Measured reality and imagined reality: between sky and land: the observation of historical city for its conservation

The observation and monitoring of territories stem historically from the man needs to know and describe the environment where he lives. Over time, methods and techniques have evolved in close connection with the scientific and technological progress. From the origin of the world, the historical city has been represented taking information from direct sites views or through manual sketching. In recent years, the introduction and the evolution of the information technology have allowed to develop analysis models more accurate and reliable that, linking territorial visions with views from the sky, offer precious tools for the identification, the knowledge, the analysis and the conservation of historic elements of the cities. If in the past, the representation of the city was done by means of orthographic views of the objects accompanied by perspective drawings, current technologies, leaving the prevailing sphere of imagination, propose themselves as a precious tool for surveying and restoration. In the virtual representation of the city, the “real space” is reviewed through a new construction composed by multifaceted forms of representation, like orthorectified raster images and textured 3D models in which the photograph "dress" the objects and the landscape of their chromaticity, highlighting the building materials texture and the degradation phenomena. New representations are no longer a simple reproduction of reality; indeed they want to highlight special features of the reality. Models composed by point clouds, obtained from laser scanning and georeferenced, are reliable databases, precious sources of morphological, geometrical, building materials information; they can record and monitor the conservation status of objects with a high accuracy, as well as any structural movements and/or degradation phenomena concerning buildings and materials. Starting from this cultural framework, this paper analyzes the performances of these technologies.