Using Photogrammetry to Reverse-Engineer a Headstone

While we employ Design X for our work with morphometrics, its primary use is for reverse-engineering parts (components or whole machines) based on 3D scan data. We are doing this with some of our ceramic scans, and it is providing us with a means to further isolate elements (lip, rim, body, neck, etc.) of Caddo ceramic design so that we might view the evolution of each, as well as their contribution to the whole (at various scales; assemblage, 25-mile increments from point of discovery, etc).

This model began as 70 images, which were subsequently processed through Autodesk 123D Catch to create the 3D model (non-invasive/non-destructive). Once uploaded and published to our gallery of markers from the Oak Grove Cemetery in Nacogdoches, Texas, we downloaded the .stl, unzipped the file, then imported it into Design X.

Fig2

We used planes as a basis for several mesh sketches that were then extruded as solids. Once extruded, each piece was merged with the other parts of the whole. For the sphere atop the marker, we opted to extract a primitive solid. The body of the marker has a draft of 1.5%, meaning that it gets smaller near the top.

Fig3

The model includes a variety of fillets at varying angles, lengths, and orientations. Using the 3D mesh as baseline data, we inferred these measurements for areas of the marker that had degraded over time. This is particularly true for the base of the marker, which has endured the business end of weed-eaters and projectiles from mowers over the last century.

Fig4

Once completed, we used the Live Transfer feature in Design X to shift to Autodesk Inventor Pro 2015, where we made a few final changes. The transfer is my favorite part of the process – since Design X is a history-based modeling software, it “builds” your model in Illustrator (or SolidWorks, or AutoCAD) while you watch.

Fig5

As you may have noticed, the photogrammetry model was not scaled (see the bottom left for any of the Design X images), so the models were subsequently scaled to the appropriate dimensions in post. The utility of this approach can contribute to conservation and preservation dialogues, while yielding accurate (+/-0.3mm in this case) models for a variety of applications. More to come on this line of inquiry soon.

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Written by zselden

Selden (PhD, Texas A&M University, 2013) is a US Marine Corps veteran, cyclist, kayaker, backpacker, hiker, climber, fisherman and general all-around outdoor enthusiast. His research is focused at the confluence of archaeological methods and digital technology, and he is particularly interested in the application of 3D technologies to archaeological problems, geometric morphometrics, network analyses, predictive modeling, archaeological theory, and archaeological science.