Three dimensional (3D) datasets have become commonplace in many archaeological projects and can result from a number of different data capture and creation processes ranging from manual modelling (e.g. reconstructions of buildings) to automated capture (e.g. 3D laser scanning of objects) and computed modelling (e.g. photogrammetry). In contrast to 2D raster images, 3D digital datasets present a number of key advantages for archaeological recording and reconstruction with 3D data being easily scaled, rotated and viewed from any direction and angle. 3D data from laser scanning, structured light scanning, or image-based modelling (e.g. photogrammetry or Structure from Motion) projects can often form the basis for complex 3D reconstructions which in turn play a role in the conservation and preservation of artefacts and monuments and, via online public dissemination, in the communication of cultural value (3D-ICONS, 2014).
3D datasets that commonly appear online as interactive models or as images within research reports and papers are often the end result of long and complex data capture and manipulation processes (often referred to as 'pipelines'). Although an understanding of data capture methodology is key to interpreting the final 3D product, this guide focusses purely on the preservation and documentation of 3D models that are created at the end of these capture processes. A key project of recent years that has focussed on 3D data creation and documentation is the 3D-ICONS project which has created detailed guidelines on the "documentation of the digitisation, modelling and online access pipeline for the creation of online 3d models of cultural heritage objects." (3D-ICONS 2014, 6). Users of this guide are therefore directed towards the 3D-ICONS Guidelines (2014) for a detailed overview of 3D capture techniques and the types of post-processing involved in the creation of 3D models (De Luca 2014). A detailed overview of the algorithms and methods used in processing and visualising 3D data is also provided by Callieri et al. (2011).
This guide additionally draws heavily on the work undertaken by McHenry et al. (2008, 2010, 2011) looking at various 3D file formats in terms of significant properties, data conversion, and information loss. Users of this guide should be aware that only a small subset of those formats looked at by McHenry are summarised here and that a number of formats have both emerged and evolved within recent years and continue to do so.
Lastly, the information in this guide should also be combined with the archiving and documentation guidelines found in other Guides to Good Practice in this series focussed on specific data capture or creation processes such as Laser Scanning, Photogrammetry, and CAD. Other relevant information may also be found in the Case Studies section, particularly on Structured Light Scanning.
As highlighted by McHenry et al. (2011, 1-2), the range of available 3D file formats is vast with each format storing various types of data in different ways. When combined with the relatively frequent release of new formats, alongside the evolution and development of existing formats, deciding on the appropriate format for long-term archiving of 3D data can be daunting. A previous guide in this series which looked specifically at virtual reality (AHDS 2002) to some extent highlights this development. While the data formats and creation processes covered by the AHDS Virtual Reality guide are now firmly dated, the guide does however highlight the importance of project planning and the documentation of 3D data alongside more general theoretical considerations involved in the creation of 3D and virtual reality datasets (AHDS 2002, sections 2, 4, 5).
While no set of guidelines can remain current forever, it is the hope of this guide that through a focus on the key elements and significant properties of 3D data within a general context of digital preservation concepts and principles, practical and lasting guidelines for 3D preservation and documentation can be created.