2.3. Image analysis
We processed images using ERDAS IMAGINE version 9.0, following the procedure developed by Forrer et al. (2000) and described in detail by Cey and Rudolph (2009). All images experienced geometric correction, background subtraction, color adjustment, histogram stretching, image classification, and finally, a visual check. Using a ratio of blue to red band, we classified an image into stained and unstained regions (Janssen and Lennartz, 2008). The blue and red band showed the stained soil pattern and the soil background, respectively. The band ratio was calculated by dividing the digital number in the blue band by the corresponding value in the red band for each pixel in the image. A threshold to distinguish between stained and unstained areas was visually determined in the band ratio image (Janssen and Lennartz, 2008). We did not further separate the stained regions into different dye classes due to their small size in most images.