Earth Analysis Techniques > Eyes on Satellite Data > Course Materials > Week 2 > Using ImageJ: Part 2

Week 2: Analyzing Change Over Time

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Using ImageJ to Analyze How Lake Mead Has Changed Over Time: Part 2

Lake Mead with bathtub ring

Investigation Questions:

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Download and Open the Images

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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

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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.

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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.

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Try it Yourself

In case you want to practice this technique more, or want 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

Here is a page of links to time series image data in NASA's Earth Observatory.

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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 Measure Lake Area
Movie Icon Measure Lake Elevation
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iPod versions

Download this version to play on your iPod or iPhone.

Movie Icon Measure Lake Area
Movie Icon Measure Lake Elevation top of page

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