Vol. IX, No. 1

May, 1996

Photogrammetry Reconsidered

H. Eiteljorg, II

Using photogrammetry at Pompeii in the summer of 1995 allowed us to get accurate information about a great many details on the walls in the Sanctuary of the Genius of Augustus. It was also very labor-intensive. Having gone through the work of preparing materials on-site for photogrammetry and then using those materials at the CSA office to work on the surveying of the building, we are now in a position to assess the utility of the method.

First, it is clear that a superior level of detail is available through this process; it is possible to obtain details that one would not normally try to obtain with a total station because of the time required. That said, however, there remain problems. One is photo interpretation; the computer operator must spend a great deal of time trying to determine what he or she is seeing, and the more the operator must consult the archaeologists in charge the lower the efficiency of the method becomes. No operator could be expected to make all decisions without help, but more assistance was required than I had anticipated. Color photographs might have helped, but they would have increased the costs as well. Color slides to augment the black-and-white photogrammetry images would have provided considerable help without adding as much to the cost, and I will, for the future, recommend using color slides to accompany the black-and-white prints. The slides should be taken at the same time of day as the black-and-white photographs so that shadows are in the same locations. They should be augmented by other slides not taken from the same vantage point or at the same time of day. (Of course, the slides and the model maker should be in the same place; we were making the model in Bryn Mawr while the best slides were in Charlottesville.)

A second problem is introduced when one uses the system on several adjacent surfaces. Each surface, standing alone, is a relatively simple project. However, when several individual photos and their respective surfaces must be connected, the problems with orientation mount quickly. The result is a very much slower process. Each photograph must be oriented in concert with others, and no single section can be completed in isolation. Not only does that add to the time required, it also makes it very difficult to segment the work efficiently. This would not be much of a problem with walls that are vertical, but walls that have begun to collapse present significant problems of this sort.

Using a total station instead of photogrammetry has different drawbacks. First, one must get every point required while in the field; there is only a second survey opportunity when one returns to the site. In the case of excavation survey, of course, there is no second chance.

Second, the time required for performing a very detailed survey of a wall, for instance, can be an enormous burden in the field, even if the total station is dedicated to that task alone.

Third, when doing detailed survey work, the operator may well need the active assistance of the archaeologist in charge while working on the survey. That is often impossible.

Weighing and comparing these two approaches requires careful consideration of the value of the system for the project at hand. Photogrammetry works best with single, flat surfaces, and it can be used to survey very irregular markings on those surfaces accurately and efficiently. A total station, on the other hand, will be more efficient on complex surfaces with, at most, moderate surface detail.

On a site with balks that should be surveyed with special precision, photogrammetry will be the best approach if the lighting will permit good, crisp images. Similarly, a mosaic found in situ would be best surveyed with photogrammetry if the surface remains reasonably flat, and a standing wall with pieces of revetment still in situ would present another good candidate for photogrammetry.

The total station will be required, even for surface details, if the basic surface has been distorted and is no longer flat enough to permit the use of single-photo photogrammetry.

Neither system of surveying is appropriate if the details to be recorded are not significant. If the technology can provide accurate survey measurements, that does not mean the archaeologists must take those accurate measurements, only that they can when appropriate. We could, for instance, measure every brick or tufa block in the walls at Pompeii with the photogrammetry system, but to what purpose? Were the walls constructed of cut stone without mortar, on the other hand, such measurements would be very valuable.

The walls of the Sanctuary of the Genius of Augustus at Pompeii provide a good case study. We needed to record the shapes of the walls and information about the surviving plaster. Originally, we thought the actual shapes of the surviving plaster patches were important enough to justify measuring them, and I underestimated the time required to do the photogrammetry work. As we worked and as the time required became apparent, it also became apparent that the dimensions of the plaster patches were not important enough to justify the time required to take them. The condition of the walls can be illustrated effectively with photographs; key dimensions can be taken, and the resulting model will provide all necessary detail. In that case, the surveying of the plaster patches seems to have been overkill.

Photogrammetry for the plaster patches was the correct technology, in the sense that it was the most efficient approach, and we will still include it in our arsenal. Indeed, we will certainly make sure that we can use photogrammetry on virtually all parts of the forum, should the need arise in the future, by taking good photographs with appropriate care. One of the important virtues of photogrammetry, after all, is the ability to return to the photos years later for more information.

But we will now rely upon photographs and fewer surveyed measurements in order to obtain the appropriate level of detail for the walls of the Sanctuary of the Genius of Augustus, and we will apply that standard to our survey procedures elsewhere in Pompeii as well.

An example of a drawing made with the aid of photogrammetry (Fig. 1) and a similar drawing with reduced detail (Fig. 2) may help make the point. A photograph of the area is also included as Fig. 3. It seems clear that the detail in Fig. 1, when compared with the detail in Fig. 2 plus the information from the photograph, does not really add information of a value equal to the cost of obtaining it.

Fig. 1 Fig. 2 Fig. 3

For other Newsletter articles concerning applications of CAD modelling in archaeology and architectural history or Pompeii, consult the Subject index.

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