Practical comparative evaluation of an integrated hybrid sensor based on Photogrammetry and Laser Scanning for Architectural Representation C. Balletti*, F. Guerra*, P. Vernier*, N. Studnicka**, J. Riegl**, S. Orlandini*** * Laboratorio di fotogrammetria – CIRCE IUAV Università degli Studi – S. Croce 1624 – 30135 Venice – Italy **RIEGL LMS Gmbh ***Microgeo S.r.l. KEY WORDS: Photogrammetry, Architecture, Integration, Modelling, Laser scanner, Orthoimage, Multi-scale ABSTRACT: In architectural surveying aimed to restoration and to conservation, close to the traditional two-dimensional vectorial representation in orthogonal projection, raster representation has gained considerable importance. Among all raster representations, the most widely-used is the digital orthophoto. The application of laser scanners as a new instrument for terrestrial survey together with firmly attached, high resolution and close range calibrated digital cameras, has permitted to obtain in straightforward way not only orthophotos but even to produce more sophisticated elaborations such as 3D orthophoto or solid image. These representations allow to get a vector representations via digitalizing in a single image. In the paper we will describe in detail a survey procedure that is based on an integrated photogrammetric – laser scanning system manufactured and marketed by Riegl LMS GmbH (high performance 3D imaging laser sensor + high resolution digital camera). A newly developed software allows to automatically extracted 3D orthophoto and so-called “solid images” from the hybrid data set (registered scan data and digital images) which can be subsequently used to easily create vector drawings by means of 2.5D digitalizing. This procedure is considered as an alternative approach to traditional digital close range photogrammetry, usually carried out in the activities of the photogrammetric laboratory of CIRCE – IUAV, to produce the well-known final representations: vectorial drawings and orthophoto. Considering that these products are in Italy the ordinary requests of a survey final user, such as architects or more generally restoration technicians, the authors have done a comparison not only on the achievable accuracy but also on time, costs and resources consumption that are necessary in real applications following the two procedures: the classical one based on traditional photogrammetry, and the new hybrid system approach based on photogrammetry -laser scanner integrated system. As test field the Arena of Verona has been chosen, such as significant application for the comparison just for the big amount of data collected by the photogrammetric laboratory of CIRCE during the photogrammetric survey campaigns (still in progress), and moreover for its own characteristics of an archaeological and architectural survey combination. The objective of the research is to offer the possibility to a surveyor to choose the best solution for representations productions, demanded by architects, restorers, etc., considering both accuracy and representation requests. Introduction Can laser-scanner survey be an alternative to topographical- photogrammetrical survey? If not, how much and to what extent can this new methodology replace or bolster the "old" method? To answer these queries, we tried to understand if the hardware and software available today for laser-scanning can obtain a satisfactory result according to the current canons of architectonic representation, which calls for not only the use of vectorial drawings in orthogonal projections but also raster images and 3D models. It was decided to carry out the survey of a single object by using the two different methods and compare them in each individual phase. The test area was a portion of the exterior façade of the Arena of Verona, consisting of three arcovoli (archways) on the south side: the XXXI, the XXXII and the XXXIII which represent an area functional to topographical-photogrammetrical survey. It was a test area significant in terms of dimensions in order to not oversimplify the problems while still maintaining complete control over the procedures. The purpose was to evaluate the techniques from an operating perspective and therefore there is the need to keep closely to the operating and professional practice of survey. The essential point is that while photogrammetric survey of a monument such as the Arena is a completely controlled technique, for which the times, costs and obtainable results are known, laser-scanning has been applied only experimentally and there is no case theory so generalized to be able to effectively design a survey whose instrumental support is based on the 3D scanner. To be able to accurately compare only the scanning and rectification techniques, we decided to keep the topographic reference system common, made up of topographical local system and detail nets. We analysed the operating methods of the two methodologies on the field and the procedures for controlling and handling the data in the laboratory, highlighting the procedures common to both techniques, the ones logically similar and those that are completely different. We then compared the vector and raster representations obtained, to evaluate the differences in the results and attempt to identify the causes of the differences. Finally, we evaluated the resources (times and costs) necessary for carrying out the survey using both techniques, to highlight any economies made possible by the use of laser scanner methods.