Part 2—Set Spatial Calibration
Step 1 – Launch ImageJ and Open MODIS Images
- Launch ImageJ by double-clicking its icon on your desktop (Mac or PC) or by clicking the icon in the dock (Mac) or the Start menu (PC).
- 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.
- Repeat the process with the remainder of the images that you downloaded, or at least these three images.
- Choose three images that represent a span of the years available. Drag the images by their title bars to place them side-by-side. Use the Magnifying glass and the Hand (scrolling) tools to adjust the view in each window so you can examine the differences between the images.
- A quick way to re-size the images is to hold down the Control key and click the on the image with Magnifying glass tool.
- 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 zoom out, hold down the Alt (PC) or Option (Mac) key while you click on the image with the Magnifying glass tool.
- To Zoom out, all the way out to the full sized image double-click the Magnifying glass in the toolbar.
- When you are zoomed in, use the Hand (scroll) tool to move to other parts of the image.
- Look for landmarks that you could use to measure and compare the distance across the sea in 2001, 2003, 2009, and 2010.
Step 2 – Set Scale for MODIS Images
So how big is the Aral Sea? The satellite images you downloaded have a spatial resolution of 500m 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.
- From ImageJ's menu bar, choose Analyze > Set Scale....
- 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.
- Click the Global checkbox to apply the scale to all open images.
- Leave the Pixel Aspect Ratio field at 1.0 since pixels represent square areas on the ground.