Photogrammetry for the Preparation of Archaeological Excavation (Part II)

4. PHOTOGRAMMETRIC DATA PROCESSING

4.1 Multi-image photogrammetry

Photogrammetric solutions (Grussenmeyer et al., 2002) are either based on the processing of single images (e.g. image rectification of plane objects), stereoplottings (for stereopairs of photos), or multi-image restitutions when a set of convergent photos of an object is available. The last solution has been chosen for our project in order to process in one block the sets of photos taken from the different types of cameras. We used the PhotoModeler software package from EOS System (Canada), well known for its applications in close range photogrammetry, architecture and archaeology.

The preparation steps in order to process the images were the following:
- definition of the “camera” file for each type of camera: the « camera calibrator » module of PhotoModeler has been used for the calibration of the digital cameras;
- for the archive images, an approximate camera file has been edited (camera with four fiducial marks) for further “on the job calibration”;
- edition of the control point table (from the geodetic survey);
- scan of the film or paper-based images (for the set of archive images and the color slides from the Nikon F3 camera).

The basic steps in this type of project performed with PhotoModeler are :
- choice two or more overlapping photographs from different angles of the object ;
- use the point and line tools to mark on the photographs control and tie points;
- reference the points by indicating which points on different photographs represent the same location on the object (homologous points);
- process referenced data to produce 3D model;
- check the adjustment and view the resulting 3D model in the 3D viewer ;
- extract coordinates, distances, curves, surfaces, textures etc. within PhotoModeler;
- export the 3D model to rendering, animation or CAD program.

4.2 Calibration of the cameras

Four types of images are used for this project:
- images from a Nikon Coolpix 5000 camera used in the extreme positions of the zoom (7mm and 21mm). This camera was calibrated in the two positions using the calibration grid proposed in the PhotoModeler software (module "camera calibration");
- images taken with a traditional reflex camera Nikon F3, equipped with a Nikkor 55mm objective. The calibration was carried out like previously after scanning the slides of the calibration grid;
- images (prints or copies of film, see paragraph 3.1) of 1930-1936 scanned with a resolution of 900dpi: unfortunately no photogrammetric information on the characteristics of the cameras used has been found. Several types of glass-platesbased cameras with focal distances of 20 to 30cm were used at that time for the acquisition of aerial and terrestrial views (Roussilhe, 1936). The focal length of the camera used for this archive images is written on the border of the image (26 cm) and the original size of the images (glass plates in the 1930’s) was supposed to be 24 cm x 18 cm.

4.3 Adjustment of the block of the images of 2003

More than 130 images were realized in spring 2003 with the Nikon F3 and Nikon Coolpix cameras (a hundred images on the ground and about thirty in the helicopter). A first block of 18 oblique views of the castle was oriented in June 2003 using the control points materialized on the ground by targets (figure 6). Standard deviations of about 10 cm on the co-ordinates of the points measured on the images have been obtained. We then selected in this project a set of points simultaneously visible on the images of 1931-1936, in order to define approximations of the photogrammetric parameters, and to calculate a block based on a selection of these archive images.

4.4 Adjustment of a block of archive images

Initially we selected 5 oblique photographs (approx. scales of 1/1000 to 1/2000) in order to document archaeological vestiges of the North and West parts (Grussenmeyer & Yasmine, 2003). The approximate values of the internal and external orientations of the archive images have been computed within the module "process & autocalibration" of PhotoModeler with the help of control points defined in §4.3.

4.5 Merger of the two projects and preparation of the restitution

At this stage, the two separately calculated projects are referred to the same reference system. The process of merging (of several projects) proposed in the PhotoModeler software allows the restitution of three-dimensional elements simultaneously on the photographs of 1936 and 2003, and to superimpose and display these results within a unique project.




Figure 9. Overview of the camera stations and the block configuration after the exterior orientation. At the right: a vertical view of 1935 allowing the location of the camera stations.

4.6 Addition of other images in the project

Depending on the parts of the castle to model, any other recent or archive image can be imported and oriented within the project by identification of homologous points.

5. PREPARATION OF ARCHAEOLOGICAL EXCAVATION

From the block of images previously oriented, the restitution initially carried out in 2D in Autocad (e.g. fig. 7 and 8) will be enriched by a 3D restitution corresponding to the objectives of the documentation. We are then able to calculate and draw the visible structures from the aerial and terrestrial photographs of 2003 by digitalizing on the images. One will proceed in an identical way on the archive photographs to measure the
archaeological structures currently covered by the remains of the war currently embanked or destroyed (fig. 10), or to plot in 3D parts of the castle (see §6).




Figure 10. Restitution of the embankment areas: on the left, the boundary of the embankment is marked on a photo of 2003 and on the right, the corresponding surface is projected on an archive photo of 1936

The superposition of the two restitutions makes it possible to highlight the areas of interest (figures 10 and 11) for archaeological work and to estimate work of civil engineering (fill, cubature, rebuilding, etc).

Moreover, the restitution of the site in wire frame and surface model can be supplemented by the addition of textures coming either from the images of 1936, or of those of 2003. From the resulting 3D photomodels, orthophotos can be calculated for the two periods, as e.g. for the frontages of the castle or the installations around the site covering the archaeological structures (fig. 11).

1936

2003
Figure 11. Archaeological structures are measured on the archive images of 1936 and the corresponding surface is shown on the photos of 2003


Figure 12. Example of orthophoto of the archaeological structure (computed from an archive image of 1936)

6. RESTITUTION OF THE HISTORICAL STRUCTURES NOWADAYS DISAPPEARED

The aim of the restitution is to help the consultant to establish the framework of the excavation and restoration tasks, by showing the destroyed historical structures.
The study is currently focused on different parts of the castle (figure 13). For some of them (as courses of bricks or stoneworks ”label B”), a 3D restitution of both archive and recent images is possible.
Parts recovered by embankment are modelled from only archive images (watering place “label A”, counterscarp “label C” and archaeological remains on the western part of the castle “label D”).


Figure 13. Vertical archive view showing some studied parts


Figure 14. Recent image showing the surroundings of the castle and the current constructions

7. CONCLUSION

This project shows the importance of photogrammetry within the framework of 3D documentation of the castle of Beaufort, for the development of the site and the landscape. By using the old aerial photographs taken by French air force of the Levant between 1931 and 1936, a comparative restitution makes it possible to locate in a precise way the visible archaeological structures on the archive photographs. This documentation helps the consultant to establish the framework of the excavation and restoration tasks by showing the destroyed historical structures. A photogrammetric restitution of the whole set of photographs carried out in April 2003 is planned in the future, as a preparation of the restoration of the castle and its surroundings. The contractor who will be in charge of this work will then have all information to release the great quantity of fill around the castle, in order to preserve intact the historical and archaeological structures which would be still in-situ below the embankment.
Such a project shows also the interest of a cooperation between the architect and the photogrammetry expert for both simple and complex tasks. On the one hand, the orientation process combining different kind of archive and digital images requires scientific tools and experience in the handling of images with different geometry. On the other hand, the photogrammetric documentation (in this project one day only was enough for the aerial and terrestrial images) as well as the restitution work can easily be done by the architect.

8. REFERENCES

References from Journals:
Grussenmeyer, P., 2003. Photogrammétrie architecturale et modélisation 3D du patrimoine. Revue de l’Association Française de Topographie, 2e trim. 2003, N°95, p.30-36.

References from Books:
Deschamps, P., 1939. Les Châteaux des Croisés en Terre Sainte II: La Défense du Royaume de Jérusalem, étude historique, géographique et monumentale, vol. "Album", Bibliothèque Archéologique et Historique, Tome XXXIV, Paris, Librairie Orientaliste Paul Geuthner, Plate LV.

Grussenmeyer, P., Yasmine, J., 2003. The Restoration of Beaufort Castle (South-Lebanon): A 3D Restitution According to Historical Documentation. In XIXth CIPA International Symposium, Antalya, Turquey. Sept. 30th. Oct 4tht, 2003. ISPRS International Archives of Photogrammetry, Remote Sensing and Spatial Information Systems Vol. XXXIV-5/C15 ISSN 1682-1750 pp. 322-327.

Grussenmeyer, P., Hanke, K., Streilein, A., 2002. Architectural photogrammetry. Chapter in « Digital Photogrammetry ». Edited by M. Kasser and Y. Egels, Taylor & Francis, pp. 300-339.

Huot, J.-L., Salem Kardous, A., 2001. Photographies du Levant. Institut Français d’Archéologie du Proche-Orient, Beyrouth, Liban, 165 pages.

Philipson, W.R. (Editor), 1997. Manual of Photographic Interpretation, 2nd Edition. Edited by the American Society for Photogrammetry and Remote Sensing, Bethesda, Maryland, 689 pages.

Roussilhe, H., 1936. La photogrammétrie et ses applications générales, Tomes 1 et 2. Encyclopédie industrielle et commerciale, Librairie de l’enseignement techniques, L. Eyrolles (Paris).

References from Other Literature:
Bessac J.-C. et Yasmine J., 2001. Etude préliminaire des chantiers de construction du château de Beaufort, in BAAL, N°5, p.241-320.

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