Digital Image Processing Techniques for Evaluating Facade Flatness in Historical Buildings

This study reports a rapid method of analysis for evaluating the flatness of the facades of historical buildings. This method applies topographic, digital photogrammetric, and image processing techniques to measurements acquired by a semimetric camera in order to provide a planar metrified and rectified image. The use of this technique allows for the virtual control of alterations of a building's geometrical and physical surface characteristics that have been induced by environmental decay and have possibly resulted in some deviation from flatness. We used the semimetric cameras Rolleiflex 3003 and Rolleiflex 6006 and a two-dimensional (2D) algorithm to verify the flatness of the main facade of a monument, the Zisa Palace in Palermo (Italy). The results of this study, which highlighted some deviations in the left edge of the facade with respect to the points of support, were obtained with an estimated 10(-3) accuracy. Accurate statistical analysis applied to the measurements showed that the facade of this historical building is not a flat surface, suggesting that decay processes have occurred. Hence, this method can be used to easily demonstrate nonflatness of a facade of a historical building and to highlight structural failures in areas of the facade.