Patterns of Reconstruction at Pompeii
IATH, University of Virginia |
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Appendix F: Photogrammetric Transparency Study
The archaeological and structural interpretation of a heterogeneous masonry wall invariably requires comparing features the wall's opposite faces, particularly to areas and features at corresponding locations on the opposite faces. Such comparisons are difficult in the field both because of the need to move from one side of the wall to the other, and also because it is difficult to determine exactly corresponding locations on the two faces. The latter difficulty also applies to comparison by photograph.
To address this problem, the study investigated the use of photogrammetric models and image transparency to allow comparison of features on opposite faces of a wall. The subject wall is W10.28, the back wall of shop 9, immediately east of the north gate: shown in figure F.1 below, generated from the photogrammetric model.
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Figure F.1: A view of the photogrammetric model showing the north face of Area B, W10.28 at shop 9.
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The question concerning this wall is whether the large stone blocks visible in the upper portion of the north face correspond to an area of ancient plaster visible on the south face. Figure 7.2 below shows a single frame from a digital video clip which makes it possible to dissolve from the image to the north face on the photogrammetric model to the image of the south face, reversed as it would look from the north. The video clip is linked to the image.
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Figure F.2: A frame from a video which dissolves from the elevation of W10.28 to the corresponding reversed elevation of W28.10. The video makes it possible to compare corresponding features on opposite faces of the wall. The image is linked to the video clip.
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While the clip illustrates the potential utility of the technique, it also illustrates the special care that must be taken in applying it effectively. The dimensional accuracy of the model shown here is low, with clearly visible errors in the range of 0.3 meters. To achieve higher dimensional accuracy, it would be necessary to include many more points in the camera shops, including placing targets (such a colored tape) on the wall surfaces to locate points more reliably. Also, there are many cases where tight spaces make it difficult to get photographs which have sufficient angular separation to achieve high accuracy.
