Dispersion assessment in the location of facial landmarks on photographs
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The morphological assessment of facial features using photographs has played an important role in forensic anthropology. The analysis of anthropometric landmarks for determining facial dimensions and angles has been considered in diverse forensic areas. Hence, the quantification of the error associated to the location of facial landmarks seems to be necessary when photographs become a key element of the forensic procedure. In this work, we statistically evaluate the inter- and intra-observer dispersions related to the facial landmark identification on photographs. In the inter-observer experiment, a set of 18 facial landmarks was provided to 39 operators. They were requested to mark only those that they could precisely place on 10 photographs with different poses (frontal, oblique, and lateral views). The frequency of landmark location was studied together with their dispersion. Regarding the intra-observer evaluation, three participants identified 13 facial points on five photographs classified in the frontal and oblique views. Each landmark location was repeated five times at intervals of at least 24 h. The frequency results reveal that glabella, nasion, subnasale, labiale superius, and pogonion obtained the highest location frequency in the three image categories. On the contrary, the lowest rate corresponds to labiale inferius and menton. Meanwhile, zygia, gonia, and gnathion were significantly more difficult to locate than other facial landmarks. They produced a significant effect on the dispersion depending on the pose of the image where they were placed, regardless of the type of observer that positioned them. In particular, zygia and gonia presented a statistically greater variation in the three image poses, while the location of gnathion is less precise in oblique view photographs. Hence, our findings suggest that the latter landmarks tend to be highly variable when determining their exact position.
KeywordsForensic anthropology Facial morphology Facial landmarks Landmark location Observer dispersion
The authors would like to acknowledge the team of the Physical Anthropology Laboratory, University of Granada, Granada, Spain, for their support during the data acquisition. We also thank the international forensic experts group and master students who kindly participated in the online poll by marking the landmarks.
This work has been supported by the Spanish MINECO under the SOCOVIFI2 project (refs. TIN2012-38525-C02-01/02, http://www.softcomputing.es/socovifi/), and the Juan de la Cierva Fellowship JCI-2012-15359. The Andalusian Department of Innovación, Ciencia y Empresa under project TIC2011-7745, the Principality of Asturias Government under the project with reference CT13-55, and the European Union’s Seventh Framework Programme for research technological development and demonstration under the MEPROCS project (Grant Agreement No. 285624), including European Development Regional Funds (EDRF).
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