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ENVI

Create Tie Points

Create Tie Points

  1. Select one of the following coregistration options:
    • Select tie points manually: The Image-to-Image Ground Control Points topic describes how to select tie points. When you are finished selecting GCPs, click Next. The Review Tie Points dialog appears.
    • Select tie points automatically: See Automatic Tie Point Selection.
  2. To use seed points, ensure that the Use Seed Points check box is enabled.
  3. Select seed points either manually or automatically. Choose one of the following:
    • To automatically generate four seed points, click Auto-Generate Seed Points. ENVI selects four seed points. The auto-generated seed points may not fall precisely on the same feature. This is acceptable, as long as they are relatively close. These seed points are discarded when tie point selection is complete. Click Select Seed Points, then ENVI opens the Time #1 and #2 images in display groups, and the Ground Control Points Selection dialog appears listing the automatically selected points.
    • To manually select seed points, click Select Seed Points. ENVI opens the Time #1 and #2 images in display groups and the Ground Control Points Selection dialog appears. Select three to five seed points using the steps in Collecting Ground Control Points (Image-to-Image).
  4. When manually selecting seed points, you can switch the Geographic Link toggle to On to move the cursor to the same area in both images, then switch the Geographic Link toggle to Off to fine tune the location.

  5. If you manually selected seed points, click Retrieve Points. If you need to clear the seed points and start again, click Clear Points.
  6. Click Show Advanced Options if you want to set additional parameters for area-based image matching methods. Typically, the default settings provide the best results, but you can edit the parameters as needed.
    • Number of Tie Points
    • Search Window Size
    • Moving Window Size
    • Area Chip Size
    • Minimum Correlation
    • Point Oversampling
    • Interest Operator
  7. Click Next to continue. The following appears:
    • One display group contains the base image.
    • One display group contains the image to warp.
    • The Ground Points Selection dialog.
    • If you selected automatic tie points, the points show in the display groups, the GCP Selection dialog shows how many points exist and the Root Mean Square (RMS) error, and the Image to Image GCP List shows the individual points.
    • The Review Tie Points panel appears.
  8. Review tie points.
  9. When you are finished reviewing tie points, choose one of the following for the warp Method in the Review Tie Points panel. Warping and Resampling Image-to-Image describes each parameter.
    • Polynomial
    • Triangulation
    • RST
  10. Choose one of the following for the warp interpolation. The interpolation is the manner in which the image is resampled to the warped grid. Warping and Resampling Image-to-Image describes the choices.
    • Nearest Neighbor: Uses the nearest pixel without any interpolation to create the warped image.
    • Cubic Convolution: Uses 16 pixels to approximate the sinc function using cubic polynomials to resample the image. Cubic convolution resampling is significantly slower than the other methods.
    • Bilinear: Performs a linear interpolation using four pixels to resample the warped image.
  11. For pan-sharpening, a nice looking image is paramount, so Cubic Convolution is the best choice.

  12. Click Next. The following appears:
    • One display group contains the base image
    • One display group contains the warped image.
    • The two display groups are dynamically linked.
    • The Check Co-Registration panel.
  13. Check the co-registration accuracy.



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