This exercise will be conducted in the Geography computer lab, using an image processing software called ERDAS Imagine. We'll be working with this software for the rest of the semester, if all goes well.Image processing involves computer manipulation of digital data (not necessarily always satellite-acquired) for a number of purposes. These include displaying and enhancing the digital data in order to create an image that yields the desired information. In this exercise, you will explore some of the ERDAS tools that allow manipulation of an image in order to see it in different ways. You are welcome to work in teams of two, but be sure to alternate computer operators so that each of you gets hands-on experience with the software.
Ordinarily, you would log on to the computer and bring up the desktop with application icons. Then double click on the ERDAS icon. However, in order to bypass some bugs on the system, ERDAS has already been opened for you. You should be seeing a window labeled Viewer #1.
With Viewer #1 open, you will have two sets of menu bars. For all of the following actions, use the Viewer Menu Bar and tool bar, directly above the Viewer window.
Load an Image File. On the Viewer Menu Bar click/hold left mouse button on FILE and drag to OPEN, then drag to RASTER and then let go. This will open the OPEN RASTER DIALOG BOX.
You will see a list of image files on the left side of the box. Highlight the "lanier.img" file. While this Open Raster box is open, observe the File Info and Display choices for the "lanier.img" file. This is a Landsat Thematic Mapper (TM) image of the Gainesville, Georgia, area, including Lake Lanier.
Question 1. How many layers does this file contain? _____ How many columns and rows? _______
Look at how the "Layers to Colors" are set up, showing which Bands (wavelength interval) will be displayed in which colors:
Red: Band______
Green: Band ______
Blue:Band ______
T hese color assignments should be (unless someone changed them) the default settings for the Landsat Thematic Mapper images. They can be changed if you want to bring out contrasts between Bands. See Table 6.7, page 141 in Avery and Berlin.
Note other Display Options: E.G. Clear Display -- when this is toggled, each time you load a new image, the old one will be closed automatically. Zoom Ratio and Fit to Frame determine how the image will appear in your viewer window. Resample Method gives you several options for statistical manipulation of digital data that affects how it is displayed. This is used if the image is reduced from its original size.
Click OK to close dialog box and load image file.
Question 2. If this were a photograph, rather than a digital image, what type of film would have been used? Why do you think so?
Note the bar at the top of the viewer, the title bar, shows you which viewer window is open (#1), the file name and type, and the bands assigned to each layer.
Note at the bottom of this viewer window is a status bar. As you pass your mouse cursor over one of the tool icons in the tool bar, information on what this tool does is displayed in the status bar.
Keep in mind that these windows work the same as other standard windows applications in terms of manipulation of pull down menus, toggling icons, changing the size and shape of the window, and moving the window. For example, if you need to move the viewer window, click on the title bar at the top, then hold and drag to a new position. If you want to make the viewer window larger, click and drag on the corners.
Pixel Data. You can find out information about each individual pixel in your displayed image by using the Utility Menu options:
Pull down the UTILITY menu, then click-hold and move to UTILITY/INQUIRE CURSOR. (Two other ways to get to this point are to click on the PLUS SIGN (+) icon on the tool bar, or to open the Quick View menu: with cursor over the image, click with the Right side of the mouse, then highlight Inquire Cursor.)
Now a dialog box opens labeled "Viewer #1: lanier.img." Also, a white crosshair is displayed over the image.
Question 3. What are the values shown in the boxes under File Pixel for each color band? You may use the scroll bar on the right to move down to where Red, Green, and Blue are displayed.
Red: ___________________ Green: _________________
Blue: __________________
Question 4. Note also the X and Y coordinates displayed. What are the coordinates of the center of this image?
X _____________________________
Y _____________________________
Question 5. How is this point different from the nadir of an air photo?
Question 6. What type of coordinates are these? What is your evidence?
Point the cursor arrow to the intersection of the crosshairs, then click and drag to a different part of the image. How do the values under File Pixel change? (rhetorical question). You can also move the Inquire Cursor around the image using the click arrows in the lower left of the dialog box. Click the circle to return to the center of the image. Close this dialog box by clicking "Close", or click the button in the upper left corner.
Measurements. Now let's try the Measurement Tool. From the UTILITY menu, click-hold on UTILITY/MEASURE. (You can also access this from the Quick View menu, or click on the Ruler icon on the tool bar.
The Measurement Tool dialog box appears, including a small blank window below the tool bar. You see a series of icons in the center of the tool bar: an arrow, a plus sign (+), a zigzag, a polygon, a square, and an oval. The arrow icon is the default cursor: each time you use one of the other tools and finish a measurement, the cursor reverts back to the arrow, which doesn't measure anything. Also, each time you make a measurement, the results will appear in the small blank window in the dialog box. When you're finished, you can save, print, or copy your results to another document, if you want to.
Click on the + icon. This tool measures a point on the image. Find the light patch on the highway that runs from northeast to southwest, in the northeast quarter of the image. Move the cursor (it looks like a white cross) to that patch and click on it.
Question 7
(a). What are the geographic coordinates at this location? (Include N and W, degrees, minutes, and seconds)
(b) On this image, (assuming north is at the top of the image) find the western-most bridge that crosses Lake Lanier. What are the coordinates for the southern edge of the lake at this bridge?
You now have two sets of coordinates shown in the window of the dialog box.
Next, Click on the Zig Zag Line icon, which measures the lengths of lines that may be made up of a number of straight segments (a "polyline"). The cursor on the image will look like a white cross.
Place the cursor on the southern banks of the western-most bridge, where you measured coordinates in 7(b), and click-hold on this spot. Then drag the cursor to the southern banks of the next bridge to the east and double-click on this point. This will end your line measurement.
Question 8. What is the distance between the two bridges?
If you want to measure a complex line, such as a highway or stream channel, place clicks (data points) at each place where the direction of your line changes. Then double click at the end of the line to finish the measurement. This is a lot like digitizing on a map.
To measure Area, click on the Polygon icon. Click once on the beginning point, then click again on each point along the perimeter where a change in angle occurs, until reaching the point of the beginning. Then double click to close the polygon. The length of the perimeter and area of the measured feature appear in the dialog box.
Question 9. Measure the area of the white patch (like two blobs of marshmallow ) just north of the highway (in the northeast quadrant of the image):
Perimeter: ______________________________
Area: __________________________________
Close the Measurement Tool dialog box, either by clicking on the upper left corner of the box, or by pulling down FILE/CLOSE on the menu bar. Choose NO to Query regarding saving changes.
Arranging Layers. It is possible to load more than one image file of a particular area into a single viewer at the same time, then arrange layers in order to look at each. In this part of the exercise, you will load in a thematic soils map of the Gainesville, Georgia area.
On the Viewer Menu Bar click on the Open File icon. The Open Raster Layer dialog box appears. Click on the file, "Insoils.img".
Then, under "Raster Options" click on the "Clear Display" box to disable this option, so that "lanier.img" won't be cleared from the Viewer when you load the "Insoils.img" file.
Click OK to display the file. The soils map will appear in the Viewer, replacing the satellite image.
The Satellite Image is still in the Viewer, but it's underneath the Soils Map. To bring the Satellite Image back to the top, Pull down the VIEW option menu and choose ARRANGE LAYERS. The Arrange Layers dialog box will open.
Click and drag the box that has the three lanier.img layers to the top of the insoils.img box, overlying the Insoils.img layer. Then click APPLY. The two images will then trade places in the dialog box. Then CLOSE. Now the soils layer is still in the Viewer, but beneath the Satellite image.
To Zoom. You can zoom in on a smaller area of the image in several ways. At this point, your image of Lake Lanier is shown at a magnification of 1. With your cursor over the image, try clicking once on the Right mouse button. This will bring up the Quick View Menu. Drag down to select ZOOM/ZOOM IN BY 2. The image is now displayed at a magnification factor of two.
You can also zoom in or zoom out by clicking on the icons that look like magnifying glasses (the one with the plus sign ZOOMS IN; the one with the minus sign ZOOMS OUT).
Question 10. What happens to the image when you Zoom In by Two? How might this display of the image be used to obtain information about the land surface?
Finish and Log Off:
First, close your image file (lower toolbar). Then, using the top tool bar, close ERDAS. When it asks you if you want to Print, click NO. This will bring you to the desktop.
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