Going Further
Variations
NASA's Earth Observatory frequently posts a single image file that shows both a before-and-after image of the same place. Follow this technique to create two separate images from the single file so you can put them into a stack. Go to Earth Observatory's before-and-after images of springtime flooding in Argentina- Read through the story to learn about the events responsible for the differences in the two images.
- Download the single image file that shows both images. Save it in an obvious place such as your Desktop or Downloads folder.
Right-click on the image (Mac users press ctrl-click) and choose the option your browser offers to save the image to your own computer.
- Launch ImageJ and open the file from within it.
From the ImageJ menu, choose File then Open... then navigate (browse) to where you saved the image.
- From ImageJ's toolbar, choose the rectangular selection tool. Place your cursor on the image and drag a selection box over the top picture. Use the arrow keys to move the selection into the upper left corner of the image. Use the handles to adjust the selection size to include the date below the image.
- From ImageJ's menu bar, choose Image --> Duplicate... and accept the default name.
- Click the large image again to activate it. Click inside the yellow selection rectangle and drag it down over the bottom image. It's essential that the selection rectangle remains the same size.
- Choose Image --> Duplicate... and accept the default name again.
- Once you have the two images in separate windows that are the exact same size, activate the large image again and close it.
- From ImageJ's menu bar, choose Image --> Stacks--> Convert Images to Stack
Download the stacked images ( 1.1MB Apr10 07). Open the file from within ImageJ.
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Other Data
EarthShots: Satellite Images of Environmental Change (more info) is a collection of registered time series images taken by Landsat instruments since 1972. You can use the same techniques presented in this chapter to annotate changes in almost any of the images on this site.
Check the Images Index at NASA Earth Observatory to find before-and-after images that they have prepared for their image of the day feature. Time-series images are not identified consistently, so they require visual inspection to locate the image pairs.
Another example of an annotated deforestation map is available from Earth Observatory.
Other Techniques
For images where areas of change are too small to outline with the freehand selection tool, you can change the color of just a few pixels at a time by selecting them with the freehand line selection tool.
This tool works similarly to the freehand selection tool, but it does not automatically enclose an area. Instead, only the pixels you drag across are selected.
Other Tools
Two non-Java versions of image processing software could be used to accomplish the tasks in this chapter. Because these applications don't use Java, they may run faster on some computers:
NIH Image is the Macintosh-only original version of Image J.
Scion Image is a Windows-capable version of the original NIH Image. The software is free, but you will need to register to download it.
Case Studies with Tool
Two other EET chapters utilize ImageJ as a tool:
- Measuring Distance and Area in Satellite Images describes how to use ImageJ to quantify change in satellite images.
- Analyzing the Antarctic Ozone Hole steps users through measuring and graphing the area of depleted ozone from TOMS images.
