(State Office for Cultural Heritage Baden-Württemberg, Esslingen am Neckar, Germany)
Keywords: aerial photography, archaeological prospection, image enhancement
Contrast or histogram stretch is a common method of image processing. In aerial archaeology, it is used to enhance the visibility of crop or soil marks in aerial photographs. In practice, the applicability and usefulness of this method is often compromised by the very inhomogeneous nature of many agricultural landscapes. The application of a single, overall contrast stretch for an aerial photograph or orthophoto commonly containing a mosaic of fields with different crops, grassland, forest and built-up areas often leads to unsatisfactory results. Readability of the image may even be reduced rather than improved. Therefore, aerial photographs are segmented by field parcel using cadastral data, and contrast stretch is applied to each field parcel. To further increase interpreter control over the image processing, histogram matching rather than simple contrast stretch is applied, resulting in similar histograms for all field parcels and a homogeneous, contrast-rich appearance of the resulting image. Tests of this approach indicate that it can be successfully applied for large-area analysis of aerial photography, in particular orthophotos, in agricultural landscapes. They do, however, also reveal some limitations, including for example non-correspondence between cadastral maps and actual crop parcels.