ES 771 Landsat MSS Image Display

DEVILS LAKE, NORTH DAKOTA

Introduction

This exercise is based on exceptional Landsat MSS scenes from Devils Lake, North Dakota. Devils Lake is a large natural lake system that occupies a series of glacially scooped basins in the northeastern part of the state. Water level and lake area have fluctuated considerably during the past two centuries. The first scene was acquired in the autumn of 1988 during a drought period.

Each MSS band consists of two files: *.RST (image file) and *.RDC (documentation file). The image file contains the digital values for each cell in the scene; the documentation file has information about the image file format, size, type, resolution, etc. The Idrisi files are named DEVIL1.*, DEVIL2.*, DEVIL3.* and DEVIL4.*. Set up a "main working folder" for this exercise on your computer. Copy these files (via FTP) into your main working folder, where you will conduct the exercise.

The scene includes several noteworthy natural and man-made features (see maps). Also examine space-shuttle stereo-photos from 1985 (on-campus only) to gain a better impression of the region.

Ground view of Sullys Hill (right horizon) and Devils Lake (far left) as seen from the southwest. Image date 10/03, © J.S. Aber.
Kite aerial photograph viewing toward the north. Sullys Hills can be seen on the left horizon, and Devils Lake appears in the distance. Image date 10/03, © J.S. Aber.
Kite aerial photograph looking toward the northeast. Devils Lake East Bay can be seen in the distance. Notice typical landuse pattern. Image date 10/03, © J.S. Aber.
Blimp aerial photograph showing Devils Lake Mountatin vicinity. Smaller lakes and pothole ponds are found throughout the region. Image date 10/03, © J.S. Aber.

The second dataset is a winter scene with ice on lakes and continuous snow over the land. In addition to snow/ice cover, winter scenes represent low sun elevation with reduced solar illumination, and active vegetation is generally lacking. Download the dataset D83 (MSS bands 1-4) from 23 Dec. 1983.

Exercise

Begin with the autumn dataset. Use Idrisi Explorer (bar on left) to set up a new project and learn about each band of the image. Note in particular the data type, file type, number of rows and columns, reference system and units, unit distance, resolution (cell size), and max/minimum x/y values, and min/max value units. Now use the Windows File Manager to find the size (bytes) of DEVIL image files.

1. How large in bytes is each image file? How does this compare to the number of pixels based on the number of rows and columns in each band?

2. What is the total ground area (in km²) represented by this Landsat scene? Hint: rows x columns x resolution squared, or max x times max y, which should give the same result.

Display (2nd icon from left) each of the four bands. Under palette click on greyscale, and accept all other default selections. Examine each of the bands to gain a general impression of the scene. Autoscaling is a quick means to view an image, but it produces rather poor displays.

3. Describe how autoscaling creates a gray-tone display. Hint: see Help: Contents.

You are ready now to perform some simple image processing. Read about and use the STRETCH module (4th icon from left) to create stretched images for bands 1, 2 and 4. Select "linear with saturation," and accept other default settings. Name your files DEVIL1S, etc. The images will be displayed automatically with a legend.

4. How does the appearance of the stretched images compare with autoscaling?

By now you should be familar with the landscape features depicted in this scene. Next read about the COMPOSITE module (under Image Processing: Enchancement: Stretch). This is the most important module for processing multispectral images. Use COMPOSITE to prepare a standard false-color composite: DEVIL1 = blue, DEVIL2 = green, DEVIL4 = red. Name the composite image DV-124. Accept default settings. Enter an appropriate title, and click OK. The image will display automatically in false color. Maximize the display--click the "end" key.

This sample image shows the Landsat false-color composite for Devils Lake. It's based on MSS bands 1, 2 and 4 color coded as blue, green and red. Active vegetation appears in red, pink and maroon colors. Your image should appear similar, but without titles and scale bar.

5. Describe the appearance of each of the following features.

6. Notice the varied appearance of certain portions of Devils Lake. What factors could cause these water bodies to look so different?

Now turn your attention to the winter dataset for Devils Lake. Download the D83 image (rst) and metadata (rdc) files. Use Idrisi Explorer to determine the ranges of data values in each band.

7. Which band has the greatest contrast (range) in data values?

The appearance of a single-band can be improved by making a stretched image. You have already done this for MSS bands 1, 2 and 4. Here you will experiment with the effects of stretching on band 3. Use the STRETCH module to make a linear stretch at 5% saturation. Name the output D83-3S; your result will be displayed automatically with the gray 256 palette.

This sample image shows the Landsat winter scene for Devils Lake. It's based on MSS band 3 in gray tones. The landscape is covered with snow and ice. Your image should appear similar, but without titles and scale bar.

8. What aspect of the landscape is emphasized and why? What landscape features are not readily apparent and why?

9. What is the apparent direction of solar illumination? Explain your answer based on Landsat orbit characteristics and time of day.

Next make a standard false-color composite (bands 1, 2, 4) as before. Name the output D83-124; it will be displayed automatically in false color.

10. Describe the appearance of each of the following features in the false-color composite.

11. Can you make out a distinction between snow and lake ice? Explain your answer.

As your final task, use the Composer (box on right) to build an image composition for the winter false-color composite. The composition should include the following elements. Open Map Properties (N arrow) to access and create each of the composition elements.

This may require some manipulation of feature size and placement in order to display the composition completely. The default is to place scale bar and north arrow on the right side, which leaves space for the insert text also on the right side. Save your composition as an Idrisi "MAP file" under the name DEVIL.

12. What declination value did you pick for the true north arrow?

Lastly save a BMP file of your map composition. BMP (bitmap) files are basic digital images, which can be converted into other, more compact image file formats, such as GIF and JPEG, using available graphics software, for example Adobe Photoshop.

Turn in:

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