3D Geometric Morphometrics of Projectile Points – Populating Splines

 

As we continue to think through the various spline configurations (see previous post here), I have been constructing a number of models based primarily on the efforts of previous analyses for both 2D and 3D geometric morphometrics. While this is representative merely of a (very) humble beginning, experimenting with the reconstruction of configurations used in other analyses could help us to better understand how we might begin to move toward a replicable consensus configuration (certainly some great examples out there).

ZSpline_1

In this example, I am using the framework of two splines that were created as a 3D mesh sketch in Design X (above). I then added 10 equidistant sections (below) between the top and bottom of the projectile point. While those would most likely be cut where they intersect with the splines above prior to populating point data, I wanted to see where the–equidistant–points would populate along the various profiles.

ZSpline_4

In this case, it may work best to cut each of the 10 sections where they intersect either the exterior or interior profiles (or both), prior to populating the LM/sLM data. I did not populate the 10th section (very near the point of the projectile) for this example, simply because it was almost impossible to view the location of Point 2 (which is defined by the confluence of the vector and the poly-vertices of the mesh at the tip of the projectile) when that particular section was populated.

ZSpline_2

Another question that we will need to ask ourselves sooner than later, is where do we reach a point of saturation or diminishing returns with regard to the number of LM/sLM data points? There is still much left to think about, and we will continue to move toward the definition of a replicable consensus configuration as we work through replicating (as closely as possible) the numerous configurations that have been used in the past.

Our tactics differ from many of our colleagues, due primarily to our efforts to devise a configuration aimed at performing an initial “sort” at the assemblage level–different configurations would then be used for each of the identified categories based on more specific attributes. In the coming months, I hope to share some of our minor successes, and–no doubt–numerous failures as we continue to work toward a suitable configuration.

As always, your comments and constructive criticisms are welcome. You can comment by clicking on Leave a Comment below, or you can email me directly at selden3d@gmail.com. 

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Toward a Replicable LM/sLM Configuration for Projectile Points

Among the many topics that warrant further attention in 3D geometric morphometric studies of archaeological artifacts is the development of a replicable method for applying landmark (LM)/semi-landmark (sLM) data points to projectile points. While I use one of our 3D Clovis points to demonstrate our progress thus far, my interest lies more entrenched in the development of a consensus configuration for dart points in general. Extending our capacity to scan and analyze diagnostic points at the level of the assemblage by employing the same LM/sLM configuration is particularly attractive. While it is likely that this approach–much like that of ceramics–would result in a hierarchically-nested method of analysis, this could potentially yield a platform that minimizes subjectivity and semantics in classifying morphological variation. Importantly, LM/sLM configurations should be constructed to address specific research questions.

Selden_2015_RawWe begin by importing the 3D file into Design X (above). Note that the point is not aligned. The consensus configuration begins with one primary assumption upon which everything else is built – the vector. The vector is inserted along the principal axis of the point, and is defined by an algorithm. We then place a reference point at the confluence of the vector and the poly-vertices of the 3D mesh at the base of the projectile. A plane is then inserted at the juncture of the reference vector, point, and poly-vertices of the mesh. This configuration of reference geometry is then used to align the mesh.

Selden_2015_AlignedWe use the aligned mesh (above) to create a mesh sketch of the point’s profile, then extrude a surface around the 3D mesh. Deviations are subsequently calculated between the surface and the mesh, identifying the single point on the projectile that lies farthest from the central vector (below). We then insert a second plane along this widest profile of the point.

Selden_2015_VecDevOnce inserted, this widest planar surface (see image at top)–along with the various reference geometry created thus far–can be employed to create a seemingly unlimited variety of LM/sLM configurations. As long as those configurations remain based upon the reference geometry, replication of the geometric elements should be possible. The geometry provides the framework upon which the splines and other vectors can be created, then populated with data points.

 Selden_2015_P1Selden_2015_MSExtending our analyses beyond typical orthogonal measurements (length, width, thickness, stem width, etc.) can help us to better characterize the dynamic nature of projectile point morphology. While there is much work left to do on this front, we have begun to test various LM/sLM configurations (see one of these below).

Selden_2015_Potential

We will keep you posted as we make progress, and welcome any/all feedback as we continue to design and experiment with the wide range of LM/sLM configurations.

Selden_Clovis