This paper describes the methodology adopted and the results obtained during the 3D documentation of the medieval capitals of St. Orso Collegiate Church, in Aosta, for reference and digital archiving. This task has been carried out through low cost technology entirely based on digital scanning of high quality images applying stereophotogrammetric principles. This technology allows to obtain point clouds with RGB information and geometries at different levels of complexity, processing a number of images taken with a limited set of constraints, with the use of a special acquisition equipment and through an image matching algorithm. Issue in this research was to obtain the accurate 3D information required to build models of artifacts of a such heritage importance, with a high degree of complexity. In future the 3D model will contribute to create a spatial information system which will assist conservation and restoration activities on the one hand and will be used for tourist information inside the St. Orso Collegiate Church on the other.

The recent survey technologies, play a leading role in the process of knowledge of the cultural heritage. In fact they are finalized to the acquisition of geometric data necessary to describe usefully geometric features and peculiarities of historical monuments necessary for their conservation and safeguard. Besides digitalisation of the acquired data, allows the same data availability by different typologies of users in order to spread and share knowledge for cultural heritage valorisation. Quick stereo-photogrammetric systems and laser scanner technology have advantages but also limits referring to the elaboration, management and possibilities of use. The recent development of digital stereo-photogrammetric systems allows the definition of 3d visualization and navigation environments within which the data referring to geometric consistency is combined, without loss of rigour, with other qualitative, morphological and colour information specific to the photographic document. The laser scanner technology likewise permits a detailed three-dimensional description of the artefacts geometry, without subjective interpretation from the operators. Therefore laser scanner opens up extremely interesting horizons to possible geometry analyses or to evaluations of monuments state of conservation and static conditions. The necessity to compare these different survey technologies becomes fundamental to verify the accuracy of acquired numeric data, to evaluate their possible integration and to define different protocols of use in relation to the artefacts morphological-architectural characteristics and to the survey finalities. The critical aspects of the described topic will be proposed through the analysis of a specific study case: the documentation of the roman triumphal arch of Augustus in Aosta.