Part 2—Set Spatial Calibration

In any step, click the Show me link to reveal extra information. If you prefer a printout of the full set of instructions for this part, choose Print from the File menu.

Step 1-
Open and Examine MODIS Images

  1. Start ImageJ by opening the folder that was installed and double-clicking its icon. Choose File > Open and navigate to where you stored the Aral Sea images and open both of them.
    2. hideHide
    1. Open the ImageJ folder and double-click ImageJ's icon to start the image processing application.
      2. Open image from within ImageJ application
    2. From ImageJ's menu bar, choose File > Open... then navigate to the folder where you stored the downloaded images. Select AralSea.A2001252.0715.500m.jpg, then click Open.
    3. Choose File > Open... again, select AralSea.A2003247.0905.500m.jpg, then click Open.
  2. Drag the two images by their title bars to place them side-by-side. Use the magnifying glass and the hand (scrolling tool) to adjust the view in each window so you can examine the differences between the images.
    • Select the magnifying glass in the toolbar and zoom in on one of the images to see the individual pixels (picture elements) or small colored squares of which the image is composed.
    • To unzoom, double-click the magnifying glass in the toolbar.
    • When you are zoomed in, use the hand tool to move to other parts of the image.
  3. Look for landmarks that you could use to measure and compare the distance across the sea in 2001 and 2003.

Step 2-
Set Scale for MODIS Images

So how big is the Aral Sea? The satellite images you downloaded have a spatial resolution of 500 m per pixel. This means that every pixel in the image represents a square area of the ground that is 500 meters on a side. Setting a scale (entering spatial calibration for the image) allows ImageJ to convert distances you measure across pixels in the image into the number of meters the measurement represents on the ground.

  1. From ImageJ's menu bar, choose Analyze > Set Scale....

    2. Set Scale dialog box
  2. In the dialog box that appears, enter a 1 in the known distance field and change the units of measure to m for meters. This is telling ImageJ that the distance across 1 pixel in the image represents a distance of 500 m in the real world.


« Previous Page      Next Page »