Divergent Views in CAD ModelsHarrison Eiteljorg, II
In the last issue of the Newsletter I discussed the need to retain competing views of archaeological material in a database (see Harrison Eiteljorg, II, "Many Heads Are Better Than One," CSA Newsletter, XX, 2; Spring, 2007; http://csanet.org/newsletter/spring07/nls0704.html.) This concern about competing interpretations of archaeological material was brought home recently in a different setting: CAD models from a project that is still on-going. In this case -- CAD models from the site of Gordion, Midas' home in Anatolia. The data sources are not only older drawings and CAD models; there is also new material coming from the site, which is still active. Important interpretations have not been fully settled in all cases because work is still on-going. As a result, the CAD files created (mostly by Dr. Phoebe Sheftel, Director of the CSA Gordion Project) are data in flux; the layer names may or may not be accurate when the site is fully understood and the phases have been sorted out.
Ms. Sheftel made the CAD models from the original drawings created in the early phases of the Gordion project. Those were the drawings on file at the University Museum of Archaeology and Anthropology, where the archival holdings from the project's first phase have been stored. For each CAD model there are many drawing layers, each with a distinct name. The names include information about phases, and Ms. Sheftel has found it necessary to do a good bit of scholarship to be sure she can apply the correct phase name to any given batch of material. This is a frustrating job because the site's chronology has been undergoing significant change over the last few years as a better understanding has been gained of the critical items that determine dating.
The current CAD models have, as a result, some layers that may need to be reinterpreted in the future. The resulting problem is similar to the one discussed in the article mentioned above about contradictory opinions in databases. How does one augment a CAD model over time as ideas, opinions, and knowledge change? Ms. Sheftel and I discussed this matter at some length and saw two distinct possibilities.
The first, preferred by Ms. Sheftel, assumes that a CAD model is a consistent view unto itself and need not -- indeed, should not -- contain competing views of the material. Rather, competing views should be put forth in separate models where the views will make each model internally consistent.
Having carefully provided for competing reconstructions in my older propylon model, I take a different position. In the Gordion example, the issue is not competing reconstructions but competing interpretations of excavated material. Nevertheless, I prefer to see one model with multiple, competing views retained in the single model. The use of well-crafted layer names makes it both possible and relatively easy to maintain different opinions. For instance, one layer may be named PISFWCLEP (indicating a plan view, P; of in situ remains, I; according to the scholar Sheftel, S; consisting of a fortification wall, FW; of cut limestone, CL; from the Early Phrygian period, EP), but another scholar might argue that the material should be seen as Middle Phrygian. In that case the same material would be on another layer with that scholar's name indicated instead of the S for Sheftel and with MP in place of EP to indicate Middle Phrygian. This makes it necessary to put identical model elements on multiple layers.
Both of these approaches to conflicting interpretations provide effective ways to permit contradictory views to remain active. Ms. Sheftel's approach makes each model the opinion of a single scholar and makes any given model internally consistent; mine allows each opinion to be retained in a single model but makes that model both more complex and more difficult to use. Another article in this issue of the CSA Newsletter, "Using AutoCAD to Construct a 4D Block-by-Block Model of the Erechtheion on the Akropolis at Athens, I: Modeling the Erechtheion in Four Dimensions," by Paul Blomerus and Alexandra Lesk, points to a different and quite possibly better way to honor scholarly differences. The next article in their three-part series will explain further. I, for one, am eager to know more about this alternate approach.
-- Harrison Eiteljorg, II