High resolution orthophoto of Milan Cathedral : latest workflow for complex architecture

Surveying Cultural Heritage is one of the most prominent subjects of 21st century and the 3D survey techniques and methods are developing rapidly ever than before. The purpose of a survey is having the most accurate and the most thorough result as possible, in order to provide the possibility of creating a work of preservation or restoration at its top. In the specific case of Milan Cathedral, for the ongoing restoration projects of the building, it is needed the creation of the technical drawings of the Cathedral. The main aim of this research is to be able to create a survey method to be used for complex geometries as in Milan Cathedral and to be able to create an orthophoto of the east façade of the building to be used for the future restoration projects. For this work the architectural photogrammetry technique was used in order to have with the same survey both an accurate and complete three dimensional model of the façade and high resolution images to create the final ortho-image. The complexities of the ‘object’ need to design a multi-scale survey in order to assure the final desired precision and to connect all the parts that are at different distances embedded in the intricate geometry. The multi-scale survey can be done using multiple lenses with different capture geometry and using external measurement with total station as support for georeferening, accurate scaling and punctual check Surveying a complicated architecture with several geometrical attributes and details is a complex work; when considering the size of the building the difficulty of the job doubles itself. Working with multiple lenses creates great deal of opportunities, such as making it possible to survey elements with different levels of details or having the possibility of varying survey positions, which is a strategic solution when the location of the survey positions are not flexible as in the case of Milan Cathedral. The thesis was developed in two stages. In the first stage a research has been done with the intent of creating a workflow for the re-elaboration of the analogue photographs, which are made for the analogue photogrammetry technique, with the contemporary digital photogrammetry methodology. For this study, the scanned analogue photographs, taken to be built the orthophoto of the main façade in 2003, have been used. Following the several trials it was possible to build a proper workflow and a result with high quality. However still it is a must for this kind of projects to have the original ground control points or building a new topographical network to have the most accurate result. In the second stage, a study was made with the aim of creating the metric orthophoto of East Façade of Milan Cathedral. To be able to create the orthophoto, the façade has been surveyed from three different locations with varying lenses. Following a great number of trials the photographs obtained from these surveys were elaborated separately and the outcomes combined to have a unique representation of the east façade. Besides the orthophoto of the east façade, this study made it possible to obtain the orthophotos of several other geometries on this façade. Each step of this process was quite challenging and to be able to realize this work a rigorous study was done from the designing of the survey phase to obtaining the orthomosaic image. Finally it was possible to reach a satisfying level of detail up to scale 1:20. The high quality result of this research shows that the workflow that was applied to this project is a reliable methodology, to be used with surveying buildings having complex geometries for future works.