Part 3—Select and Measure Distances
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-Select Distance to Measure
Look for a feature that you can identify in both images that will allow you to make a fair comparison of the width of the lake in 2001 and 2003.
- Zoom in or out on the 2001 image to focus on the area you want to measure across.
- From the ImageJ toolbar, choose the Straight Line selections tool.
- Click down on your mouse button on the image at the location where you want to start your measuring line and keep the mouse button down as you drag your cursor to the end of your line. Let up on the mouse button at the end of your line.
- You can adjust your selection by dragging the "handles" that appear at each end and in the center of the selection line.
- To delete a selection, simply click the image.
Show me an example selection
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- In this example, the widest part of the sea on the right side of the image is selected.
- You can decide for yourself if you should include what appears to be wet land or if you would rather measure what is clearly water. Be consistent in how you measure each image.
- Hold the Shift key down as you make the selection if you want to keep the line horizontal.
- For a larger view of the example, click the image.
Step 2-Measure Distance
- Select Analyze > Measure.
- The distance you selected is displayed (in meters) in the Results window. Divide the results by 1000 to get the number of kilometers across the lake. If the Results window does not appear, select Window > Results to bring it forward.
Step 3-Compare Measurements
Repeat Steps 1 and 2 for the 2003 image. Record your measurements on paper and use them to answer the following questions.
- How much did the lake's width change in two years?
- If the lake keeps shrinking at the rate you measured, how long would it take to disappear?
- Over the two years, did the area covered by water on the left side of the lake shrink by the same amount as the water on the right side of the lake? Why do you think there might be a difference in the amount of change in the distance across each side of the lake? Use evidence in the images to support your answer.
- The widest portion of the east side of the Aral Sea appears to have shrunk by about 8000 m or 8 kilometers in the 2 years between September of 2001 and September of 2003.
- At this rate (4 km per year), it would take approximately 20 years for the rest of the 79.5 kilometer width of the lake to disappear. This calculation is only theoretical: distance across the lake at only one location is not a direct proxy for volume of the lake.
- The difference in the distance across the left side of the lake from 2001 to 2003 is less than on the right side, perhaps because the left side of the lake is deeper. Changes in the volume of a three-dimensional shape such as a lake cannot be characterized very accurately by measuring only one dimension.
Measuring the distance across the Aral Sea is one way to document how it has changed; measuring the area of land covered by water is another way. For example, you could use ImageJ's Freehand selection tool to trace along the shoreline and measure to find out how much area the sea covers in each image. Tracing along the shoreline by hand can be quite tedious though-Part 4 will step you through a technique to automatically highlight the pixels that represent water so you can easily measure the area covered by water in each image.
