CAA Conference

Traditionally the archaeological recording is based on direct measurements made upon a grid set up on field that supports manual drawing done on sheets of paper. The setting out of the grid could be accomplished with the use of a theodolite. Total stations allowed not to materialize a grid on field and to consider it only conceptually. Either way, the recording is always based on the discretization of the archaeological structure towards a two-dimensional representation.In recent times, the use of terrestrial laser scanning (TLS) resulted in a much faster and reliable way of recording with the advantage of three dimensional data acquisition. The issue is that it is a very expensive technology both in hardware and software and requires a high level of expertise.The very recent developments of three dimensional recording image based techniques brings an all new set of opportunities for the archaeological recording that, in some circumstances, present advantages with respect to TLS.This study pretends to demonstrate that automatic photogrammetric techniques such as structure from motion (SFM) and multi view stereo (MVS) enable the production of documentation with comparable quality to the one produced with traditional methods, or even comparable with the one produced with TLS. In this paper we present two case studies: i) "Pátio dos Carrascos", and ii) "Paços do Infante". Both sites are part of the "Convento de Cristo" UNESCO World Heritage site in Tomar, Portugal.In both cases we used the state-of-the-art software VSFM (Visual Structure From Motion) and CMVS (Clustering Multi-View Stereo) for the site three dimensional reconstruction.In the first case we demonstrate that the metric quality of the resulting documentation is comparable with the one produced with traditional methods. In the second case we merged the image based survey with a previous TLS survey of the same site. This operation allowed showing that both surveys have comparable performances both in metric and semantic aspects. It further allowed to visually depict the amount of excavation that was done. It can be suggested that image based three dimensional reconstruction techniques, such as the ones referred, can present themselves as a very effective and very low time consuming ways of maintaining a site recording always up to date, using only standard uncalibrated digital cameras for data acquisition. It should be added that the three dimensional reconstruction is almost unattended and automatic, what doesn't happen with TLS.We describe the image acquisition strategy, the processing workflow, the results obtained and the comparisons with the other recording techniques referred (traditional techniques and TLS).