Towards a 3D representation of Archaeological Layers

1. Towards a 3D representation of Archaeological Layers Juan A. Barceló, Oscar de Castro, David Travet, Oriol Vicente UNIVERSITAT AUTONOMA BARCELONA SPAIN

2. ABSTRACT In this presentation, we present how to acquire 3D data from the field, and the process of modelling an archaeological excavation. Once a model of the excavation has been obtained, then we can use the model to obtain an stratigraphic matrix in 3D, which gives much more information than standard approaches. The method also allows volume calculation, and the simulation of archaeological formation and transformation processes.

3. Archaeological sites do not exist before excavation.

4. Archaeological artefacts are not waiting for the patient archaeologist

5. ......but they are a consequence of the complex interplay of natural and human process.

6. An archaeological site is the place where social action “was” performed

7. Therefore,... Any physical space modified as a result of human agency is an archaeological site.

8. DIGGING FOR HISTORICAL EXPLANATIONS When digging archaeological sites, we are not concerned with the mere unearthing of objects, but with the explanation of social dynamics, that is the complex sequence of social actions being performed at a specific location through time.

9. LOOKING FOR DATA We are able to definehuman action only in terms of observable modifications in the natural features of physical spaces, that is, in terms of observable discontinuities.

10. LOOKING FOR DATA A wall, a pit, a garbage accumulation are discontinuities in the natural properties of physical space, and they can be described as spatially distributed values of a qualitative variable, that is a feature which has positive value if it is present, and negative value in case of absence.

11. LOOKING FOR DATA Our main assumption is that an archaeological site can be decomposed in terms of observable discontinuities.

12. LOOKING FOR DATA The contents of the information sources can be reduced to three basic data types: variables, observed discontinuities, and coordinates VARIABLES: such as concentrations of archaeological materials and other quantitative information properties OBSERVED DISCONTINUITIES: qualitative variables used as classifiers to differentiate spatial locations

13. LOOKING FOR DATA The preliminary step in any archaeological project is to define the variables and site components relevant to the project, in terms of type, identity, reporting attributes and functions. In the case of discontinuities, we should differentiate between:…..

14. 1) A pattern of changes in light wavelength and surface-reflectance, that is colour discontinuities

15. 2) A pattern of changes in edge orientation (curvature), that is shape discontinuities,

16. 3) A pattern of changes in the surface properties, that is texture discontinuities. We perceive this continuities in terms of luminance variations in an image with non-uniform reflectance,

17. 4) A pattern of discrimination between locations, that is, topology discontinuities

18. DATA ACQUISITION Observation The idea is to take digital pictures on the field, and through a careful processing, to extract observed discontinuties and to map relevant site components

20. DATA ACQUISITION 2. Measuring Digital images can be integrated with topographic points captured by means of a total station. Image and coordinate information is all we need to define a spatial model of the site.

21. DATA ACQUISITION 2. Measuring We measure the coordinates of the contour or outline of the observed discontinuity, as well as the elevation of its surface. Each discontinuity is then defined in terms of its location in a 3D space.

23. DATA ACQUISITION 3. Pre-Processing. Orthorectification Ortho-rectification is simply the process of removing scale variations from any image. Scale variations are caused by the natural point-to-point variations in the elevation of the terrain being imaged. Scale variations are also caused by the varying distances of objects out from the principle point of the camera, as it is a perspective projection.

24. DATA ACQUISITION 3. Pre-Processing. Ortho-rectification Once these variations in scale are removed from a photo, the photo becomes a true image map of the ground, where “map” is defined as a constant scale representation of a portion of the Earth’s surface.

25. SITE COMPONENTS It is the minimal spatial unit defined by a unitary value on a qualitative variable, with distinct boundaries Therefore, when we decompose a site, we are qualifying the subsurface space in terms of some relevant discontinuities, such as lithology, mineralogy characteristics or the presence/absence of some archaeological structure.

26. SITE COMPONENTS Site components are then defined in terms of the spatial relationships between observed discontinuities We can use geometric primitives (point, line, area, surface, volume), to represent changes in colour, texture, shape and topology values, and standard geometric functions for analysing spatial relationships between discontinuities.

27. VISUALIZING SITE COMPONENTS To be able to translate spatial relationships between observed discontinuities into a mapping of spatial components, we require a vector based data structure that translates the empirical into the virtual. A preliminary solution is a geometric model in 2D.

29. VISUALIZING SITE COMPONENTS Archaeological information is intrinsically 3 Dimensional. We need a volume data structure that accommodates complex, irregular 3D volumes associated with specific characteristic values.

30. VISUALIZING SITE COMPONENTS In the geometrical model of an archaeological site, each observed discontinuity is represented as a surface, and it is defined as composed of squared cells, similar to pixels, each having a colour and value that represent some condition. All the cells in a surface or DEM having a given value constitute a zone, and are displayed in the surface with the same colour.

31. Observed Discontinuity C1

33. Geo-Referentiation A normal byproduct of the modelling process is that observed discontinuitiesare also geo-referenced: the x and y axes of the surface arealigned with the east and north axes of the map projection being used, and each pixel in the component is geo-coded with its associated coordinate in the map projection.

34. VISUALIZING SITE COMPONENTS

35. VISUALIZING SITE COMPONENTS To build a spatial model of an archaeological site means much more than a simple correlation between observed discontinuities. Changes in shape, texture, colour and topology are represented in terms of surfaces, but site components are much more than simple discontinuities, and therefore they cannot be defined by only one surface.

36. VISUALIZING SITE COMPONENTS The principal objective of the volume data structure is to provide a means of qualifying subsurface space in terms of a relevant characteristic, such as lithology, mineralogy or the presence/absence of specific archaeological structures.

37. VISUALIZING SITE COMPONENTS The subsurface distribution of site components is represented by discrete, contiguous, irregular volumes, with uniform value throughout each volume. The structure of the site is then defined in terms of faults, contacts and other discontinuities.

42. Visualizing Site Components These representations are called VOXEL MODELS Here a volumetric data set is represented as a 3D discrete regular grid of volume elements (voxels) and is commonly stored in a volume buffer (also called cubic frame or 3D raster) which is a large array of voxels

44. Visualizing Site Components The requirement for interactive interpretation is satisfied by allowing volume elements to be defined from whatever sectional orientation is most appropriate to subsurface conditions. The requirement for spatial control of geostatistical estimation is satisfied by a volumetric integration with the grid data structure

45. Solids

46. VISUALIZING SITE COMPONENTS Every volume element in a volume data structure is associated with a value of the relevant site component. The characteristic value of a volume element serves several functions: * it controls the parameters used for the geostatistical estimation of a variable at points within the element volume * it also controls colour selection for visualization

47. VISUALIZING SITE COMPONENTS Voxel models are not “photographs” of archaeologicalcomponents, but visual models of the geometry of three dimensional data. Because they are not single pictures, geometric properties (curvature, length, thickness, height, volume, etc.) can now be measured on these models.

48. CONCLUSIONS Archaeological sites are not naturally divided into spatial units, nor topo-stratigraphical divisions are meaningful in themselves. That means that stratigraphic units, activity areas or any other spatial component do not exist as individual thing before excavation. They are analytical concepts we use to understand the spatial variability of observable discontinuities.

49. CONCLUSIONS It is the archaeologist who should decompose the site in order to define the best decomposition into meaningful units. And those units can be defined as those regions in 3D space were the probability of a specific formation process is the highest.

50. CONCLUSIONS The primary objective of decomposing the archaeological 3D space is concerned with the analysis of the spatial variation of one or more variables. In this context, a variable is a property of the archaeological record within a geological subsurface that exhibits spatial variation, and can be measured or sampled, in terms of real numeric variables