Practical comparative evaluation of an integrated hybrid sensor based on Photogrammetry and Laser Scanning for Architectural Representation

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.