Landsat Data Searching
Old Wives Lake, Saskatchewan

Introduction

The primary purpose of this exercise is to become familiar with online sources for Landsat datasets and to learn how to select, download, and process those datasets. The U.S. Geological Survey operates two search routines, EarthExplorer and Glovis, for this purpose. Your instructor prefers the latter, the Global Visualization Viewer, but both are equally useful.

Old Wives Lake is a large, shallow, saline lake in south-central Saskatchewan, Canada (lat/long ~50°N/106°W), just north of eastern Montana. It gained its name from an Indian legend (see historical sign). It is the fourth largest saline lake in North America. Given its great surface area and shallow depth, it is subject to large changes in size from year to year during drought and flood cycles. Sodium-sulfate is present both in the brine and in salt deposits, mainly as the mineral mirabilite (Glauber's salt), which was produced commercially in the past from Frederick Lake at the southeastern end of Old Wives Lake. For more information, go to the Encyclopedia of Saskatchewan.

Overview of Old Wives Lake seen from the southeastern margin looking toward the northwest. A high shoreline is marked by the sinuous brushy line across the scene middle. Mud/salt flats are visible to right, and open water can be seen in the distance. This scene depicts only a small portion of the lake basin. Kite airphoto © by SWA and JSA, July 2010.

Exercise—part A

Begin by using one (or both) of the USGS search routines. Target your initial search for Landsat 7 (SCL-on, 1999-2003). The relevant scene is located at WRS-2 path 36, row 25. Find and download the following scene: Sept. 9, 2002 (LE70360252002252EDC00). To place orders or download scenes, students may need to set up personal accounts with the U.S. Geological Survey. It's simple and free, as part of the online procedure at EarthExplorer or Glovis.

Browse image of scene LE70360252002252EDC00 acquired on 9 Sept. 2002. The large lake in the southwestern quadrant is Old Wives Lake (*). Clouds (and cloud shadows) cover the southeastern corner of the scene, which is otherwise cloud-free. Browse images are false-color composites based on ETM bands 3, 4 and 5 color coded as blue, green and red. Active vegetation appears in green and yellow-green colors. Image modified from USGS Glovis.

Downloading Landsat datasets in Glovis. This part is a bit tricky. When you get to this screen, click on the active link under "Entity Id" then scroll to bottom of next page and click on "Download." Select the "Level 1 GeoTIFF Data Product" option.

Once you have obtained the file, you should observe that it is stored in an unusual, compressed format with a "tar.gz" extension. In order to extract this file into its components, special software is necessary. Download and install 7-Zip free shareware. Most students should need the "exe" 32-bit for Windows version (unless you have an unusual computer).

With 7-Zip, extract and decompress the files contained in the dataset. This requires a few steps to accomplish. Using 7-Zip, first "open" the file, which should remove the "gz" extension from the file name. Then "extract" the tar file. Double click on the resulting tar file, which should display the individual file contents. Select all files and "extract" again.

You should obtain nine, large individual data files, one for each band (B10, etc.), plus some small metadata files. Once you have extracted these files, you may delete the tar.gz and tar files to save disk space. Each band is a "tif" file, or more specifically, a GeoTiff file. This is a basic image file type with the addition of georeferencing information.


Idrisi can import these files directly (under File, Import, Government, Landsat ETM). Click GeoTiff as the data type, and enter the B10 file for band 1. All other bands will be added automatically. Click OK and wait for the large files to be imported. A sample image (band 4) should display with default settings, as the import routine continues to convert additional bands.

The browse image shown above is not georectified. The downloaded dataset is georectified in the UTM projection, as you should notice upon initial display of the individual bands; check metadata for any band. The whole scene is rotated several degrees (~15°) clockwise to account for the satellite orbit path at this latitude. This rotation introduces large black borders around the actual image. At this stage, the image is much too large for our purpose, and the black border is distracting.

As your next operation, create a small subscene for the Old Wives Lake region, similar to the example below, approximately 1400 columns by 1100 rows in size. Locate the Window function (under Reformat). ETM band 4 (near-infrared) has the best distinction between water and active vegetation, so it's often best to begin with this band. Enlarge it and determine row/column (or x/y) values to use for the window. This may take a some trial windows to achieve one you like. This window, then, may be used as the template for extracting identical windows for the other bands. Create subscenes for bands 1, 2, 3, 4, 5 and 7; you may want to experiment with bands 6 (thermal) and 8 (pan) on your own later.

Upon extracting windows for the Old Wives Lake region, create a false-color composite based on ETM bands 3, 4 and 5 color coded as blue, green and red, which should be similar to the browse image above.

Sample subscene for Old Wives Lake. False-color composite made from ETM bands 3, 4 and 5 color coded as blue, green and red. Your image should appear similiar, but without annotation and scale bar. Asterisk (*) indicates location of kite aerial photograph shown above.

1. Describe the appearance of Old Wives Lake and its surroundings paying particular attention to the water body itself, the shoreline, and adjacent features. What aspects do you find most striking?

2. Give the basic geospatial information about your window: projection, area (km²), and resolution.

3. What are the approximate dimensions of Old Wives Lake—length and width?

4. A distinct green patch appears in the southeastern portion of the lake. What do you suppose this is? Explain your answer.

5. Describe the apparent drainage system into and out of Old Wives Lake.

The 2002 scene depicts Old Wives Lake during a full-water stage. During extended drought periods, however, the lake becomes much reduced or may disappear completely, which has happened several times.

Space-shuttle color-visible photograph, southern Saskatchewan. 1 = Old Wives Lake, 2 = Lake of the Rivers, 3 = City of Regina. Notice that Old Wives Lake and Lake of the Rivers are both mostly dry mud and salt flats in this scene. STS28-152-164, date 8/89.

Exercise—part B

Your next task is to locate an appropriate Landsat TM image that depicts Old Wives Lake during a dry phase with little or no water present. You may search through Landsat 4 and 5 TM, Landsat 7 ETM+ (SLC-on), and Landsat 8 OLI datasets, but do not use Landsat 7 ETM+ (SLC-off). Identify, request and download a suitable dataset. Process a subscene equivalent to the Sept. 2002 subscene, and create a false-color composite image, as above, based on TM/ETM bands 3, 4 and 5 (or equivalent OLI bands).

6. Describe the appearance of Old Wives Lake and its surroundings paying particular attention to the water body itself, the shoreline, and adjacent features. What aspects do you find most striking?

7. Compare the 2002 composite with your "dry" composite image. What features have changed? What features are relatively constant?

The TM/ETM 345 composite is a good "first look" for visually examining Landsat TM/ETM imagery. It includes visible (3), near-infrared (4), and mid-infrared (5) bands. Active vegetation appears green, and water bodies are blue/black. Now, experiment with other bands and color assignments for both subscenes. Note: bands 1-7 are directly comparable between Landsat TM and ETM; however, band 8 (pan) is found only with ETM and OLI datasets. OLI datasets have additional bands and slightly different band numbering. Select the false-color composite that you like the most.

8. What bands and color assignments did you select? Why do you like this particular combination?

As your final task, create map compositions for both subscenes with suitable titles, subtitles, scale bar, and other components. Save image files, named OWL_WET and OWL_DRY to turn in with your answers.

Turn in

Note: this is a double-credit exercise, worth 20 points.

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© ES 771 is presented for the use and benefit of students enrolled at Emporia State University. Any other use of text, imagery or curriculum materials is prohibited without permission of the instructor, J.S. Aber.