Image Rectification

When you are got the image from Remote Sensing data, that image is having rough Latitude and Longitude Values. This information is available from the Leader L-3 File (D:\ERDAS_Exercise_Files\trpc20026j099-059\product1\Leader L-3). Now the image is to be rectified with this Lat/Lon Values. This is called First Stage of Rectification of the Image or Pre-Rectification of the Image.

i. Locating the Lat/Lon Values

If the Import/Export Window is not active then

1. Click on IMPORT/EXPORT Option from the Main Tool Bar
2. Since we are importing the Images into ERDAS Environment Keep the Default Option IMPORT
3. Click on the TYPE list option box. Then it will display all ERDAS supported file formats
4. Select GENERIC BINARY type as the option for Image Type
5. Similarly, select the Media as FILE
6. Select Input file i.e LEADER L-3. This is available in this path D:\ERDAS_Exercise_Files\trpc20026j099-059\product1\LEADER L-3
7. Click on DATA VIEW Tab. The Lat/Lon information in Data View Window is displayed as shown below.

8. Now type the following at appropriate places

Record: : 7
Record Size : 1024
Data Type : ASCII

The five Lat/Lon values and Pixel Size are displayed in the first few lines

9. Block the First few lines (Preferably Six or Seven Lines) and Copy the selected text.
10. Open NOTEPAD Program and PASTE these value
11. Close the DATA VIEW Window
12. Now edit this text in NOTE PAD. After editing the table is looks like as follows

13. Save this reference points as image 99-59ref.txt
14. Minimize Note Pad Program
15. Close the IMPORT Window

ii. Viewing the Image

1. Click on VIEWER Icon of the Main Tool Bar
2. Select Viewer type as CLASSIC VIEWER
3. Click OK to get a blank Viewer Window
4. Click on OPEN
5. Select RASTER LAYER option
6. Then Browse for the File IMAGE99-59.IMG and select the same file
7. Click on OK to display on the viewer
8. Click on the RIGHT CLICK of the Mouse and Select FIT IMAGE TO WINDOW option to display the entire image on the screen. Now you can observe that RASTER Menu is added to the Viewer Menu Bar, since presently Raster image is displayed on the window

iii. Changing the Layer Band Combination for Display
User can change the selected layers (bands) that are displayed in Red, Green, and Blue on the monitor. The selected bands may be from different files. It also allows you to turn off selected bands.

1. Click on RASTER Menu of the VIEWER Window
2. Click on the BAND COMBINATIONS Option. Then it will display the SET LAYER COMBINATION Windows As shown below

3. Set the Bands for RED, GREEN and BLUE as desired. In this exercise Select RED: Band 1; Green: Band 2 and Blue: Band 3 to view the image clearly on the screen
4. Click on APPLY to implement the selected Band combination for the Image
5. Click OK

iv. Geometric Correction

In remote sensing systems, the image of a stationary grid on earth is not perfectly reproduced by the sensor. Instead, the geometric characteristics of the scene change as a function of geodetic and intrinsic properties of the imagery system, such as satellite orbit, position, attitude, and scan angle. The results from these imperfections contribute to the overall Geometric Distortion of the image, and are the essential motivation behind the modeling of Geometric Correction algorithms.

The distortions involved are of two types:
1. Non-systematic Distortion: These distortions are caused due to variations in spacecraft variables.
2. Systematic Distortions: Geometric systematic distortions are those effects that are constant and can be predicted in advance

Rectification is the process of projecting image data onto a plane and making it conform to a map projection system. Registration is the process of making image data conforms to another image. A map coordinate system is not necessarily involved. However rectification involves rearrangement of the input pixels onto a new grid, which conforms to the desired map projection and coordinate system. Rectification and Registration therefore involve similar sets of procedures for both the distortions.

1. Click on RASTER Menu of the Viewer Window
2. Click on GEOMETRIC CORRECTION. Then it will display the SET GEOMETRIC MODEL Window as shown below

3. Select POLYNOMIAL as the option for GEOMETRIC MODEL from the above displayed list of model
4. Click on OK. Then it will display the POLYNOMIAL MODEL PROPERITY Window as shown below

5. Click on the PROJECTION tab. Then you may observe that the displayed window is looks like above. As per the selection of the option it will display. By default this will be on PARAMETERS Tab
6. Click on the ADD/CHANGE PROJECTION option. Then it will display the PROJECTION CHOOSER Window as shown below

7. Click on the CUSTOM Tab option, of the Projection Chooser Window. By default this will be on Standard Tab
8. Click on PROJECTION TYPE list box
9. Select GEOGRAPHIC (LAT/LON) as the option for the Projection Type from the displayed list.
10. Click on the SPHERIOD NAME list box
11. Select EVEREST 1956 as the option for Spheroid
12. Click on the DATUM NAME list box
13. Select UNDEFINED as the option for Datum
14. Click on OK. Then it will be back to POLYNOMIAL PROPERITY Window
15. Click on SET PROJECTION FROM GCP TOOL option. Then it will display the GCP TOOL REFERENCE SETUP window displayed as shown below

16. Select KEYBOARD ONLY as the option for COLLECTION REFERENCE POINT FROM
17. Click on OK. Then it will display the REFERENCE MAP INFORMATION Window as shown below. In this window

It shows the Current Reference Map Projection as follows

Projection : Geographic (Lat/Lon)
Spheroid : Everest 1956
Datum : Undefined
Map Units : Degrees

18. Click on OK. Then it will open five windows as shown below

i. GCP Tool Window;
ii. Viewer 1 Window
iii. Viewer 2 Window it is also called Magnifier Window for Viewer 1 Window
iv. Polynomial Property Window
v. Geo Correction Tool Window

19. Click on APPLY on the Polynomial Property Window
20. Close the Viewer 2 Window or Magnifier Window

GCP TOOL

The file coordinates for a pixel value in an image are in the Column and Row indices used to locate that value in the raster array of the image. An un-georeferenced image is normally displayed so that the upper left corner of the image represents file coordinate (0,0). The positive Column (X) direction is to the right, and the positive Row (Y) direction is downwards. When using the GCP Tool in early versions of ERDAS IMAGINE (such as V8.2), the coordinates for an un-georeferenced image were reported using the file coordinate system

21. Activate the GCP TOOL Window by clicking on its Title Bar. In this GCP Tool Window

22. Click to turn off and on the automatic GCP editing mode

23. Click to create new GCPs . Then the cursor will turn into a cross hair in the Viewer.

24. Then Click on Five different locations spread all over the image in Viewer 1. For each click it will create a GCP Point with continuous Number
25. Then Observe in the GCP TOOL Window, The GCP Points X, Y locations are automatically filled as the X input and Y input values.
26. Now Activate the Text file IMAGE 99-59REF.TXT in NOTEPAD to fill the values in GCP Tool Window
27. Now replace the X INPUT, Y INPUT, X REF and Y REF value as you can see in the IMAGE99-59REF.TXT file in the Notepad. Here one precaution is to be taken that, whenever you typing Y INPUT values add Negative sign (-) to that value except for the values 1. Then GCP Tool will looks as shown below

28. If you want to change the color for GCP points, Then select all the ROWS and block the COLOR Column, right click on COLOR Column and pickup required color for GCP Points.
29. Click on to solve the geometric model with control points

30. Activate the GEOCORRECTION TOOL Window

31. Click on to open the Resample dialog

From this dialog, you can choose the desired output file, resample method, and output information.. Then it will display the RESAMPLE Window as shown below

32. Type the OUTPUT File Name as PRE_RECT_IMAGE_99-59 and save this file in the folder D:\ERDAS_Exercis_Files
33. Keep the default option i.e. NEAREAST NEIGHBOUR as the option for RESAMPLE METHOD
34. To define output cell size, Click on NOMINAL option
35. Type X: 23.5 and Y: 23.5
36. Click on APPLY Button on the Nominal Cell Size window
37. Click on CLOSE to close this window
38. Check the option for IGNORE ZEROS IN STATS
39. Click on OK. Wait for few Seconds to complete the process of sampling
40. Click on OK when JOB STATE: DONE
41. Activate GCP Tool Window,
42. Click on SAVE AS option in the File Menu
43. Type the name as IMAGE99-59. The extension of this file is GCC
44. Close this window
45. Click on EXIT on the GCP Correction tool. And also Close all other windows which are opened during this process

v. Viewing the Geo Metric Correction Image

1. Click on VIEWER Icon of the Main Tool Bar
2. Select Viewer type as CLASSIC VIEWER
3. Click OK to get a blank Viewer Window
4. Click on OPEN from the FILE Menu
5. Select RASTER LAYER option
6. Then Browse for the File PRE_RECT_IMAGE99-59.IMG and select the same file
7. Click on OK to display on the viewer
8. Click on the RIGHT CLICK of the Mouse and Select FIT IMAGE TO WINDOW option to display the entire image on the screen.
9. Now Observe, that wherever you place the cursor the corresponding LAT/LON values of that point will be displayed.

vi. Layer Information

1. Click on the UTILITY Menu of the Viewer Window
2. Click on the LAYER INFO… option. Then it will display the Image Info Window as shown below

And then study the layer information. After that CLOSE this window and Also CLOSE the Viewer window