ABSTRACT:The research illustrated in this article aimed at identifying a good standard methodology to survey very narrow spaces during 3D investigation of Cultural Heritage. It is an important topic in today's era of BIM modelling applied to Cultural Heritage. Spaces like staircases, corridors and passages are very common in the architectural or archaeological fields, and obtaining a 3D-oriented survey of those areas can be a very complex task when completeness of the model and high precision are requested. Photogrammetry appears to be the most promising solution in terms of versatility and manoeuvrability also considering the quality of the required data. Fisheye lenses were studied and tested in depth because of their significant advantage in the field of view if compared with rectilinear lenses. This advantage alone can be crucial to reduce the total amount of photos and, as a consequence, to obtain manageable data, to simplify the survey phase and to significantly reduce the elaboration time. In order to overcome the main issue that arise when using fisheye lenses, which is the lack of rules that can be employed to design the survey, a general mathematical formulation to precisely estimate the GSD (Ground Sampling Distance) for every optical projection is presented here. A complete survey of a real complex case study was performed in order to test and stress the proposed methodology, and to handle a fisheye-based survey from beginning to end: the photogrammetric survey of the Minguzzi Staircase. It is a complex service spiralstaircase located in the Duomo di Milano with a total height of 25 meters and characterized by a narrow walkable space about 70 centimetres wide.

ABSTRACT:In the architectural survey field, there has been the spread of a vast number of automated techniques. However, it is important to underline the gap that exists between the technical specification sheet of a particular instrument and its usability, accuracy and level of automation reachable in real cases scenario, especially speaking about Cultural Heritage (CH) field. In fact, even if the technical specifications (range, accuracy and field of view) are known for each instrument, their functioning and features are influenced by the environment, shape and materials of the object. The results depend more on how techniques are employed than the nominal specifications of the instruments. The aim of this article is to evaluate the real usability, for the 1:50 architectonic restitution scale, of common and not so common survey techniques applied to the complex scenario of dark, intricate and narrow spaces such as service areas, corridors and stairs of Milan's cathedral indoors. Tests have shown that the quality of the results is strongly affected by side-issues like the impossibility of following the theoretical ideal methodology when survey such spaces. The tested instruments are: the laser scanner Leica C10, the GeoSLAM ZEB1, the DOT DPI 8 and two photogrammetric setups, a full frame camera with a fisheye lens and the NCTech iSTAR, a panoramic camera. Each instrument presents advantages and limits concerning both the sensors themselves and the acquisition phase.

Narrow spaces and passages are not a rare encounter in cultural heritage, the shape and extension of those areas place a serious challenge on any techniques one may choose to survey their 3D geometry. Especially on techniques that make use of stationary instrumentation like terrestrial laser scanning. The ratio between space extension and cross section width of many corridors and staircases can easily lead to distortions/drift of the 3D reconstruction because of the problem of propagation of uncertainty. This paper investigates the use of fisheye photogrammetry to produce the 3D reconstruction of such spaces and presents some tests to contain the degree of freedom of the photogrammetric network, thereby containing the drift of long data set as well. The idea is that of employing a multi-camera system composed of several fisheye cameras and to implement distances and relative orientation constraints, as well as the pre-calibration of the internal parameters for each camera, within the bundle adjustment. For the beginning of this investigation, we used the NCTech iSTAR panoramic camera as a rigid multi-camera system. The case study of the Amedeo Spire of the Milan Cathedral, that encloses a spiral staircase, is the stage for all the tests. Comparisons have been made between the results obtained with the multi-camera configuration, the auto-stitched equirectangular images and a data set obtained with a monocular fisheye configuration using a full frame DSLR. Results show improved accuracy, down to millimetres, using a rigidly constrained multi-camera.

Abstract. The task of digitalizing meandering complex spaces in 3D is a challenging one even with the most advanced instrumentation like lightweight terrestrial laser scanner or portable/wearable Mobile Mapping Systems (MMSs). The complexity and extension of architectonic spaces such as staircases, corridors and passages are such that the acquisition time using static devices becomes prohibitive and the accuracy using mobile devices gets affected by drift error leading to warped models or requiring abundant control measurements. This paper presents a photogrammetric portable fisheye multicamera solution for the 3D survey of complex areas that aims at being both handy and fast in the acquisition as well as more reliable ad accurate than common MMSs. The paper showcases a stress test conducted on five complex reconstruction trajectories selected from the meandering connection passages of Milan’s Cathedral. The tests are constructed as worst-case scenario to evaluate the accuracy and drift error amount of the proposed system in open-ended unconstrained paths. The results, though still suffering from moderate drift error, highlights the potential of the solution, especially in retaining the overall shape and orthogonality of the architectonic elements acquired.

Since 2008, an intense survey has been underway in the Milan Cathedral. The operations, over the years, have been conducted with laser scanner and photogrammetry choosing or integrating the different methodologies according to the environment, the necessary "drawing" to support the different sites' maintenance operation but always concurrently with the evolution of the method and the software. The "principal actor" of conservation activities is the "marble block." The Veneranda Fabbrica organizes the activities identifying the areas that are in need of intervention and identifying which blocks will be affected by replacement operations, tessellation, or consolidation. Thus, the objective of the survey activities and the subsequent modeling phase was to build a detailed 3D model in which the marble blocks are easily recognizable in terms of their shape, size, position, and texture (only for the outer part). All the elaborations produced, the 3D models, the two-dimensional representations, and the orthophotos, allow for the identification of the blocks, as to provide proper technical support for site operations. In parallel with the survey and modeling activities, an ad hoc online information system was created to support the construction site activities in a smart manner. The system allows for the consultation and the sharing of the 3D models and all the data necessary for maintenance operations of the entire Cathedral.