Abstract
In this study, we assess the precision, accuracy, and repeatability of craniodental landmarks (Types I, II, and III, plus curves of semilandmarks) on a single macaque cranium digitally reconstructed with three different surface scanners and a microCT scanner. Nine researchers with varying degrees of osteological and geometric morphometric knowledge landmarked ten iterations of each scan (40 total) to test the effects of scan quality, researcher experience, and landmark type on levels of intra- and interobserver error. Two researchers additionally landmarked ten specimens from seven different macaque species using the same landmark protocol to test the effects of the previously listed variables relative to species-level morphological differences (i.e., observer variance versus real biological variance). Error rates within and among researchers by scan type were calculated to determine whether or not data collected by different individuals or on different digitally rendered crania are consistent enough to be used in a single dataset. Results indicate that scan type does not impact rate of intra- or interobserver error. Interobserver error is far greater than intraobserver error among all individuals, and is similar in variance to that found among different macaque species. Additionally, experience with osteology and morphometrics both positively contribute to precision in multiple landmarking sessions, even where less experienced researchers have been trained in point acquisition. Individual training increases precision (although not necessarily accuracy), and is highly recommended in any situation where multiple researchers will be collecting data for a single project.
Highlights
Over the last decade, landmark based three-dimensional geometric morphometrics (3DGM) utilizing digital specimen scans has become an increasingly integral tool in the fields of physical anthropology and paleontology. 3DGM allows researchers to analyze complex shape data through the application of landmarks to anatomically homologous points on multiple specimens [1]
We present results of an error study comparing compatibility of scan types–which vary by instruments and scan acquisition protocol–on user-gathered landmark data to determine the extent to which error within and among individuals can influence the outcome of a geometric morphometric study
Our results suggest that interobserver error is of much greater concern than intraobserver error for different scanners or scan iterations in 3DGM studies using landmarks collected on virtual specimens
Summary
Landmark based three-dimensional geometric morphometrics (3DGM) utilizing digital specimen scans has become an increasingly integral tool in the fields of physical anthropology and paleontology. 3DGM allows researchers to analyze complex (i.e., nonlinear) shape data through the application of landmarks to anatomically homologous points on multiple specimens [1]. Landmarks can be acquired either directly from a physical specimen, as with a Microscribe digitizer, or digitally via a computer program, such as Landmark Editor [2], on a virtual rendition of a bone. The latter method has become popular recently with the decreased price and increased ease-of-use of surface scanners, which allow researchers to create a permanent digital copy of a specimen for later use in landmark-based analyses and/ or for storage and sharing with other researchers via an online database There is no broadly consistent protocol for rendering digital scans or for applying landmarks to digital models, and the possibility that landmark-based 3DGM studies can potentially suffer from problems of inter- and intraobserver error as a result of these variables has not been thoroughly investigated (but see [7])
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