CONCLUSION

In our study we compared manual and automated photogrammetry and micro-CT scanning in terms of accuracy, reproducibility, and efficiency in 3D surface generation of a typical vertebrate fossil. The following summary presents our conclusions and recommendations that are in general accordance with the recommendations made by Mallison and Wings (2014) and Sutton et al. (2014).
3D surface comparisons and documentation of processing and training times revealed that manual photogrammetry has a high degree of reproducibility and is the most efficient and least costly method of those tested, given that appropriate lighting and camera settings are chosen in order to ensure accuracy. Manual photogrammetry is generally recommended if only the external surface of a specimen is of interest to the researcher, and especially in case of a small budget. However, one needs to account for training and practicing time prior to the application of this method.
Automated photogrammetry yielded results similar to manual photogrammetry but larger data volumes. If one is interested in an online object panorama (presentation and curation purposes) or in a 3D surface, but does not have the time for practicing or data processing, MDS Witikon offers a convenient digitization method ranging in costs between manual photogram-metry and micro-CT scanning.
Generally, when using photogrammetry, back-ground color should contrast the color of the specimen, allowing for correct distinction between the two during surface generation.
CT scanning (including micro-CT) has its strength in the reconstruction of internal structures. While it is possible to extract external surfaces from CT scans, extremely long post-processing times, lack of photo-realistic surface colors, and high costs make this method less suitable for external 3D surface generation than the other methods tested. (Micro-)CT scanning should only be used for external surface generation if no other means is available. We would also recommend the additional use of manual photogrammetry, even if CT scans are available, if both internal and external surfaces are to be digitized.
 

ACKNOWLEDGMENTS

First, we thank Oliver Hampe and Thomas Schossleitner (Museum für Naturkunde Berlin) for access to the specimen, and valuable background information and discussion. We also thank Irina Ruf (Senckenberg Naturmuseum Frankfurt am Main, formerly Steinmann-Institut Bonn) and Georg Oleschinski (Steinmann-Institut Bonn) for micro-CT scanning, and Daniel Baum (Zuse Institute Berlin) for discussing surface registration techniques in ZIB-Amira. Heinrich Mallison (Museum für Naturkunde Berlin) is thanked for taking one set of photos used for photogrammetry and for helpful suggestions. We furthermore thank René Guráň, Róbert Sičak, Paul Safko and Mira Silanova (EDICO SK, Bratislava) for digitizing our specimen with MDS Witikon, and Daniel Girardeau-Montaut (Grenoble, France) for constructive comments regarding the use of CloudCompare. Funding was provided by the Deutsche Forschungsgemeinschaft (DFG FA 889/2-1) and the Alexander von Humboldt Foundation (Feodor Lynen Return Fellowship for JMF in 2012).
Publication costs were covered thanks to a grant from Transmitting Science to the Journal of Paleontological Techniques.
 

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