Part 1: Download Landsat Data3
Register for USGS Data
The United States Geological Survey (USGS) is a goldmine for free geospatial science data. The USGS is responsible for managing and disseminating data to other government agencies, the research community, and the public. Much of this data comes from Earth-orbiting satellites such as Landsat, Terra, and Aqua. Typically, the images you find on the Internet have been heavily processed to create pretty, press release quality images. Unfortunately, these images are of limited use for scientific research. Fortunately, the USGS provides free tools and services to access minimally-processsed satellite data for research.
To download data files from certain USGS sites, you need to register for a free account. If you already have an account, use it. If not, complete the registration process before continuing. After registering, log in with your username and password. You can close the login window, but don't log out until you are finished downloading data.
Download Landsat Data Using the USGS Global Visualization Viewer (GloVis)
The Global Visualization Viewer is a tool for previewing, selecting, and ordering data from a variety of USGS satellite image collections. One advantage of this site is that it provides data for all of the spectral bands produced by the satellite.
Why not use Landsat 7 data?
Landsat 7, launched in April 1999, is the newest satellite in the Landsat series. Why, then, don't we use recent Landsat images in our investigations? On May 31, 2003, a component called a Scan Line Corrector (SLC), which removes the "wobble" from the rotating image sensor as the satellite moves forward in its orbit, failed. The satellite continues to operate normally in other respects, but the SLC problem produces black stripes of missing data across all but a narrow strip down the center of each scene. To learn more about this problem and see a sample Landsat 7 SLC-off image, see the explanation here.
For this reason, you will download data from Landsat 5, which has a working SLC.
You will look at a portion of the Colorado Plateau in Northeastern Arizona that includes a famous tourist attraction and one of the youngest and best-preserved meteorite craters in the world, Barringer Meteor Crater.
- Click here to open the GloVis page.
- The GloVis window has its own menus. To view data from Landsat 4 and 5, choose Collection > Landsat Archive > Landsat 4-5 TM.
The TM or Thematic Mapper instrument produces images through seven different filters, at visible and infrared wavelengths. These wavelengths were chosen because of their importance in identifying different types of ground cover and surface features and processes.
Landsat orbits
Landsat orbits are fascinating (you can read more about them here), but the key thing to remember is that the satellites' orbital tracks allow them to image the entire planet between 82 degrees north and south latitude every 16 days. Each orbit follows a roughly north-south path as Earth spins underneath the satellite. The satellite images continuously along this path. For convenience, each path is divided into 248 smaller chunks called rows, each row corresponding to about 24 seconds of satellite time. This path and row coordinate system is called the Landsat Worldwide Reference System (WRS). Landsat scenes are located by their path and row numbers, and are further identified by the date.
GloVis allows you to preview the desired scene (and the eight surrounding scenes, in case you're off a little) by entering either its WRS path and row number or the latitude and longitude coordinates of the feature you're interested in. The Scene Information field tells the scene ID, the percentage of cloud cover (ideally 0% if you can find one), and the date.
- Click the four direction arrows to choose adjacent scenes, and click the Prev Scene and Next Scene buttons and note the dates of the scenes. How many days apart are they?
- According to Google Earth, Meteor Crater is located at about 35.03 North and 111.03 West or WRS Path 36, Row 36. Enter either set of coordinates in the viewer window and click the appropriate Go button. (Hint: Use + for North and East, and – for South and West. Thus, enter the coordinates as 35.03 and -111.03. If you get these signs wrong, you'll wind up in the wrong hemisphere!)
- The scene containing Meteor Crater should appear in the center of the Viewer window. You won't be able to see the crater—it's too small to see at this scale. Using the Prev Scene and Next Scene buttons, look for a recent scene with 0% cloud cover that is labeled "Downloadable" in red in the upper left corner of the preview window.
- To order the data for that scene, click the Add button below the Landsat 4-5 TM Scene List, then click the Send to Cart button.
- In the cart window, click the download icon for the file you selected.
- Choose the Level 1 Product (165.0 Mb Geotiff) download option, then click the Download button. Your download should begin soon.
- It's a LARGE file, so the download time will depend on network speed and traffic conditions. As the file (LT50360362011173PAC01.tar.gz) downloads, look at its name—it may look like a bunch of random letters and numbers, but it's not! Can you figure out what it means?
- When the file finishes downloading, it needs to be unzipped and "un-tarred." On Macs, this should happen automatically. Under Windows, right-click the downloaded file and extract the files using your favorite unzip application.
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Explore the Data
Metadata
Before examining the images themselves, look at the metadata files. Metadata means "data about the data." Thus, the metadata files are text files containing information about the images and their source. Another useful ImageJ feature is its ability to open plain text files.
- Locate the downloaded and unzipped LT50360362011173PAC01 folder and open it so you can see its contents. It should look something like this. (Change the file view to the detailed list view shown here.)
- Using the drag-and-drop file opening technique you learned earlier, drag the README.GTF file onto the ImageJ status bar to open the file. (Yes, ImageJ does test files!) The file should open, looking something like this.
- Scan through the file to the line that says: For information on TM products (Landsats 4-5):. Select and copy the URL from the next line of text, choose File > Import > URL, paste in the address, and click OK. What happens?
- Close the web page and continue scrolling down the README file to the section titled ORGANIZATION. What letters would you look for to identify the actual metadata file?
- Keep scrolling down to the Data File Names section. How is the date encoding of the file names different from the date encoding of the folder name?
- Keep scrolling. How would you identify the image showing Landsat Thematic Mapper Band 3 (0.63 - 0.69 micrometers)?
- Close the README.GTF file, and use the drag-and-drop method to open the metadata (MTL) file in ImageJ.
- Find the width (PRODUCT_SAMPLES_REF) and height (PRODUCT_LINES_REF) of the images, in pixels.
- What is the scale of the images (GRID_CELL_SIZE_REF), in meters?
- Finally, what map projection is used? (Tip: This is useful information if you're using the image in a GIS!)
- Close the metadata file.
- The last text file is the GCP (ground control points) file, which describes how close the points in the image are to the corresponding points on the ground. A quick scan of this file shows that all of the pixels in the images are within about 6 meters of their true positions on the ground.
Metadata files are important and useful, but they're not much fun to look at. You don't always need to read through the metadata before opening an image, but it's useful to know it's there and what information it can tell you. Now it's time to look at the images.
Opening Landsat images
- Using any method you're comfortable with, open the Band 1 (B10) image in ImageJ. Confirm that the dimensions of the image (width, height, and bit depth) match what you read in the metadata.
- Zoom and scroll around the image and look for Meteor Crater. It's small in these images, but stands out because of its round shape and the bright rocks exposed around the rim.
Set the Scale
- Choose Analyze > Set Scale and use the information from the metadata to set the scale for the image. In the Set Scale dialog box, check the Global option. This will automatically set the scale of the other Landsat images when they're opened. When you're finished working with Landsat images, be sure to open this dialog box again and turn off the global option.
- Using the Straight Line selection tool, measure the east-west diameter through the center of the crater.
- If you wanted to line the entire crater with aluminum foil to make a giant solar reflector, how much foil would you need? To measure the area, use the oval selection tool to select, then measure the approximate area of the crater. Tip: Normally, the selection tools drag from corner to corner. To select from the center, hold down the command (Mac) or control (PC) key, then click in the center of the crater and drag outward.
- Practice using other tools to measure the diameter, circumference, and area of the crater, and compare these measurements with your initial results. The selection tools with the small red triangles have multiple options. To choose from these optional tools, right-click (PC) or control-click (Mac) the tool button and choose the appropriate tool from the drop-down tool selector.
Opening Additional Band Images and Measuring
Remember, as you work with these large images, memory will fill up fast. Use either the Monitor Memory macro utility or click the ImageJ status bar frequently to check the available memory. To clear unused memory, double-click either the Monitor Memory window or the ImageJ status bar.
- Open the Band 2 and Band 3 images.
- To see the image windows side-by-side, choose Window > Tile
- Confirm that the scale is set correctly for bands 2 and 3 by measuring the diameter of the crater in each image. Here's another neat trick: zoom into the crater on one image, select the crater, measure it, activate the next image, then choose Edit > Selection > Restore Selection to apply the selection from the original image to the current image. If you measure the crater on the second (and third) image this way, you should get exactly the same area as the first.
You will learn more about selections and measuring on Day 3.
Comparing bands with stacks
- Open bands 4 through 7, in order. You should now have all seven bands open.
- If you have tried all the memory tricks (like "garbage collection") and you still can't open and stack all seven Landsat bands of the Meteor Crater scene, download this file. Here's another neat trick—you don't even need to unzip the file! ImageJ can open zipped TIFF files (well, some of them anyway) without having to unzip the file first. In fact, you can just drag the link below and drop it on your ImageJ status bar to open the stack. Small Meteor Crater stack (Zip Archive 42.3MB Jul18 11)
- Choose Image > Stacks > Images to Stack, and name the new stack "Meteor Crater Stack".
- Look at the information in the image window status bar. Use the left and right arrow keys to flip through the stack, and note what information changes and what stays the same. How much of your computer's memory does this stack occupy?
- Use your keyboard or the stack buttons on the ImageJ toolbar to flip back and forth through the stack. Notice both the general appearance of the image (Example: Is it low or high contrast?) and the appearance of specific types of features in the image (water, snow, rock, forest, etc.). To see the wavelengths of the Landsat 5 bands, go here .
Is this a spatial, temporal, or spectral stack?
- Choose Image > Stacks > Stack to Images to separate the images into their own windows.
Your Assignment: Download A Landsat Image
- Create a folder (directory) on your computer for your Day 2 files.
- Download at least one band of Landsat image data (from GloVis or Click and Pick) and save it to your Day 2 folder.
- Locate a feature of interest, set a scale, and measure the feature.
- Then go to the Part 1: Share and Discuss page and post a screen shot of the image, demonstrating that you can download a Landsat image.
Screenshot Instructions for Mac Users
- Press Command-Shift-4 (Command key = Apple key) all at the same time and drag a box over the area of the screen that you want to capture. Depending upon your operating system, this will produce a file named Picture1.png or Screen shot Date Time.png on your desktop. Move the screenshot to your Day 1 folder or to a place where you can easily find it. Double click on the file to open it in Preview. Rename the image and save it as a jpeg, giving it a name that describes it, such as Lake_Mead_pixel_zoom.jpg.
Screenshot Instructions for PC Users
- Press Alt and Printscreen at the same time. This will save an image of the screen to the computer's clipboard.
- Launch Paint and choose Edit > Paste.
- Save the image as a jpeg, giving it a name that describes it, such as Lake_Mead_pixel_zoom.jpg.
Sources
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
4From Remote Sensing Math: A Brief Mathematical Guide by Dr. Sten Odenwald, NASA 2011.