3-D mapping
By Greg Liggett

Introduction

Before becoming interested in vertebrate fossils, I worked as a land surveyor and gained valuable experience in mapping techniques. Combining the two interests, I have, therefore, been interested in detailed paleontological site documentation.

Excavation is a process of evidence destruction. While digging out fossils, we are destroying the context of the very thing we wish to study. Museums are filled with fossils which were "trophy hunted," excavated to get the prize specimen out of the rock for study or display, but without regard to the sedimentologic and taphonomic context.

Even in more recent years when the importance of the taphonomic context is recognized, the techniques of site documentation have been limited. For example, most researchers record field sketches and quarry maps, but the information from a two dimensional map is limited and can be misleading.

 

Click on the image to see a single bone example (the rotating "pork chop") of a 3-D bone as recorded in the field. This one is from the hip of a sauropod dinosaur excavated on the Comanche National Grassland.

I have been working for several years to develop a protocol for 3-D digital site reconstruction. I was thrilled to see a presentation at the 2002 SVP by Art Chadwick et al. on their work using ArcGIS for site reconstruction. This basic idea is to use detailed 3-D locations of excavation elements along with digital field photographs to make a digital model of the site that can be viewed from any spatial orientation.

The Comanche project marks the first time I have been able to pull together all the data for this type of modeling, and here are the first results of those efforts. Bones (or anything else for which you wish to record the spatial location) are mapped. Surface detail of the bones is mapped to record the surface topography. Imagine each bone as a mountain that you wish to represent on a topographic map. An "aerial" photograph is taken of the bone, then in the ArcGIS program, the photograph is wrapped around the topography of the bone, creating a psuedo-three-diminsional bone in space. There is some distortion of the bone, especially around the edges, as the software converts the 2-D picture to the 3-D model, but the ease of doing this procedure makes up for the loss of some detail.

In this way, the quarry map is built in digital 3-D space using photographs of the actual bones themselves, not just field sketches and this map preserves the context of the excavation elements for all time.

 

Click the figure to see the "road cut" site of the grassland in animation. The surface is the original hillside surface so the bones are seen "under ground."
The following is an example of how preserving the 3-D aspects of the site can yield more information. At the "Road Cut" site (see animation above) we collected numerous polished stones in situ (gastroliths?). Below is a 2-D map view showing the distribution of the stones (green dots). The red squares are bone fragments that were located, but not photographed for detailed placement on the map. Note that while we can see the 2-D distribution of the stones, we cannot see their relationships in the third dimension.
In the 3-D model at right, the complete spatial relationship can be clearly seen. (The green dots represent the stones; blue dots are bone elements that are red above).

Thanks to Art Chadwick for ideas and inspiration on this technique.


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