This study introduces a new approach for taking areal measurements of soiling on historical walls and buildings. Using a grayscale incorporated into each image, close-up photographic surveys were taken in central Oxford in fine (clear sky) and overcast conditions in 2006. Spectrophotometric measurements were taken congruent with the former survey. These data were used to compare the method, which is an outdoor application of the integrated digital photography and image processing (IDIP) method, for validation of outdoor lighting conditions across photographic surveys. Using a grayscale as a constant in the photographic surveys, it was possible to acquire color measurements on flat surfaces out-of-doors to track changes in the lightness (or soiling) and chroma of building surfaces. This outdoor application of IDIP proves to be a useful tool, itself with many applications for deciphering cross-temporal changes in the soiling of building facades. Simple grayscale calibration improves the comparability of surveys to spectrophotometric measurements. (c) 2007 Wiley Periodicals, Inc.