Remote Sensing: Change Detection
Images taken at different times and co-registered to ensure accurate alignment of features can be compared to detect changes in surface reflectance between the two scenes. The upper image compares "before" and "after" Landsat TM scenes of a forested mountain region, acquired about 1 year apart. After the first image was acquired, a wildfire burned the forest on the hillside in the upper-left quadrant of the image, producing a mostly unvegetated bare slope that shows up prominently in the second image. While changes such as these may be easy to detect by visual inspection, more subtle changes, such as the increased vegetation on the small landslide scar in the far lower-right corner are easier to detect by digital image subtraction (right), which highlights major changes in brightness between the two images.