Abstract
Abstract. 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.
Highlights
This Paper tackles the problem of surveying narrow spaces in cultural heritage (CH) using image-based techniques
There are some examples described in the literature (Roncat 2011, Bonacini 2012, Rodríguez-Gonzálvez 2015), to survey a narrow tunnel with terrestrial laser scanners (TLSs) require a great number of scan stations that commonly results in a very timeconsuming process
Since the aim is to reduce to the minimum the number of ground control points (GCPs) that are required to achieve accurate high-quality results, the goal has been that of get low errors – within the 1cm threshold that corresponds to error of 1:50 scale – on CPs, when GCPs are picked only at one of the extremities of the narrow staircase. 4.2 Raw Images Multi-camera: 4.2.1 No Constraints: The first test sees the raw fisheye images from iSTAR aligned without any kind of multi-camera constraints
Summary
This Paper tackles the problem of surveying narrow spaces in cultural heritage (CH) using image-based techniques. There are some examples described in the literature (Roncat 2011, Bonacini 2012, Rodríguez-Gonzálvez 2015), to survey a narrow tunnel with terrestrial laser scanners (TLSs) require a great number of scan stations that commonly results in a very timeconsuming process Both range- and image-based solutions usually employed in CH are not fine-tuned to address the task of mapping narrow areas. The narrow environment of many passages hampers the operator ability to perform the survey e.g. placing a topographic tripod and carrying heavy instruments The shape of those areas is usually characterised by a high ratio between space extension and width, bringing to a configuration that suffers greatly from the problem of propagation of uncertainty. The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018 ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4–7 June 2018, Riva del Garda, Italy earlier instead, other authors presented applications of photogrammetry for narrow areas but using regular rectilinear wide-angle lenses (Roncella et al, 2012; Arles et al, 2013)
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