geolog

The purpose of this lab is to give the student some basic experience in interpreting airborne imagery and other forms of remote sensing. The first part of the lab will focus on a particular form of aerial photos called a stereogram. The second section will use a series of web pages to illustrate concepts of satellite remote sensing.

Pre-lab reading topics: Stereoscopes, albedo, air photographs, satellite remote sensing

AERIAL PHOTOGRAPHS

Aerial photographs were one of the first forms of remote sensing imagery, and they remain widely used today. Topographic maps are made from aerial photos, and many engineering projects employ them as well. Soil conservation studies, agricultural crop inventories, city planning, geologic mapping and exploration all use aerial photos. To know the techniques of air photo interpretation is to have an essential background for understanding other remote sensing images.

The film in an aerial camera is sensitive to reflected solar energy at wave-lengths in the visible and near visible spectral range. Land and ocean surfaces reflect some short-wave radiation directly back to the atmosphere. The percent of radiant energy reflected back by a surface is its "albedo." Surfaces which reflect more energy (high albedo) appear to be lighter on the photograph than surfaces which do not reflect as much energy (low albedo).

In this lab we will be using overlapping photos called stereopairs or stereograms. They were taken successively along a single flight line which thus enables the viewer to be able to produce a 3-D image through the stereoscopes. The remarkable vertical relief that can be seen is partly a result of "vertical exaggeration". Because the perceived vertical scale is larger than the horizontal scale, the vertical images appear to be four times the equivalent of horizontal distances. Therefore keep in mind that the stereoscopic view is not 100% realistic.

Plate 1

1. What stage of the erosional cycle is this area in? Support with three reasons. Use the “Cycle of Erosion” explained in class to determine your answer.

2. Calculate the relief, or change in elevation, between the Colorado River and the surrounding plateau (The topographic map will help you with this; values are in feet).

Plate 2

3. Describe the gradient (steep/moderate/flat), Strahler order (high/moderate/low), and shape (sinuous, oxbow, etc.) of the streams and rivers in this area.

4. Name the type of drainage pattern which is apparent in the image (dendritic, trellis, parallel, radial, rectangular, deranged).

Plate 3

5. Why are there so many lakes here?

6. What are these type of lakes called?

Plate 6

7. List several possible explanations for the line-like features that cut across the ridges. Do they appear to be natural or man-made (consider the relief)?

8. What information does the airphoto show that the topographic map does not? List three.

9. What information does the map show that the airphoto does not? List three.

Plate 12

10. Describe the tone (light, medium, dark), texture (coarse, intermediate, smooth) and pattern (regular/intermediate/geometric) for each of the following features:

a. Agricultural fields

Tone__________________________________________

Texture________________________________________

Pattern_________________________________________

b. Residential neighborhoods

Tone__________________________________________

Texture________________________________________

Pattern_________________________________________

c. Vegetated highlands

Tone__________________________________________

Texture________________________________________

Pattern_________________________________________

d. Water

Tone__________________________________________

Texture________________________________________

Pattern_________________________________________

Plate 27

11. What is the curly line on Mount Capulin?

12. Sometimes, three-dimensional visualizations will have vertical exaggeration to more easily display vertical change. Are stereophoto images verticall exaggerated?

Plate 33

13. Describe the gradient (steep/moderate/flat), Strahler order (high/moderate/low), and shape (sinuous, oxbow, etc.) of the streams and rivers in this area.

14. Overall, which direction is the river flowing and how do you know?

SATELLITE REMOTE SENSING

Currently, sensors on orbiting satellites detect energy reflected and emitted from the Earth's surface at a wide range of different wavelengths. An important area of geographical research is the study of interpreting these images, and determining what physical features and processes are responsible for variations in the remotely sensed data. In this section you will examine world wide web pages pertaining to remote sensing in order to introduce you to some of the applications of satellite remote sensing.

Go to the USGS’s Earthshots homepage, which is at http://earthshots.usgs.gov/

Once the page is loaded, you should see a map of the world with several red markers on it that provide links to remote sensing articles, which are also listed to the left of the map.

Scroll down to the list of article subjects and click on the tab for Rosso, Mauritania (“Deserts”). You will see a Landsat image of an area in western Africa. Click on the image for a larger view. Click the grey tabs above the description (“Desertification” or “Irrigation”) to zoom in on certain areas and get further descriptive information.

1. What are the rectangular red patches near the center of the 1972 image? What do you think explains their presence here?

2. Now click on “1989” at the bottom of the image to see a more recent image. Notice how the red areas have expanded. How do you account for this expansion? Describe and explain the patterns of the red patches.

Click on the blue “Table of Contents” button just above the satellite image to return to the main menu. Now select the images for Mount St. Helens, USA.

3. Examine the images of Mont St. Helens for the four years of 1973, 1983, 1988, and 1992. Use the grey tab to view zoomed imagery. What are the blue/gray patches scattered across the western portions of the mountain the 1973 image?

4. The large ash flow to the northwest of the mountain appears blue/gray in the 1983 image and has a pinkish tinge in the 1988 image. What accounts for this change in color?

Finally, take some time to look through several of the other images on the list (choose whichever interest you the most).

5. How does the spatial resolution of the satellite images compare with the resolution of the aerial photographs looked at earlier? What factors may account for difference?

6. In general, what sorts of features are most apparent in these satellite images?

7. What sorts of features might not appear in satellite images (hint: think about pixel size)? How might this limit the usefulness of these images?

8. Based on your knowledge of physical geography, what sort of research applications might these images be most suited for?