Documentation and detection of colour changes of bas relieves using close range photogrammetry

The digitalization of complex buildings, findings or bas relieves can strongly facilitate the work of archeologi sta, mainly for in depht analysis tasks. Notwithstanding, wheter new visualization techniques ease the study phase, a classical approach for determining changes or surface alteration could bring towards several drawbacks. The research work described in these pages is aimed at providing experts with a workflow for the evaluation of alterations, allowing a more rapid and objective monitoring of monuments. More in deep, a pipeline of work has been tested in order to evaluate the color variation between surfaces acquired at different époque. The introduction of reliable tools of change detection in the archaeological domain is needful: in fact, the most widespread practice, among archaeologists and practitioners, is to perform a traditional monitoring of surfaces that is made of three main steps: production of an handmade map based on a subjective analysis, selection of a sub-set of regions of interest, removal of small portions of surface for in depth analysis conducted in laboratory. To overcome this risky and time consuming process, digital automatic change detection procedure represents a turning point. To do so, automatic classification has been carried out according to two approaches: a pixel-based and an object-based method. Pixel- based classification aims to identify the classes by means of the spectral information provided by each pixel belonging to the original bands. The object based approach operates on sets of pixels grouped together by means of an image segmentation technique. The methodology was tested by studying the bas relieves of a temple located in Perù, named Huaca de la Luna.despite the data sources were collected with unplanned surveys, the workflow proved to be a valuable solution useful to understand which are the main changes over time.