RC-HELI AND STRUCTURE & MOTION TECHNIQUES FOR THE 3-D RECONSTRUCTION OF A MILAN DOME SPIRE M. Scaioni, L. Barazzetti, R. Brumana, B. Cuca, F. Fassi, F. Prandi Politecnico di Milano, Dept. BEST/SITE Group, piazza L. da Vinci 32, 20133 Milano, Italy e-mail: {marco.scaioni; luigi.barazzetti; raffaella.brumana; branka.cuca; francesco.fassi; federico.prandi}@polimi.it KEY WORDS: Cultural Heritage, 3-D Reconstruction, Structure and Motion, UAV ABSTRACT: The paper describes a complete workflow for the 3-D photogrammetric surveying and modeling of complex architectonical objects. The application is focused to the major spire of the Dome of Milan, that is characterized by the well known striking vertical geometry and a high level of complexity. Because no positions from which capturing images and scans exists, a 5 kg payload model helicopter has been used to lift and carry a calibrated digital camera, which has been adopted to capture images for the reconstruction of highest part of the spire. The huge quantity of image data has been partially oriented with a new algorithm able to detect automatically corresponding tie points among the overlapping images by using a Structure & Motion approach, but extended to blocks with a generic shape. The algorithm can identify homologous points with the SIFT operator alternated to a robust estimation of camera poses to remove wrong correspondences. Then a progressive resection alternated with triangulation is carried out to determine the orientation parameters of each image. Finally a photogrammetric bundle adjustment is computed to derive final exterior orientation parameters. 1. INTRODUCTION 1.1 UAV: a new paradigm for photogrammetry In the latest five years one of the most emerging technology in photogrammetry is undoubtedly represented by unmanned aerial vehicles or UAVs (van Blyenburg, 1999). The interest for these flying platforms in motivated by several grounds related to several fields of possible application. In aerial survey, they allow a high resolution image acquisition, because of the possibility to fly at very low height on the ground with respect to manned aircraft and helicopters. The possibility of mounting sensors in tilted position can be exploited for imaging some kinds of objects which are difficult to capture in standard aerial imagery. Typical examples of this are building façades, waterfronts or vertical rock faces (Eisenbeiss, 2008). So far, digital or video-cameras have been installed for the most on UAVs, even though multi-spectral sensors have been already tried (Nebiker et al., 2008) and their use is increasing on such vehicles. The presence of on-board navigation/positioning systems enable the most evoluted UAV to follow a predefined flight-plan and to record orientation parameters. These sensor are usually based on GPS/INS technology, but alternative solution integrating other kinds of data (scans, images) have been explored (Eugster & Nebiker, 2008; Steffen and Förstner, 2008; Wang et al., 2008). UAVs can be deployed with ease and require a small plot of land for take off and landing, properties that encourage their use for data acquisition just after anthropogenic or natural disasters. Thus they are an ideal platform to capture images needed in emergency response (Kerle et al., 2008), without any risk for operators. Studies and efforts to improve autonomous flying capability, data acquisition and control make this platform capable of reducing cost and time for production of mapping products (Zongjian, 2008). The new scenario that UAV are opening is so relevant that Colomina et al. (2008) state that a new paradigm for photogrammetry is forthcoming, following aerial and satellite ones. Agriculture represents another potential field for the extensive use of UAVs (Grenzdörffer, 2008). But also in large-scale or close-range applications of photogrammetry (e.g. for Cultural and Environmental Heritage documentation and conservation), the UAV technology might solve many problems that currently cannot be coped with effectively by using terrestrial or aerial surveys. UAVs were successfully applied to map archaeological sites, where other surveying techniques were not suitable (see e.g. Eisenbeiss, 2004; Bendea et al., 2007). Let us consider for example the completion of the 3-D survey of a construction, where the roof is to be capture at high resolution as well, or when a high-rising building has to be imaged in its top. In many cases, suitable images cannot be taken neither from the ground, nor from other buildings or scaffoldings. On the other hand, aerial imagery might not be suitable due to the insufficient scale, while the used of manned helicopter usually is largely expensive. The availability of a micro or mini UAV (see Bento, 2008 for classification of UAVs) equipped with a high resolution digital camera enables one to capture images from unconventional point-of-views and to respond to the above-mentioned critical requirements. The application described in this paper just belongs to this category and it will further analysed in the next Sub-section. 1.2 Project overview The inquired “object” is the major spire of the Milano (Italy) Cathedral (see Fig. 1) that is characterized by the well known striking vertical geometry and a high level of complexity. Information on the Cathedral can be found at Veneranda Fabbrica del Duomo di Milano (2009). The 3-D geometric measurement of this object cannot be done with a combination of classical topography and manual survey, because the density of architectural and structural details and the richness of decorations make this approach very intricate, time consuming and in some cases also impracticable. For example total station measurements require the creation of a complex geodetic network and the collection of a huge number of measured