Course Lectures and Labs
Getting Started with ERDAS Imagine
1. Open ERDAS IMAGINE from your desktop by going to Start --> Programs --> Lieca Geosystem -> ERDAS IMAGINE When ERDAS opens, a Menu Bar running across the top of the screen and a Viewer window below the Menu Bar appear. All available functions for ERDAS can be activated from the Menu Bar; however, not all functions can be activated from the Viewer.
2. In the Viewer window go to File --> Open --> Raster Layer..... and find the bbcgis folder. Open the "pnw98_bbg.img" file from the folder. Make sure that the file type is ".img".
3. On the toolbar inside the the Viewer window, click on the Zoom In button . Zoom in on the image a few times. Click on the image with the right mouse button and choose "Fit Image to Window" from the menu that appears. This command is analogous to the "Zoom to Theme Extent or Full Extent" buttons in ArcView. This part of the exercise is to illustrate the point that ***Clicking on the right mouse button gives you access to many of the functions in ERDAS*** So.....if you cannot find a function in the menu or on the tool bar, try clicking once on the right mouse button.
4. Change back to the arrow button and click on . The following menu will appear
and white cross-hairs will appear on the image. Use your pointer to move the cross-hairs by clicking once in the intersection of the two lines and dragging the center of the cross around. As you do this, notice that the X and Y values in the table change. Zoom in on the image a few times so that you can see each individual pixel.
Move the cross-hairs again, but this time stay within one pixel. Notice that the X and Y values in the table still change, but that the values in the File Pixel and LUT (Look Up Table) Value columns do not change. The columns assign to each pixel a value from 0 to 255 (called a DN number). The File Pixel columns are the actual pixel values of your data. The LUT Value column assigns a number to each pixel based on a combination of red, green, and blue used to make the colors on your screen.
5. Changing Band Combinations. Zoom back out to fit the image to the window and close the table that is now open to get rid of the cross-hairs. From the Viewer menu choose Raster --> Bands Combinations and a table will appear:
Change the values on the right-hand side of the table to 5,2,1
and notice that the forested and grassy areas now appear green. You can play around with this band combination to give the image some really funky colors, but this feature is also useful. For example, by knowing that soils shows up really well with a certain band combination you can use this combination to see them more clearly than you would with another combination of bands. Forests might show up more clearly in a different band combination than soils.
6. The purpose of the next part of the exercise is for you to see how ERDAS can help you see that different features (i.e. water, urban areas, forested areas, etc.) reflect different wavelengths. Change The band combination back to 4,3,2. Now click on the graph button in the toolbar and a menu with appear:
Choose Spectral from this menu and hit OK. A new screen will appear:
Click on the cross button on this NEW Spectral Profile menu and then click somewhere in the water on the image.
A line will appear on your graph that describes the spectral profile at given point. Now choose a point over an urban area and see that you will get a different spectral profile on your graph.
Finally, choose a forested area and repeat the procedure.
As you have probably figured out by now, the forest has a different spectral profile. With your new knowledge about the spectral profile for a given feature, you can go back and change your band combinations to make that feature show up more clearly on the image.
8. This part of the exercise shows you how ERDAS can help you visualize how different wavelength reflect along a transect of the land surface. Close the Spectral Profile window to get rid of your profile points. Click again on the graph button in the toolbar in the Viewer window, but this time choose Spatial and hit OK. A similar looking window as for the Spectral Profile will appear; however, notice the the axes are different.
Click on the line button . On the image, click once with the left mouse button to start drawing a straight line. Each time you want to change the direction of the line click on the left mouse button.
To stop drawing the line and to display the results on your graph, double click with the left mouse button.
9. The next exercise will use the Spectral Profile Surface, which is similar to the Spatial Profile, except that it is in 3D. Close the Spatial Profile table and click again on the graph button in the Viewer toolbar. Choose surface and hit OK. You guessed it.....another graph will appear:
Click on the square button in the graph window and choose and area of interest (AOI - get use to this term) on the image. To do this, click once on the image with the left mouse button and a little x will appear . Click and hold with the left mouse button on this x and drag the resulting rectangle to the areal extent you desire. Release the mouse button and a graph will appear.
Cool! Now change the "PLOT LAYER" value in the Surface Profile window to display the surface of each of the bands of reflectence within the profile AOI.
10. Finally, we will learn about geo-linking image with ERDAS. This means that when two images have been geo-registered in a common real world projection and coordinate system they therefore have both spatial information (the X,Y locations for each pixel or raster grid cell) and they have descriptive data (the value of reflected energy for a given number of wavelengths). This means that the geo-registered image data may be considered "geographic" data. Geographic data my be "linked" through their common locational data and quired about their variation or differences. In image processing, this is most often done to aid in understanding the nature of the data by linking "high" resolution data with "coarse" resolution data.
Lets start by fitting the existing image to the window of viewer #1. Open a new viewer and load the iconisbbc.img image file. Your desktop may look like this
Now use you RIGHT MOUSE BUTTON in the viewer #2 (the iconisbbc.img) to select the GeoLink/Unlink option. You'll see the following window.
The instructions tell you to click the left mouse button in the viewer with the image you wish to "link" with. The mouse will change symbols to inform you what action will take place in each window. Select viewer #1 with you left mouse. An "index box" (or some call it the "zoom box" ) will appear. Now zoom in and out in BOTH viewers and see what happens. Change the size of the "index box" (use the tool).
That's it for now. Close both viewers, and select EXIT IMAGINE from the session menu on the top tool bar.
IMPORTANT: When you close ERDAS, you will be asked if you want to print the Log File. Say NO, unless you really have something against trees.
more to come ...
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Last modified: 11/06/2009 4:23 PM