geomorphology” returned only seven results. This apparent lack of interest is surprising, given the abundant and sometimes drastic impacts of conflict on the Earth surface. Often this is due to the fact that geomorphological impacts of conflict are not explicitly discussed. The few papers so far published explicitly on conflict geomorphology have mainly been concerned with the impact of bombturbative processes on the soilscape (Hupy and Schaetzl, 2006, 2008). In the field of conflict research, environmental concerns have largely focused on ecology, negative impacts on biological diversity and pollution (e.g. Machlis and Hanson, 2008; Hanson et al., 2009; Francis, 2011; Hanson, 2011; Gorsevski et al., 2013) rather than on geomorphological impacts. The aim of this paper is to outline the intersecting field of conflict archaeology and geomorphology (“conflict archaeogeomorphology”), to provide an overview of geomorphological impacts of past conflicts and to emphasize the potential of high-resolution digital elevation models (DEM) in the study of morphological traces of past conflicts. It will become apparent that landscape approaches, in which such DEM are valuable tools, are indispensable in the study of past conflicts. Finally, the paper addresses aspects of the management of negative or “dark” heritage related to past conflicts. 2. High-resolution digital elevation models in archaeogeomorphology Accepted manuscript, published in Applied Geography 46 (2014) 11-20 2.1 Acquisition techniques In recent years, high-resolution digital elevation models have rapidly gained importance in the fields of both archaeology and geomorphology. This is due to rapid technological advances leading to an increasing availability and quality of such data and to growing possibilities for data manipulation and analysis. The main technological advance has been the development of airborne lidar (light detection and ranging), also known as ALS (airborne laser scanning). It can rapidly provide high-resolution topographic data sets for very large areas. Canopy penetration by the laser beam and subsequent analysis of sequential signal returns or of the full waveform of the individual laser signals allows filtering algorithms to remove nonsurface points (e.g., vegetation) from the data sets (cf. Doneus and Briese, 2011, for an overview of recent progress). As the work with high-resolution lidar data and DEMs entails the acquisition and processing of enormous amounts of data, the increasing computing power of modern PCs plays an important role in the growing use of such data sets. The readily available computing power has also led to a surge in software products and applications of multi-view photogrammetry (structure from motion), often using consumer-grade digital cameras (Westoby et al., 2012). In particular in areas lacking vegetation cover, this approach can be used to generate digital elevation models of very high resolution. Other sources for intermediate- to high-resolution digital elevation models are airborne or satellite synthetic aperture radar (SAR), for example the results of the SRTM mission with a ground resolution of approximately 30-90 metres (USGS, 2006) and the more recent TerraSAR-X / TanDEM-X with a ground resolution of 12 m in regular acquisition mode (Krieger et al., 2007) and a ground resolution better than 2 m in spotlight acquisition mode (Maurer et al, 2012). While the resolution of these SAR-based elevation models is still coarser that that provided by airborne lidar, they are useful for larger features and can be an important supplementary data source for the study of topography and landscape around known sites. 2.2 Visualisation techniques In addition to the increasing availability of lidar-based and other high-resolution elevation data and progress in the fields of spatial resolution and vegetation filtering algorithms, there Accepted manuscript, published in Applied Geography 46 (2014) 11-20 has been rapid development of new and adoption of existing visualisation techniques. Besides the “conventional” Shaded Relief (cf. Imhof, 2007), high-resolution DEM can be visualised using numerous techniques. Each of these techniques has advantages and disadvantages with respect to particular types of relief features and landscapes. For example, Shaded Relief is very adaptable to different topographic settings and relief features but suffers from poor visibility of linear features aligned parallel to illumination azimuth and from optical illusions (inverted relief) for azimuths between 90° and 270°. Sky-View Factor, on the other hand, is well suited for the visualisation of small topographic depressions and features on slopes but will produce poorer results for low relief features on horizontal surfaces. Below, an overview of a variety of currently used visualisation techniques is given. Shaded Relief visualisation simulates directional visualisation of the DEM from a point light source at a specified illumination azimuth and elevation (Imhof, 2007). By changing illumination direction, visibility of selected relief features can be enhanced. Principal Component Analysis can be applied to a set of multiple Shaded Relief images; the first few principal components can be used as visualisations which combine the visible relief features from all input images (Devereux et al., 2008). Like Shaded Relief, Exaggerated Relief simulates directional illumination from a point light source; however, it is a multi-scale approach in which illumination elevation is locally adapted to maximize feature visibility at each scale (Rusinkiewicz et al., 2006). In contrast to Shaded Relief, Sky-View Factor visualisation simulates a diffuse illumination of the DEM from a homogeneously bright hemisphere centred over each DEM pixel (Zakšek et al., 2011; Kokalj et al., 2011). Similar to Sky-View Factor, Openness visualisation is based on diffuse illumination of the DEM; however, it is extended to also allow illumination from negative elevation angles, i.e. based on a full sphere instead of a hemisphere (Yokoyama et al., 2002). Trend Removal algorithms (e.g. subtraction of a low-pass filtered DEM from the original DEM) can be used to highlight small topographic differences. Local Relief Models are computed by an advanced trend removal algorithm (Hesse, 2010). Local Dominance visualisation depicts how dominant an observer is with regards to its local surroundings, i.e. the average steepness of the angle under which an observer placed at a DEM pixel would look down onto the surrounding pixels within a specified radius range (Hesse, subm.). Cumulative Visibility depicts the percentage of the area (surrounding each pixel within a given radius) which is visible for an observer positioned at that pixel (Hesse, subm.). Accessibility visualisation is based on an algorithm which, for every pixel in the DEM, computes the maximum radius of a sphere that could be Accepted manuscript, published in Applied Geography 46 (2014) 11-20 placed on the surface at this position without being impeded by the heights of surrounding pixels (Miller, 1994). Multi-Scale Integral Invariants (MSII) is an analysis in which for n spheres with different diameters, centred on each DEM pixel, the percentage of each sphere above and below the DEM surface is computed. The resulting sets of n values for each pixel are interpreted as n-dimensional vectors, and the distance to a reference vector (e.g. the origin of the n-dimensional coordinate system) can be computed (Mara et al., 2010). Most of the above visualisation techniques are implemented in the Open Source software LiVT (Hesse, in press). Selected visualisation techniques have been compared by Kokalj et al. (2012) and Bennett at al. (2012). 2.3 Applications Increasing data availability, processing power and highly adaptable visualisation techniques have made high-resolution DEMs almost universally useful data sets. The fields of application in archaeology include archaeological prospection on local to regional scales (e.g. Doneus and Briese, 2001; Hesse, 2013), documentation as well as on-site and off-site topographic analyses like slope, aspect, visibility, cost surfaces and least cost paths (e.g. Davis et al., 2012; Posluschny, 2012; Mlekuž, 2013). These approaches are followed both in archaeological research and in heritage management and protection. The availability of and engagement with data for such analyses has contributed to an increasing preoccupation with landscape in recent archaeological research (e.g. Kluiving and Guttmann-Bond, 2012). Many applications of high-resolution DEM in archaeology have in common that they are concerned with anthropogenic relief changes, i.e. with archaeogeomorphology. The abundance and multitemporality of such anthropogenic traces has led to a view of “messy landscapes” (Mlekuž, 2012). 3. Geomorphological impacts of conflict 3.1. Traces of prehistoric to early modern conflict Geomorphological traces of prehistoric to early modern conflict are almost exclusively related to defensive structures. This is largely due to the lack of powerful explosives capable of displacing large amounts of material. Pre-modern conflict geomorphology is thus largely limited to the study of defensive earthworks. These include fortified settlements such as Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Neolithic hilltop enclosures, Iron Age oppida (Figure 1) and medieval town fortifications as well as larger structures protecting frontiers or state borders such as the Roman limes, the Danevirke or the Great Wall of China, stretches of which consist of an earthen wall (Clapp, 1920). In many post-medieval conflicts, earthworks were constructed along front and siege lines and around the perimeters of contested towns and areas such as during the American Seven Years War (Millard et al., 2009), the War of the Spanish Succession (Lynn, 1999) or the War of the Polish Succession (Figure 2). It has been disputed whether the existence of fortifications can generally be interpreted as an indicator of conflict or warfare. However, while the construction of earthworks does not necessarily indicate violent conflict, it documents at least a perceived threat or a demonstration of strength and power (Armit et al., 2007). Due to erosion, intentional removal as well as unintentional levelling and infilling by later agriculture, many pre-historic to early modern fortifications are reduced to low banks and shallow ditches. Well-preserved earthworks such as the Heidengraben oppidum with its 2.5 m high bank and 1.5 m deep ditch are clearly visible in Shaded Relief or Sky-View Factor visualisations (Figure 1). More shallow features like the early modern fortifications near Philippsburg, most of which have elevation differences of less than one metre, often require visualisation techniques such as Local Relief Model (Figure 2 b) or Local Dominance (Figure 2c) to allow unambiguous delineation. 3.2. Primary traces of modern “industrial” warfare As a result of the development of powerful explosives and of increasingly capable means of delivery to the target such as airplanes and artillery, together with industrial mass production and immense armies, many violent conflicts since the late 19th century can best be described as “industrial” warfare (e.g., Archer et al., 2002). “Industrial” warfare has culminated in the notion (and realisation) of “total war” (cf. Ludendorff, 1935) which became part of the Nazi ideology of racial superiority and domination (Chickering et al., 2005). In the form of “mutual assured destruction”, it was also the underlying concept of the “deterrence” strategies of the superpowers during the Cold War (Jervis, 2002; Sokolski, 2004). The range of morphological traces of conflict now encompasses not only fortification structures along front lines such as trenches and bunkers but includes direct evidence of Accepted manuscript, published in Applied Geography 46 (2014) 11-20 warfare such as bomb, mine and artillery craters. In particular due to the mass-use of bombs and artillery grenades, conflicts have gained the potential to considerably alter the geomorphology of contested areas. This is perhaps most conspicuous on the large battle fields of World War I, for example Verdun (Hupy and Schaetzl, 2008), Kemmelberg (Stal et al., 2010) or the Somme battlefields (Hertzog, 2012). Historical aerial photographs have been used together with geophysical investigations (electromagnetic induction) to assess the archaeological heritage of World War I in Belgium, including bunkers, ruins, fire and communication trenches, mine craters and military roads and railways (Stichelbaut et al., 2011). While most geomorphological traces of warfare have been obliterated in areas which have since been ploughed or built over, many such traces still exist in forested areas. Without the aid provided by historical documents such as legacy aerial photographs, the mapping of such features until recently required time-intensive field survey, an often prohibitive approach given the large spatial scale of modern warfare. Irrespective of present-day forest cover, highresolution DEM based on airborne lidar now provide the means to efficiently detect and map morphological traces such as trenches, bunkers and bomb craters along the World War II “Siegfried Line” (Figure 3). As for prehistoric to early modern features, the optimum choice of lidar visualisation depends on the state of preservation. While conventional Shaded Relief visualisation can be used to map well-preserved, more than one metre deep bomb craters (Figure 4a), this is often not sufficient for shallow or poorly preserved features (Figure 3b) where techniques such as Local Dominance (Figure 3c) or combinations of different visualisations (Figure 3d) are much more appropriate. 3.3. Secondary traces of warfare Besides the abovementioned primary geomorphological expressions, 20th century warfare has left many less self-evident traces. In the case of cities heavily destroyed by bombing during World War II, for example, large amounts of rubble were commonly deposited at one or a few dedicated localities within or near the city, creating rubble mountains containing commonly several million cubic metres (Forßbohm, 2011). Examples for this are “Birkenkopf” and “Grüner Heiner” near Stuttgart, Germany (Figure 4b). High-resolution digital elevation models can be used to determine the volume of material deposited at these sites. Secondary use of such localities to dump household or industrial waste in some cases obscures the Accepted manuscript, published in Applied Geography 46 (2014) 11-20 primary, conflict-related character of these anthropogenic landforms. Other secondary impacts of conflict on geomorphology include changes in the functioning of the landscape, i.e. changes in geomorphological processes due to primary conflict-related topographic changes (Hupy and Schaetzl, 2008). 3.4. Warfare traces without war Conflicts do not only leave their geomorphological traces on battlefields or along frontlines. Closely related to modern warfare are military training and weapons testing facilities, commonly located far away from any contested areas. Examples include military training areas (e.g., tank tracks, Gilewitch, 2003), shooting ranges and bomb and artillery training sites (Figure 5). Exceptionally large and conspicuous examples of such facilities are nuclear weapons test sites in the USA (Powell, 2012) and the former USSR. The craters created by underground nuclear tests are recognisable in the relatively coarse-resolution SRTM data (USGS, 2006; approx. 30 m for the USA) (Figure 6). Sedan crater, the largest anthropogenic crater, has a diameter of 390 m and a depth of 98 m (Hutchings et al., 2005). 3.5. Traces of conflict sustenance It can be argued that the field of conflict geomorphology could or even should also include morphological traces conflict sustenance. This would mainly have to encompass conflictrelated mining activities (e.g. uranium mining during the “Cold War”) as well as military roads and other infrastructure. However, such a broad scope could lead to an allencompassing classification of any anthropogenic structure in heavily militarised countries (such as Germany during World War II, where almost the entire economy was geared towards war-related production) as conflict-related. This would hardly provide a means for further analysis. In other cases or under a more constrained definition of “conflict related” features, such a perspective could be taken to quantify the total geomorphological impact of a conflict. 4. Conflict archaeogeomorphology and heritage management Traces of past conflicts – from prehistoric to modern – are part of the cultural heritage. Many heritage management issues apply similarly to conflict archaeogeomorphology as they apply to other types of archaeological heritage. However, due to the role of violent conflicts in the Accepted manuscript, published in Applied Geography 46 (2014) 11-20 shaping of nations and states and the immense human suffering inflicted by warfare, the material remains of violent conflict are much more emotionally and politically loaded: they are typical examples for negative or “dark” heritage (cf. Biran, 2011). While “dark heritage” is a relatively recent term, its principle may have been in operation at least since classical antiquity when monuments were erected to commemorate wars or battles; however, generally with a focus on triumph (i.e., with a positive connotation) rather than defeat or wrongdoing (Hope, 2003; Holliday, 2002). Traces of conflicts in the distant past today only rarely have a negative connotation. For example, the Roman Limes in south-western Germany is presently not perceived as a negative monument of foreign occupation or oppression. Traces of more recent conflicts like World War I and II (including monuments of fascist oppression) or the “Cold War” do have a negative connotation for much of the population in many countries. This has led to a classification as negative or “dark” heritage or even as “evil places” (Porombka and Schmundt, 2005) or even as “no-place” as in the case of the Nevada test site (Powell, 2012; cf. Figure 6). In recent years, there has been a growing interest (scientifically as well as public interest and heritage tourism) in such places and traces subsumed as negative heritage (e.g., Ashworth and Hartmann, 2005; Stone, 2006) even to the level of World Heritage (Rico, 2008). For the World War II “Westwall” or “Siegfried Line”, the sometimes conflicting issues of heritage management, interpretation and protection as well as public presentation and perception have been the topics of a dedicated conference (Fings and Möller, 2008b). Notably, the primary reason for the recent heritage management interest in the “Westwall” is not so much a preoccupation with historical or heritage issues but concerns about misinterpretation and misrepresentation by small private museums as well as publications and websites focussing mainly on decontextualised perspectives such as bunker architecture and military technology. It is argued that the presentation of the “Westwall” should instead focus on its historical context and emphasise its role in the fields of warfare, propaganda and ideology of expansion and racial superiority of Nazi Germany. The heritage management task is mainly seen as having to counteract the perpetuation of Nazi propaganda regarding the “Westwall” (Fings and Möller, 2008a; Fings, 2008; Otten, 2008). The challenge of managing heritage resources therefore goes well beyond physical landscapes and requires dark tourists and planners to confront (and at times challenge) historical narratives and propaganda in place. Thus, the management of the “dark” heritage (including heritage tourism) related to past conflicts clearly is an endeavour embedded in the scientific as well as political discourse. Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Irrespective of the sometimes contested interpretation of conflict-related heritage sites, one issue which has been gaining importance in the past years in both archaeological research and heritage management is the increasing recognition of and focus on landscapes rather than individual sites. This has been reflected for example by the biannual Landscape Archaeology conferences (e.g. Kluiving and Guttmann-Bond, 2012). Because of their spatially extensive nature and spatio-temporal developments, violent conflicts can only be fully understood by studying them on the landscape (or, for some modern conflicts, regional, continental or even global) scale. The application of high-resolution DEMs – which allows to visualise and map geomorphological traces of conflicts irrespective of present-day vegetation cover – can therefore provide valuable insights to improve our understanding of past conflicts. Beyond these scientific aspects, however, the management of heritage sites on a landscape scale poses challenges due to the conflicting interests of site protection, tourism and agriculture and forestry. While individual sites can be easily delineated, protected and presented for heritage tourism, this is usually not the case for landscapes, even where traces of warfare are densely spaced such as in the World War I battlefields (cf. Hertzog, 2012). 5. Conclusions Geomorphological traces of past conflicts are common archaeological features in many landscapes. An overview of commonly encountered impacts of violent conflict on geomorphology illustrates the broad scope of the overlapping fields of conflict archaeology and conflict geomorphology. Traces of past conflict in the prehistoric to early modern periods are largely limited to defensive earthworks. In the era of modern “industrial” warfare, violent conflicts have left widespread and large-scale traces. These include trenches, bunkers, flak dugouts, bomb, mine and artillery craters. Besides these primary traces of modern warfare, secondary traces, in particular rubble mountains, are identified. Furthermore, geomorphological traces of modern conflict are not constrained to contested areas or battlefields but include military training and weapons testing facilities as well as, arguably, traces of conflict sustenance such as war-related mining. As high-resolution digital elevation models become increasingly available and play an increasingly important role in both archaeology and geomorphology, they will increasingly benefit the detection, documentation and analysis of such traces. The application of Accepted manuscript, published in Applied Geography 46 (2014) 11-20 appropriate DEM visualisation techniques enhances the visibility of archaeogemorphological features. Detecting and mapping such features is a necessary first step for scientific analysis, protection and touristic valorisation. In contrast to traces of conflict from the more distant past, in particular the traces of twentieth century conflict such as World War I and II generally have a negative connotation, making them part of what has in recent years been recognised as negative or “dark” heritage. 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Remote Sensing 3, 398-415. Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Figures Figure 1: A section of the fortification of the late Iron Age oppidum Heidengraben near Grabenstetten, Germany. (a) Shaded Relief image, (b) Sky-View Factor visualisation. Lidar data provided by LGL. Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Figure 2: Early modern fortification near Philippsburg, Germany, related to the War of the Polish Succession 1733-1738. (a) Shaded Relief image, (b) colour-coded Local Relief Model draped over Shaded Relief, (c) Local Dominance visualisation. Lidar data provided by LGL. Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Figure 3: Trenches (T), bomb craters (C) and earth-covered bunkers (B) of the “Westwall” near Hügelsheim, Germany. (a) Orthophoto, (b) Shaded Relief, (c) Local Dominance and (d) Local Dominance draped over Shaded Relief. Lidar data and orthophoto provided by LGL. Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Figure 4: (a) World War II bomb craters in Ulm, Germany, are clearly visible in a vertically illuminated Shaded Relief image. (b) Lidar-based 3D rendering of the “Birkenkopf” near Stuttgart, Germany, a ~750,000 m3 artificial hill where rubble from the war-damaged city was deposited. Note bomb craters in the foreground. Lidar visualisation: Local Dominance draped over Shaded Relief. Lidar data provided by LGL. Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Figure 5: Former military training area near Münsingen, Germany, showing a heavily cratered surface caused by artillery training. Cratering has partially obliterated traces of medieval settlement and field patterns. Lidar visualisation: Local Dominance draped over Shaded Relief. Lidar data provided by LGL. Accepted manuscript, published in Applied Geography 46 (2014) 11-20 Figure 6: SRTM elevation model (USGS, 2006) of the Nevada nuclear weapons test site, USA (visualisation: Shaded Relief). Sedan crater is the largest crater on the northern margin of the site.