GIS 4021c Aerial Photograph Interpretation

Introduction to ERDAS Imagine for Air Photo Interpretation

Objective

To introduce basic ERDAS Imagine information, display, screen cursor, and measurement control procedures.
To conduct rudimentary and familiar aerial photography analysis with ERDAS Imagine.

Note lmb = left mouse button, and rmb = right mouse button.

Introduction to ERDAS IMAGINE

Leica Systems’ ERDAS IMAGINE image processing and GIS software is the industry standard for remote sensing and aerial photo interpretation. ERDAS (Earth Resource Data Analysis System) is a mapping software company specializing in Geographic Imaging solutions since 1978. Software functions include importing, viewing, altering, and analyzing raster and vector data sets. For more information on ERDAS, you can browse their company web page http://www.erdas.com/. This link is redirected to http://gis.leica-geosystems.com/. Leica bought ERDAS a few years ago.

1. We will be practicing with two images in this exercise. Login to the system. The aerial photos that you will work with this week are currently in the network data directory

G:\classes\GEO 4021c Aerial Photo Interpretation\data

2. After you have successfully logged onto the system, create a new folder in your G:\share\GIS 4120c Air Photo Interepretation\yourname\workshop_5_data directory in which to store and analyze data.

3. Launch IMAGINE 9.x by selecting it from the Common Program list. Wait for all menus to appear (the IMAGINE icon panel along the top of the screen and IMAGINE Viewer #1). This may take a minute or two, then examine the options on the icon panel along the top of the screen. These icons represent the various components and add-on modules purchased with the system. You have the option of displaying the icon panel horizontally across the top of the screen or vertically down the left side of the screen using the Session - Flip Icons menu item.

4. Familiarize yourself with the five menus located along the top of the icon panel in the left corner: Session, Main, Tools, Utilities, and Help.

a. The Session menu controls many of the session settings such as user preferences and configuration.

b. The Main menu allows access to all the modules located along the icon panel.

c. The Tools menu allows you to display and edit annotation, image, and vector information, access surface draping capabilities, manage postscript and true type fonts, convert coordinates, and view Erdas Macro Language (EML) script files.

d. The Utilities menu allows access to a variety of compression and conversion algorithms including JPEG, ASCII, image to annotation, and annotation to raster.

e. The Help menu brings up the On-Line Help documentation as well as icon panel and version information. An index of keywords helps you to quickly locate a help topic by title. A text search function also helps you find topics in which a word or phrase appears.

5. The menu you will probably use the most under the Session menu is the Preference Editor. The Preference Editor is accessed under Preferences. It allows you to customize and control many individual or global IMAGINE parameters and default settings. Use the left mouse button (lmb) on the scroll arrows on the side of this menu to examine the available categories. With the User Interface & Session category open, change the Default Data and Output Directories to (for example): G:\share\GIS 4120c Air Photo Interepretation\yourname\workshop_5_data to access your personal space on the system, or G:\share\GIS 4120c Aerial Photo Interepretation\yourname for placing assignments so the instructor can read them.

6. Scroll down until you see the Delete Session Log on Exit and Delete History File on Exit. Click on both of these check boxes and make sure they are on if you haven’t done so previously. Leave all other options in their default settings. Save the changes using the Save To - User Level option under the File drop down menu in the Preference Editor. You may now exit the editor by selecting Close under the File drop down menu. One of the first things you should do whenever you use IMAGINE is to check and set these preference settings in the Preference Editor.

Image Display

1. Now you are ready to display the first image. First copy all the files in G:\classes\GEO 4021c Aerial Photo Interpretation\data\ to your folder in your folder G:\share\GIS 4120c Aerial Photo Interepretation\yourname\workshop_5_data. There should be 7 files in total: 5 with the name DOQQ_1999_NE_gainesville_false_infrared and 2 with the name DOQQ_2004_NE_gainesville_false_infrared. Note that they are of two different image formats: jpg and sid.

2. Move the cursor back to the IMAGINE Viewer and select the File dropdown menu with the lmb. In the file menu select the Open option and then slide to the menu which opens to the right and select the Raster option to get the corresponding menu. You can also type Ctrl -R to access the open raster layer menu if the cursor is over the Viewer or you can click on the Viewer icon that looks like a manila folder that is half open. Additional Viewers may be opened by clicking the Viewer icon on the IMAGINE icon panel.

3. If you did not change the preferences, you must navigate to your folder that holds the data in the usual manner. On the left side of the menu you should see a list of files in your folder.

4. Change the “Files of Type” to JPEG2000 so you can see all the files of this type. The normal Imagine file type is .img, but the air photo file type is a kind of JPEG format.

5. Position the cursor over the file you want to display (DOQQ_2004_NE_gainesville_false_infrared.jp2) and click the lmb once (do not double-click). The file name should appear in the file name window in the Viewer. If you do not see the correct files in your account then you are either not looking in the correct directory or you do not have the Files of type specified as JPEG2000 (*.jp2).

6. Before clicking OK, you can assign the spectral bands of the image to the color planes red, green, blue (RGB). Click on the Raster Options folder tab and assign band 3 to red, band 2 to green, and band 1 to blue. This is simply using an RGB color model much like the ones used in Photoshop or other image analysis program. Make sure that the Display option is set to True Color. You also have the option of making the image fit the Viewer frame by depressing the small box next to Fit to Frame. Note the other options as well (all of these can be changed in the Preferences menu). Now you are ready to click OK. If the air photo requires less space than the IMAGINE Viewer (there are large black borders on the sides) then you can resize the IMAGINE Viewer to use your screen desktop area more efficiently. This will become important in future exercises when many IMAGINE Viewers will need to be open at once. To remove an image displayed in the IMAGINE Viewer move to the File dropdown menu in that Viewer and select it with the lmb, then find the Clear option and select it. You can also click on the "eraser" tool icon in the Viewer.

7. To find out additional information about this image, go to the ‘Utility’ drop down menu in the open Viewer. Choose Layer Info and wait for the Image Info dialog box to appear. You can also access Image Info by clicking on the "info" icon in the Viewer icon menu (third one from the left). Now answer the following questions:

Questions will be shown in Bold Type

1a. What is the pixel size in the X and Y direction? (2 points)

1b. What are the units of measurement? (2 points)

1c. What is the image georeferenced to (the projection)? (2 points)

1d. What are the corner coordinates of the air photo? Use whole numbers and not the decimals. (4 points)

1e. How many rows and columns does the air photo have? How many total pixels? (4 points)

8. Now exit the Image Info dialog box by choosing Close under the File drop down menu and return to the IMAGINE Viewer #1.

9. Zoom in to some area that looks interesting to you using the +magnifying glass icon from the icon bar. You can zoom by either clicking the +magnifying glass icon on a spot, or by holding the lmb while dragging a rectangle. Stop zooming when the pixels become too large for you to determine what the object is.

10. Now position the cursor over the Viewer and press the right mouse button (rmb) to access the Quick View menu. Examine the options and move the cursor over Fit Image to Window and select it. The Quick View menu should then disappear. This will affect only the Viewer you are currently using. If you have multiple Viewers open you will need to repeat the process. You can additionally use the View - Fit Image to Window command to achieve the same result.

11. Open a second Viewer and load the file DOQQ_1999_NE_gainesville_false_infrared.sid. Note that you’ll have to change the Files of Type window to MrSID format.

2a. What is the difference between .jp2 and .sid formats? You might have to look up this difference. (5 points)

2b. What differences between the two years do you see? Write generalizations, not specifics. (10 points

12. Now open the same file (DOQQ_1999_NE_gainesville_false_infrared.sid) in Viewer 1. Be sure that “Clear Display” in “Raster Options” is not checked.

13. Click lmb on “Swipe” in the “Utility” menu of Viewer 1. Play with the controls in the window that pops up to control the swipe function. Try both vertical and horizontal swipes and the automatic functions.

2c. Now describe the differences between the two years again, emphasizing new differences that you see and differences that you no longer notice (5 points)

2d. Has your interpretation of the changes that occurred between 1999 and 2004 changed by using the swipe function? How? (10 points)

14. Close the swipe window and try both the blend and the flicker functions in the Utility menu.

2e. Describe how each of the three different methods for comparing two different air photos enables you to see different things. (10 points)

15. You are using Digital Orthophotos that are already rectified (geometrically corrected to a known coordinate system, projection, and datum) so you can make linear and area measurements quite easily. Click on the little ruler on the icon bar to run the measurement tool.

2f. What is the length of the main runway of the airport? (5 points)

2g. How much forest area was clearcut between 1999 and 2004? (15 points)

2g. How much forest area regrew between 1999 and 2004? (20 points)

16. Sometimes it is necessary to determine the coordinates and brightness values of specific pixels on the displayed image. The inquire cursor allows you to do this. Go into the Quick View menu of the IMAGINE Viewer and select Inquire Cursor or click on the crosshair icon on the icon panel. This will open a pixel information menu that allows you to move a crosshair cursor on the Viewer. You can use the black arrows to move the crosshair cursor in any pixel increment you set. For now leave the increment at 1.00 and note that the increment is variable between the file and map coordinate system. You can move the crosshair cursor using the black arrows or by pressing and holding the lmb while the mouse cursor touches the crosshair cursor. For "fine tuning" use the keyboard arrows to move the cursor. The black circle will move the crosshair cursor back to the center of the Viewer. Play with the Inquire Cursor for a minute or so.

17. Coordinate values for the image can be obtained in either map, paper, file, or latitude, longitude as long as this data exists in the image file. Both files DOQQ_1999_NE_gainesville_false_infrared.sid and DOQQ_2004_NE_gainesville_false_infrared.jp2 have map, file, lat/long, paper (inches), and MGRS (Military Grid Reference System ) coordinates, any of which can be selected by clicking on the button in the top left of the Inquire Cursor box that says Map. Notice that the coordinate system is defined for you. The image projection is also shown but if you have not selected the Map option that may not necessarily be the x, y coordinate system. The table shows the R,G,B pixel brightness values for both the image file (FILE PIXEL) and the color lookup table (LUT VALUE). Move the Viewer cursor and notice how the values change. To move the crosshair cursor using the mouse you must initially place the arrow cursor at the center of the crosshairs and click on the lmb. Keep the lmb depressed to move the crosshair cursor.

3a) What are the coordinates in all of the systems for the fountain pool at the GRU water treatment and distribution center? (5 points)

3b) Which of the coordinates would you use to describe a pixel location to someone working on a different software system? (i.e. not Imagine) Why? (10 points)

3c) Position the crosshairs on a representative pixel and record the actual data values (“File Pixels”) in each band (1-3) for the following features: (2 points each = 8 points total)

a. Urban

b. Roads

c. Forests

d. Grass

3d. There are three values for each “File Pixel.” What are these data values? What do they mean? (5 points)

As usual, hand in your answers via printed copy or as a .doc file placed in your folder in the G:\share\GIS 4120c Air Photo Interepretation\yourname folder.