- Display files
- Start Geometri correction tool
- record Geometric(ground) Control Points(C+GCPs)
- resample the image
- Start the Geometric Correction tool from the viewer displaying the file to be retified.
- Select RASTER > Geometric Correction from the viewer's menu bar and Polynomial from the dialog box.
- Each of these geometric models relate to specialized tasks. The polynomial model has the most general application.
- The first one is Geo Correction Tools Window and the other is Polynomial model properties
- From the Polynomial model properties click on Projection Tab-->click on Add/Change projection.
- Select Datum--> Geographic Lat/Long
- Spreriod--> Everest 1956
- Datum name--> India Nepal
Click on GCP tool Reference Setup from the Polynomial model properties. You would observe the following dialog box:
- Select the option Keyboard Only. This option differs when you are Georectifying Image/Topo sheet from an existing Geometrically corrected Image/Topo sheet.
- Click on Ok
You are now ready to begin assigning real world coordinates to the non-geographic registered image.
- First, each viewer now has it's own "zoom" window above it. The zoom window display that part of the image which is inside the "box" (some time called the link box). The Ground Control Points (GCPs) are the small targets which are place on the image using the GCP tool .As you observer there are two viewers , one in which the image to be rectified is displayed and in the other a magnified part of the image is displayed.
- In the Viewer1 you would observe a magnifier box pointing to some area and the respective area is visible in the Viewer2 at a best zoom level.
- Now adjust the Maginifer box in the viewer to the top left corner of the topo sheet where you can see a cross hair mark having coordinates.
- From the GCP tool bar, select GCP tool and click on the cross hair in the Viewer1(Top Left Corner).
- As soon as you click on the viewer paper coordinates are added to the GCP control. You have to manually add the Geographic coordinates to the control, i.e, longitude and Latitude.
Note: In ERDAS Imagine you have to first add Longitude and later Latitude.
You need not convert Geographic coordinates whhich are in Degree Min Sec to Decimal coordinates.
Add them as 17 45 50 in the table provided X and Y Reference, they will be added as Decimal Coordinates.
- Repeat this for Top-Right, Bottom Left and Bottom Right Corners in the Topo sheet(Anti clock wise direction)
Save the Input from the table file menu
- Our next task is to resample the image . Resampling is the process of calculatin the values for the rectified image and creating the new file. All of the raster data layers (all bands) in the source file are resampled. The best known algorithms for resampling are Nearest Neighborhood, Bilinear Interpolation, and Cubic Convolution. We'll use the Nearest Neighborhood algorithm.
- Click on the resampling icon from the Geo Correction Tool Menu . The resample dialog opens. Enter a file name for the output (know where on the drive you will put the file) - select the Nearest Neighborhood resampling method - click to exclude zero file values in the statistics.
A job status dialog opens to let you know when the processes is complete.
- Verify the rectification by displaying the new image in a viewer and Geo. Link the reference image with the new image and use an Inquire Cursor to check that they are georegistered to each other.