Earth Analysis Techniques > Image Analysis Modules > Introduction to Image Analysis > Day 5—Apply Image Analysis Techniques to Investigate Research Questions > Part 1: Use ImageJ to Analyze How Lake Mead Has Changed Over Time

Part 1: Use ImageJ to Analyze How Lake Mead Has Changed Over Time2

Drought in the Desert Water and Energy for a Thirsty Southwest

Lake Mead 2000 Labeled

Except for some coastal areas, the southwestern United States is mostly desert beautiful in it own way, but dry and inhospitable. In the early 1920s, the southwestern states realized that they needed to plan for the water and energy needs of their rapidly expanding population and for agricultural development. The Bureau of Reclamation began making plans to build a series of dams along the Colorado River to store water and generate electricity.

Hoover Dam Aerial View

The Bureau began the construction of Boulder Dam, later renamed Hoover Dam, in 1931. The dam was completed in 1936 and soon formed one of the world's largest reservoirs, Lake Mead. Over 97% of the water that flows into Lake Mead comes from rain and snowfall in the Colorado River watershed of the Western Rocky Mountains. Historical records show that patterns of severe drought frequently strike this region.

As the graph below shows, the water level of the lake has fallen dramatically since 2003. Today, a visit to Lake Mead and Hoover Dam shows a lake surface far below the high water mark, surrounded by a broad, white "bathtub ring" of mineral deposits left along the shoreline by the retreating water. As of January 2010, the elevation of the lake surface has dropped by over 130 feet, and the lake is at 44% capacity. Planners project that if the drought continues, the dam will eventually be incapable of generating a reliable supply of electricity. Communities that depend on the water and power supplied by the lake will be forced to look elsewhere for these important resources.

Lake Mead Water Levels Lake Mead Water Levels Historical and Present
Source: http://www.arachnoid.com/NaturalResources/


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).

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.


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.

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.

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

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.


Select a Region of Interest to Measure

In this investigation, the region of interest is the lake. To measure the area of the lake, you need to select or highlight it in some way. There are many ways to do this. In your own investigations, you may need to experiment with different techniques until you find the one that works best for you. Using the selection tools to outline the lake would be tedious and probably wildly inaccurate.

The key question is: What is it about the lake that makes it stand out from what isn't lake in the image? The color? The brightness? The water in these images appears much darker than the land. ImageJ has tools that can use these differences to highlight and select just the lake.

Specifically, you're going to use the Wand (tracing) tool Wand tool to select the pixels that represent water. When you click on the image with this tool, ImageJ selects all of the pixels within a contiguous area - touching each other - that have values within a specified range, called the tolerance. For example, if the pixel you click on has a value of 25 and the tolerance is set to 20, all adjacent pixels with values from 5 to 45 would be selected. Using this tool takes a little practice, but you should get the hang of it quickly.

Set Tolerance

Measure the Region of Interest

The yellow selection outline is still visible, showing the shoreline of the lake in 2000 superimposed on the 2004 image. This is another technique for making visual comparisons. You don't want to do it now, but you could draw this outline on the image to make it permanent.

You have measured the surface area of Lake Mead in 2000 and 2004. Write down your results or print out the ImageJ Results window. (Or, you could export it as spreadsheet data, but that seems overkill with just two areas.) To estimate the volume of the water lost over this four year period, you need to know the change in elevation of the lake surface that corresponds to this time interval.


Quantify the Change in Depth

Hoover Dam Intake 10-26-2004

Ground-based images often complement the views we get through our eyes in the sky. To determine the change in the elevation of the lake, we'll use good old-fashioned snapshots of Lake Mead.

Hoover Dam was completed in 1936 and continues to be a popular and scenic tourist stop. Visible in almost every tourist snapshot of the dam and the lake behind it are one or more of four white concrete structures, connected to the dam by concrete walkways. These are the very tops of four huge water intake towers that feed lake water to the electric generators below the dam.

In these historical photos taken during the construction of the dam, you can see the true nature of the intake towers. The two on the left are on the Arizona side of the dam and the two on the right are on the Nevada side. (The boundary between the two states runs right down the middle of the river and the dam.)

Hoover Dam Intake Towers 1935

Each tower is 395 feet high, 82 feet in diameter at the base, and 48 feet in diameter at the very top. The highest water level ever recorded 1226 feet above sea level just touched the bottom of the concrete walkways leading from the top of the dam to the tops of the intake towers.

This image shows the two intake towers on the Nevada side of the dam. You will use the information about these towers to set the scale for measuring the water elevation.

Tips for measuring Hoover Dam intake towers
Most satellite images are nadir images - that is, they look straight down on Earth's surface. Every point (pixel) in the image is approximately the same distance from the satellite, so setting a scale or spatial calibration based on one part of the image is valid for the entire image.

The same can't be said for photographs like this one. The features in the image are at different distances from the camera. When you set a scale on an image, it is only meaningful for features at one distance. Be sure to use the same intake tower for setting the scale and measuring the distance from the high water mark (the bottom of the walkway) to the lake surface.


To continue the investigation, here is a set of intake tower images from different years, courtesy of the U.S. Bureau of Reclamation. Additional images are available online, but can't be distributed here due to copyright restrictions.

If you enjoyed this and want to practice this process more, here are some additional intake tower images you can download and measure. Click the thumbnail to open a full-size version of the image and right-click (Win) or control-click (Mac) the image and save it to your Week 2 folder. You may want to create a sub-folder for just these images.

Hoover Dam Intake 03-24-80
March 24, 1980

Hoover Dam Intake 03-25-83
March 25, 1983

Hoover Dam Intake 07-20-1983
July 20, 1983

Hoover Dam Intake 04-08-97
April 8, 1997

Hoover Dam Intake 09-27-2005
September 27, 2005

Hoover Dam Intake 04-26-2006
April 26, 2006

Hoover Dam Intake 03-23-07
March 23, 2007

Hoover Dam Intake 07-05-2009
July 5, 2009

Time-Series Pairs From the Earth Observatory

Aquaculture


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17204

Coastal change


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17211

Deforestation


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17943 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17404 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17358 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17240 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17062 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16274

Dust, haze, and air pollution


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16997 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16725 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17882

Fires


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=4027

Flooding


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=18011 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17776 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17585 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17256 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17246 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17202 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17075 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16881 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16857 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16794 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16680 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16324 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=15781 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=10316

Glacial retreat


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17905 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17158 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16625 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=15341

Hurricanes


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17024 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17020 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16310

Ice shelves & sea ice


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17513 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16339 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=10894

Invasive species/water plants


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17778 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17671

Lakes


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17700 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17634 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17623 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17532 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16986 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16913

Landslide


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17081 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16656 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=10781

Mining


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=10772 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17870

Reservoirs


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=18009 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17802 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17674 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17632 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17357 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17280 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16302 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=15320

Rivers


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17383 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17305

Seasonal variability


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17656 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17584 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17448 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=15270

Snow cover


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17699

top of page

Suspended sediment


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17792

Tornadoes


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17684

Tsunami


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16777 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16774 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16778 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16892 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16777

Urbanization/human presence


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=16495 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17735 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17647 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17296 http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17113

Volcanoes


http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17555



Time Series Images in World of Change Collection

  • Earth Observatory: World of Change: time series of images that demonstrate change over time.


  • Your Assignment: Stack, Calibrate, and Animate an Image Set to Investigate Change Over Time

    1. Browse through one of the two collections above and find a set of images that interests you.
    2. Download the files to your Day 5 directory or folder and use ImageJ to open, stack, and flip back and forth between the images.
    3. Use the same process as in the investigation above to set a scale and make measurements on your image.
    4. Share your results and describe how the features in your image that have changed over time.
    5. Go to the Part 1: Share and Discuss page and post a montage of the images you used along with your results.

    Source

    1Adapted from Earth Exploration Toolbook chapter instructions under Creative Commons license Attribution-NonCommercial-ShareAlike 1.0.
    2Adapted from Eyes in the Sky II online course materials, Copyright 2010, TERC. All rights reserved.
    3New material developed for Earth Analysis Techniques, Copyright 2011, TERC. All rights reserved.


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