Eyes in the Sky II > GIT Web Course > Module 1 > Week 2 > Using ImageJ: Part 1

Week 2: Analyzing Change Over Time

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Using ImageJ to Analyze How Lake Mead Has Changed Over Time: Part 1


Lake Mead with bathtub ring Lake Mead in recent years. Notice the white coating on the rock. The line is what is known as the "bathtub ring".

Investigation questions:

Lake Mead was created to store water for agricultural and domestic use, as well as to generate electricity. It has also become an important recreational resource in 1964 Lake Mead became our country's first National Recreation Area. As the water level drops, all of these uses are affected.

Some experts predict that if Lake Mead falls more than about 50 feet below its January 2010 level, power generation will not be possible.
Lake Mead Elevations

Lakes are 3-dimensional features kind of like upside-down mountains. As the water level drops, the volume of the lake decreases dramatically. As of January 2010, the lake was at only 44% of capacity. To get a real sense of how the water level of Lake Mead has changed in recent years, and what this means in terms of the volume of water lost, we need to view the lake from two different perspectives from above to get a sense of the change in surface area, and from the ground to measure the change in elevation (depth).

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Download and Open Images

You're going to use a pair of high resolution Landsat images one taken in May 2000 and the other in May 2004 to measure changes in the surface area of the lake.

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Make a Visual Comparison

These true color satellite images show the exact same view of Lake Mead, four years apart. Slight color differences between the two images are mostly due to differences in atmospheric conditions on the two dates.

Another way to compare the images is to display both images in the same window and flip back and forth between them. In ImageJ a window containing multiple images is called a stack, and the individual images are called slices. Stacking images is a useful way to look at images that show change over time. In addition to flipping through images, you can also animate them. (You'll learn a lot more about stacks in Week 3!)

What are the main differences you see between the images of the lake on the two dates? Look for specific features in and around the lake that change dramatically. Think about how these changes affect both people and wildlife.

The lake is noticeably smaller and you can see the "bathtub ring". It is now much more difficult for people to launch boats and for fish to survive in the shallower water. Some islands are now connected to the shore, making once-safe habitat more accessible to predators. Elsewhere, new islands have appeared, creating new habitat for some animals.

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Select What You Want to Measure

You are interested in measuring the surface area of the lake. You'll also need to measure straight line distances when setting the scale.

  1. Choose Analyze > Set Measurements....
  2. In the Set Measurements dialog box, check the Area and Perimeter (length) options. Since you are working with a stack, check the Stack Position option, then click OK.
  3. Analyse set measurements stack chckd
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Set the Scale

Now you'll set the scale on these images using the distance you measured between the two islands. One reason for stacking the two images is that setting the scale for one image in a stack automatically sets the scale for the others.

The scale is now set for both the 2000 and 2004 images.

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Movies on This Page

How to download movies

  1. Click the link to go to the SERC media library listing for the movie. The record will open in a new window.
  2. On the SERC media library page, right-click (Win) or control-click (Mac) the link (below the movie on the Flash version pages) to download the movie file to your hard drive.
  3. Look below the movie window for the file download link.

    Save Movie from CMS listing

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Flash video versions

Download this version to play on your computer. You'll need an appropriate movie player to view the file, such as Flash Player, Real Player (Mac / Win), or Adobe Media Player.
Movie Icon Animate Lake Mead Stack

Movie Icon Set Scale Stack

Movie Icon Make Scale Bars

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iPod versions

Download this version to play on your iPod or iPhone.
Movie Icon Animate Lake Mead Stack

Movie Icon Set Scale Stack

Movie Icon Make Scale Bars

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