Photo-based decay mapping of replaced stone blocks on the boundary wall of Worcester College, Oxford

Abstract

Abstract In this study on the boundary wall of Worcester College, Oxford, the decay mapping in Adobe Photoshop (DMAP) approach is introduced to test the use of simple daylight photographs in the long-term monitoring of stone decay. This is conducted primarily through measured changes in surface brightness and roughness based on close-up photographic images of walls. The Magic Wand Tool was applied to greyscale images in Lab Color Mode to select proportions of pixels with a lightness ( L ) value of 77%. This paper shows the effectiveness of the calibration procedure used to validate lightness between surveys so that cross-temporal comparisons have a greater validity. It also outlines and discusses errors associated with the method as well as its limitations. The DMAP approach proves to be particularly useful when applied to long-term monitoring exceeding 5 years of survey.