2. Methodology

The application of digital photogrammetry develops a long history of georectification in archaeological imagery, as both a means to record sites, monuments and portable material culture, and as a means to research spatial aspects of archaeological data. Digital photogrammetry uses structure-from-motion as a means to produce 3D models of archaeological objects. Applications of archaeological photogrammetry have developed significantly in recent years, as access to greater processing power, personal computing, and digital photographic solutions have become more commonplace. With the advent of high-quality digital photography, photogrammetry has come to be used alongside laser-scanning to provide reliable texture mapping for the untextured models produced by the scanning equipment (e.g. Al-Kheder et al. 2009; Ergun et al. 2010; Lerma et al. 2010). Photogrammetry is now being used professionally to record and research a variety of archaeological subjects, from Galician petroglyphs (Sans et al. 2010) to the Dome of the Rock (Al-Ruzouq et al. 2012) and the cathedral of Santiago de Compostela (Martinez et al. 2013).

As far back as 2004, it was recognised that the speed and low-cost nature of digital photogrammetry meant that it had the potential to become the technology of choice for 3D modelling in all but the most high precision of applications (Boehler and Marbs 2004, 298). The latest developments in processing software, for example Autodesk's 123D Catch (first published in 2009) and Agisoft Photoscan (first published in 2013) allow the automatic recognition and combination of images without the necessity for reference points or known dimensions from the subject being modelled.

Our project uses Agisoft Photoscan, a commercially available computer programme, to identify common points in multiple digital images. The metadata contained in the unadulterated file allows the program to adjust for barrel distortion and depth of field, rendering the camera calibration required for previous photogrammetric approaches unnecessary. By changing the camera position between different images, it is possible to generate many common points, with differences between the location of common points allowing the program to generate volumes for photographic subjects. In addition, photographic subjects can be associated with known geographic coordinates, which facilitates accurate scaling and geolocation. This in turn allows landscape approaches to the analysis of the models by positioning them in 3D space in relation to other models or features, in order to investigate the setting of sites or monuments with reference to their orientation, or the wider geographical context of sites. Rendering an accurate volumetric model of a site or monument means that it is possible to produce metric measurements that may not have been recorded or analysed before.

In a study comparing this automatic method with earlier software reliant on reference metrics and stereo-pair images, it was found that automatic algorithms delivered highly comparable levels of detail and accuracy, without requiring advanced knowledge of surveying or 3D modelling software (Krasic and Pejic 2014, 282). Automatic digital photogrammetry therefore opens the way for a democratic and low-cost approach to generating archaeological data. The accessibility of these approaches provides the means for members of the public to produce really high-quality research data as part of a digital citizen science project, which can make 'real' contributions to archaeological understandings.

HeritageTogether differs from many digital public archaeology projects in our use of large-scale digital data generated by members of the public (see Griffiths et al. this issue). Our data sample has given us scope to explore the methodologies of co-producing 3D photogrammetric models, and of the challenges and affordances of producing data in different ways. Critically, by working with a range of users on diverse sites, by designing guidelines for the project, and by responding to queries at workshops, lectures, and on the online forum, our methodologies have been developed by citizen scientists. As the project developed, we realised that as well as the more standard research outputs (the site models that provide condition surveys, and site-specific research data), the development of the method with members of the public was an important project output in itself (Moshenska 2014). The conversations that we had, and the ways we learnt through practice with members of the public, were processes that were due to the citizen science emphasis of the project. Increasingly as the project developed, our practice (meaning everyone involved with HeritageTogether research) was co-produced.

Three case studies are presented here to illustrate a range of learning outcomes and methodological issues that were developed in consultation with members of the public who worked on sites for the HeritageTogether research. We detail them as a series of practical approaches for doing digital photogrammetry. As our project developed, so did our methodology. This article represents the co-produced outcomes of this approach to doing digital public archaeology in practice.