Part 3—Compare Processing Results

Step 1 Create a Stack of Images

  1. Select File > Open, navigate to the Forests folder, and open all three images saved at the end of Part 2 (map1.tif, map2.tif, map3.tif).
  2. Choose Images > Stacks > Images to Stack to create a stack from these images. This process takes the three individual images and puts them into one image containing all three "slices" in a "stack" so that each one is on top of the other and can be moved through frame by frame or animated.
    image to stack menu

  3. The stack should look like this image. Notice the 1/3 in the upper left corner of the stack. This indicates that you are looking at the first of three images in a stack. The scroll bar below the image lets you navigate to other images in the stack.


  4. Use your mouse to click and drag the scroll bar across to change the image that you are looking at. In addition, you can move forward and backward through the stack using the Next Slice (>) and Previous Slice (<) keyboard shortcuts.
  5. Observe each slice. How do the deforested areas (the bright white areas) change through time?
    The bright white areas change position and appear to increase over time.

Step 2 Create an RGB Color Composite Image from the Stack

It is possible to assign each image for each of the three years to a different color channel in order to use color to compare the specific changes in deforestation that have taken place from 1994 to 1995 to 1996.
  1. Use Image > Color > Stack to RGB to create an RGB (red, green, blue) color composite image from the stack.
    stack to rgb menu

  2. The color composite image should look like this.

  3. RGB image


Step 3 Interpret Results

To understand what each color on this final image means, first examine the figure below. Remember the concept of additive colors; red, green and blue are the three primary colors which together add up to make white.

As is clear from this figure:
Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
Red + Green + Blue = White


  1. Make the 3-slice stack active again by clicking on it. The deforested areas appear as bright white in each slice of the stack.
  2. Arrange the image stack and the color composite image so that you can see both side by side.
  3. The color composite image you created is a combination of three individual images that are displayed in three primary colors.
    The red areas are those that were bright white in the 1994 image.
    The green areas are those that were bright white in the 1995 image.
    The blue areas are those that were bright white in the 1994 image.
  4. However, notice that in the color composite image, the four most prominent colors are black, white, cyan and blue.
    Given how primary colors add together, what is your interpretation of what the colors mean on the color composite image?
    • Black means that these areas are dark on all the three images. Thus, black represents areas that are forested in 1994, 1995, and 1996.
    • White means that these areas are bright on all the three images. Thus, white represents roads that were developed prior to 1994 and that continued to exist through 1996 and possibly beyond.
    • Cyan, which is a combination of green and blue, represents areas that were dark (forested) in 1994, but were cleared between the time the 1994 and 1995 images were acquired. These areas are bright in 1995 and were assigned to the green channel and are also bright in 1996 and were assigned to the blue channel. Therefore, these areas show up as cyan in the color composite image.
    • Blue shows us the most recently deforested areas. Because the 1996 image was assigned to the blue channel, blue in the color composite image shows areas where deforestation took place after the 1995 image was acquired.

  5. A careful look at the color composite image reveals that there are several white linear areas that have adjacent cyan and blue edges. What do think might be the reason for this pattern?
  6. If you were responsible for traffic in your city, what would you do to accommodate the needs of growing population?
    These areas show the most recent road expansion.

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