ES 771 Lab 2

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 Andes 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.

Devils Lake: A connected system of saline lakes, which occupy depressions of variable size and depth that were scooped out by glaciers. Devils Lake is part of an internal drainage basin that has no surface outlet at present. Lake level fluctuates with climatic conditions. In 1988, the lake was at a relatively high elevation. Extensive marshes are developed in some parts of the system, and algal blooms are common. The lake is famous for both summer and winter (ice) fishing.
Crow Hill and Sullys Hill: Parts of a major ridge created by ice-pushing of bedrock (shale) and sediment scooped out of the depressions of Devils Lake. These ridges were formed by advances of two ice lobes, one from the north and another from the northeast. As much as 200 m of relief exists between the crest of Sullys Hill and the bedrock floor beneath the central depression of Devils Lake. Deciduous forest covers much of the ice-pushed ridge.
Spillway valleys: Deep valleys south of the ice-pushed ridges, including Sheyenne River, Big Coulee, and other valleys. These valleys were eroded by melt-water floods from the North Viking ice margin. Some forest and scrub vegetation grows in these valleys.
City of Devils Lake: County seat of Ramsey County, a regional transportation and service center that caters to tourists and sportsmen. The city's sewage treatment plant is located near the head of Creel Bay. Several other smaller cities are included in the scene.
Agriculture: Cropland and pasture cover most of the low-relief portions of this scene. Ground water from the shallow Warwick aquifer (southeast corner) is pumped for center- pivot irrigation of spring wheat and other summer crops. No winter wheat is grown in this area, but about one-quarter to one-third of the cropland is summer fallowed (bare soil).

	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 and 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 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 Glossary under the Help, contents menu.

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 (5th icon from left). 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.

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

Devils Lake (Main Bay, center)
Devils Lake (West Bay)
Sullys Hill
City of Devils Lake

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.

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.

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.

Devils Lake
Sullys Hill
City of Devils Lake
Agricultural cropland

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" to access and create each of the composition elements.

Title including location, date of Landsat image acquisition.
Subtitle with your name and date of composition creation.
Scale bar, 10 km long, divided into 5 intervals.
North arrow. Notice that true north is different from the image grid north. Show true north on image. Hint: consult maps and road network to determine the angle between grid and true north. Note: this has nothing to do with magnetic declination.

This may require some manipulation of feature size and placement in order to display the composition completely. Position the north arrow in the upper right corner of image, and put the scale bar in the lower left corner. Title and subtitle should be at top, above the image scene. Add the north arrow as the last step. 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:

Written answers (1-12).
Digital image file for DEVIL composition (jpg format preferred).

ES 771 schedule or ES 771 homepage

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