Different Photogrammetric Approaches to 3D Survey of the Mausoleum of Romulus in Rome

In recent years, digital photogrammetry has enjoyed a renewed approval in the field of Cultural Heritage. This is due both to the relative cheapness of the instruments (a high resolution camera, possibly a reflex with good lenses) and to new algorithms and software that simplified the use, perhaps at the expense of the necessary knowledge of its principles. The 3D survey of the Mausoleum of Romulus, along the Via Appia Antica, within the European project 3DICONS, provided the opportunity to test different photogrammetric techniques, with the aim to verify the results and to evaluate the positive and negative aspects. In particular two different approaches have been applied: spherical photogrammetry and dense image matching. The first technique is based on traditional photogrammetric principles, applied on panoramic images instead of frame images. The second one, the most recent and very widespread, is inspired by traditional photogrammetry and computer vision. In order to have a significant and correct comparison, a topographic support has been realized for the Mausoleum, to have all surveyed data in a single local reference system. The comparison has been made by using, as a reference, the point cloud acquired by laser scanner. In this paper, after a description of the funeral monument and its complexity, the two techniques will be described in order to investigate pros and cons, their algorithm and application fields. The acquisition and processing stage will be described in order to give all the necessary elements for the final judgement. At the end of the restitution and modelling process, the comparison will take into account many parameters: the scheme of image acquisition, the time required (on-site and in laboratory), the hardware (for data acquisition and post-processing), the results that can be obtained (2d and 3D representations with texture) and the metric accuracy achieved. Finally there will be some hints about different applications of these methods as concerning above all the visualization of data. For example, the exploration of the Mausoleum can be done through the navigation of bubbles, obtained by spherical photogrammetry.