ArcMap Image Difference: Help interpreting/utilizing results

Question

I'm working for the first time with ArcMap's relatively new image analysis window, and am using the "image difference" function to calculate change between two images which I have performed a supervised classification on.

Each of the two images have only two types of land cover, forest and pasture; however, when I conduct the "image difference" analysis, I receive a raster with a ramp of 1 to -1. My question is how to interpret it? I.e. I'm looking to be able to symbolize where forest remains forest, forest changes to pasture, pasture remains pasture, and pasture changes to forest. I know this should be easy, and is so in ERDAS, but can't figure it out in ArcMap. Thanks in advance for any suggestions, David

Answer 1

Since you are comparing two classified images with a small range of values, instead of using the image analysis window functions (especially given the cryptic result), why not just use spatial analyst functions?

There are a number of ways to do this, but I suggest using Combine: This will output a new grid with a single value for every unique combination of input values, e.g. forest-forest, pasture-forest, forest-pasture, pasture-pasture, etc. The output attribute table will show all original and new values, allowing you to assign new names to the mapping (e.g. deforestation, regrowth, no change, etc.).

Answer 2

Essentially what you are doing is called change detection. When you select the "image difference" function, ensure that you are comparing the same respective bands within each image. I.e. Band 4 vs Band 4, Band 3 vs Band 3 and so on.This will create a raster which you can interpret as follows. Positive numbers represent positive change, (i.e. Areas getting brighter) while negative numbers equal negative changes (i.e. Areas getting darker). I would suggest using band 3 for your comparison. Since vegetation absorbs nearly all red light (it is sometimes called the chlorophyll absorption band) this band can be useful for distinguishing between vegetation and soil and in monitoring vegetation health.