Comparison of coordinate digitizers for cranial surfaces.

Abstract

Four commercial coordinate digitizers are evaluated for measuring external cranial vault surfaces in our study of papionine metric, morphological and molecular diversity (Norris et al. 2006, this meeting). Primary concerns are fidelity of surface geometry and ease of use, with cost a secondary consideration. The four instruments employed are: electromagnetic Ascension Bird with standard receiver and pencil stylus (SB); electromagnetic Ascension miniBird with two 8mm receivers (MB); Immersion Microscribe G2 mechanical arm (MS); and Polhemus Fast- SCAN laser stripe digitizer (FS). Two tests were conducted. First, points (N=28 - 30) on the upper hemisphere of a hard plastic ball (nominal 55mm radius, 0.05mm asphericity) were digitized with each device. Radius (R), root mean square errors (RMS) and maximum radial deviation (MD) of data points from a spherical numerical model were computed (all measurements in mm). For SB, R = 54.7675, RMS = 0.4162, MD = 1.8111; for MB, R = 55.9208, RMS = 0.3274, MD = 0.7991; for MS, R = 56.6342, RMS = 0.1375, MD = 0.3442. Second, we digitized points (N>204) on the exocranial surface of an adult female Macaca mulatta calvarium with left and right porions and right infraorbitale establishing a spatial orientation plane. Radial deviations of data points were computed for a spherical model, an ellipsoid model and a tapered superquadric model. For example, using MB, superquadric model, RMS error = 0.8612 and maximum asymmetry = 1.2673; sagittal/transverse squareness = 0.1037, coronal squareness = 0.0643. With device precisions differing within acceptable limits, particular applications dictate equipment choices. Support for this investigation was provided by the Australian Research Council, the Pennsylvania State University College of Health and Human Development, and the Department of Kinesiology’s Davis Fund for the Encouragement of Innovative Research.