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Registration

Use Registration to reference images to geographic coordinates and/or correct them to match base image geometry. You can select ground control points (GCPs) interactively from Image windows and/or Vector windows. ENVI performs warping using polynomial functions; Delaunay triangulation; or rotation, scaling, and translation (RST). Resampling methods include nearest neighbor, bilinear, and cubic convolution. Comparison of base and warped images using ENVI's multiple Dynamic Overlay capabilities allows quick assessment of registration accuracy. For step-by-step image registration instructions, see the "Image Georeferencing and Registration" ENVI Tutorial.

Image-to-Image Ground Control Points

Use Select GCPs: Image to Image to interactively select ground control points (GCP or Tie Points) to use in image-to-image registration. Use the Zoom windows of two displayed images (image-to-image registration requires that two images be displayed) to select the GCPs. When selecting GCPs, you can select sub-pixel (fractional) coordinates. When enough points to define a warp polynomial are selected, GCP locations in the warp image can be predicted. You can save and restore GCPs to/from files and you can change the color labels and ordering of the GCP markers. See Warping and Resampling Image-to-Image for descriptions of the warping methods.

Single band or multiband images can be warped. The GCP selection tool allows prototyping and testing of different GCPs and warp options.

Opening and Displaying Images


Tip
To rotate or flip images prior to registration, use Basic Tools  Rotate/Flip Images (see Rotating Images).

  1. Open the base image and warp image files and display them in two windows.

    2.  

  2. With the two images displayed, select Map  Registration  Select GCPs: Image to Image. The Image to Image Registration dialog appears.

    3.  

  3. In the Image to Image Registration dialog:
    • In the Base Image list, click on the base (reference) display.

      •  

    • In the Warp Image list, click on the display containing the image to warp.

     

  4. Click OK. The Ground Control Points Selection dialog appears.

    5.  


    Tip
    See the "Image Georeferencing and Registration" ENVI Tutorial for step-by-step descriptions of image registration.

Collecting Ground Control Points (Image-to-Image)

Identify Ground Control Points (GCPs) in the two displayed images by locating pixels in the Zoom windows. Pixel information is loaded into the Ground Points Selection dialog. The upper-left of a pixel is the position of the whole number coordinates and the X and Y values increase to the right and bottom of the pixel, respectively. The pixel fraction available in the Zoom window is proportional to the zoom factor. For example, at a zoom of 4x, the pixels are divided into 4 sub-areas. At a zoom factor of 10x, positioning is possible to 1/10th pixel. The GCP marker is placed showing the subpixel position in the Zoom window. Subpixel locations are supplied to provide higher accuracy in selecting GCPs.

  1. In the Image window of each image, position the Zoom box over the desired GCP area (see Controlling the Zoom Window from the Image Window).

    2.  

  2. In the Zoom window, click the left mouse button on a specific pixel to position the cursor over that pixel or portion of that pixel (see The Zoom Window).

    3.  

    The coordinates of the selected location (in both images) display in the Ground Control Points Selection dialog, in sample and line order in the fields labeled Base X, Y and Warp X, Y, respectively. Positioning in the Zoom window provides subpixel coordinates so floating point coordinate values are shown in the dialog.

     


    Note
    Select Tools  Pixel Locator from the Image window menu bar to select pixel positions. Provide whole number GCP coordinates by clicking the Export button (see Using the Pixel Locator).

     

  3. When the desired pixel is selected in both images, click Add Point in the Ground Control Points Selection dialog to add the selected GCPs to the list of selected X, Y (sample, line) pairs.

    4.  

    To view the list of GCPs, click Show List. The Image to Image GCP List appears with the GCPs listed in a table. For a description of the GCP List, see Using the Image to Image GCP List.

     

    When the GCPs are added to the list, a marker is placed in the Image displays of both the base and warp images. The GCP marker in the Image is the identifying number of the GCP displayed next to an encircled crosshair.

     

    The marker indicates the selected pixel (or subpixel location). The center of the marker (located under the crosshair) indicates the actual GCP location.

     

  4. Add additional GCPs by following the same procedure.

    5.  

    When four or more GCPs are selected, the predicted X, Y coordinates for the selected warp, the X and Y error, and the RMS error are listed in the GCP List table.

     


    Tip
    For the best results, try to minimize the RMS error by refining the positions of the pixels with the largest errors or by removing them (see Minimizing RMS Error). Errors can be reduced by using more points. If you only have a few points, place them near the image corners or widely scattered in the image.

Minimizing RMS Error

The number of GCPs selected displays in the Ground Control Points Selection dialog. After a number of GCPs sufficient enough to conduct a 1st degree polynomial warp are selected, the total RMS error displays in the Ground Control Points Selection dialog and the RMS error for each point is listed in the Image to Image GCP List table.

The degree of polynomial used for calculating the error displays in the Degree box in the Selection dialog and can be changed when a sufficient number of GCPs are selected.


Tip
To see which points have the highest RMS errors, select Options  Order Points by Error in the Ground Control Points Selection dialog. The points in the CGP List are reordered so that those with the highest errors appear at the top of the list.

Using the Image to Image GCP List

Use the Image to Image GCP List to edit and update positions, turn points on and off, delete selected points, and predict point locations.

Hiding/Showing the Image to Image GCP List

To display the Image to Image GCP List, click Show List in the Ground Control Points Selection dialog.

To hide the Image to Image GCP List:

Positioning the Zoom Window

To center the Zoom window over any of the selected GCPs, click on the desired GCP in the Image to Image GCP List and click Goto, or click on the GCP number in the Image to Image GCP List.

The marker outlining the selected GCP is positioned at the center of the Zoom window for both the base and warp images.

Ignoring GCPs

To have GCPs selectively ignored during registration, click on the point to ignore and click On/Off.

The RMS error and the spatial transformation are recalculated without using the selected points. The + next the GCP number changes into a - in the Image to Image GCP List and the color of the GCP marker changes.

To turn the GCP on again, click on the point in the list and click On/Off again.

Editing GCP Locations

You can edit the location of a GCP by changing the X and/or Y values in the Image to Image GCP List, or by interactively selecting new locations in the Zoom windows.

To change a GCP from the Image to Image GCP List:

  1. In the Image to Image GCP List, double-click on the value to edit.

    2.  

  2. Enter the new value and press Enter key or enter a new value and click Update.

    3.  

    The changes are reflected in the Image to Image GCP List and in the base and warp images.

     


    Note
    If several GCPs have already been selected, a significant delay may occur while the GCPs are redrawn and the error is recalculated.

To interactively move a GCP:

  1. In the Image to Image GCP List, click on the GCP to move.

    2.  

  2. In the base and warp images, center the Zoom boxes over a new location.

    3.  

  3. In the Image to Image GCP List, click Update.

The location of the GCP is replaced with the new pixel positions in the Image to Image GCP List. The Zoom windows for both images are centered over the new location.

Deleting GCPs

To permanently remove any single control point from the list in the Image to Image GCP List, click on the GCP to remove and click Delete.

Warping Options

Use the Options menu in the Ground Control Points Selection dialog to warp from the currently displayed band or to warp from a file. You can also select image-to-map type warping if your base image is georeferenced. This allows you to change the output pixel size and projection type of the warped image.

The Registration Parameters dialog appears. The details of the various warp options available in ENVI are discussed in Warping and Resampling Image-to-Image and Warping and Resampling Image-to-Map.

GCP Options

Use the Options menu in the Ground Control Points Selection dialog to control the ground control points labels, colors, and ordering; to reverse the base and warp images; and to set other preferences.

Reversing Base/Warp Images

To reverse the positions of the base and warp GCPs, select Options  Reverse Base/Warp.

Selecting RST Calculation

For 1st order polynomials, you have the option of selecting RST (Rotation, Scaling, and Translation) calculation of errors, in the Ground Control Points Selection dialog, select Options  1st Degree (RST Only). A check mark next to the menu option indicates it is enabled.

Predicting GCP Locations

To predict the location of a GCP in the warp image based on the warping determined by the current GCPs and the selected polynomial degree:

  1. In the base Zoom window, position the crosshairs over a pixel.

    2.  

  2. In the Group Control Points Selection dialog, click Predict. The warp image Zoom box and the crosshairs move to the predicted pixel.

    3.  

  3. Refine the prediction by selecting the correct pixel in the warp image.

    4.  

  4. Click Add Point to enter the GCP.
Automatically Predicting GCP Locations

To have the GCP location in the warp image predicted automatically:

  1. Select Options  Auto Predict. A check mark next to the menu option indicates it is enabled.

    2.  

    When you adjust the pixel location using the Zoom window or the Zoom box in the Image window, the Zoom box and crosshairs of the warp Zoom window move to the predicted pixel location.

     

  2. Click Add Point to enter the GCP.
Turning Labels On/Off

To turn the GCP labels off or on, select Options  Label Points. A check mark next to the menu option indicates it is enabled.

Setting Ordering Preferences

To select whether the GCP pairs display in the list by their GCP ID# or by RMS Error, select Options  Order Points. A check mark next to the menu option indicates ordering by RMS error is enabled. No check mark indicates the list order is by GCP ID#

Changing Label Colors
  1. Select Options  Set Point Colors. The GCP Colors dialog appears.

    2.  

  2. Select the color of the active GCP markers from the GCPs which are On in the drop-down button (initially labeled Red).

    3.  

  3. Select the color for the inactive GCP markers from the GCPs which are Off in the drop-down button (initially labeled Green).

GCP ID order is the order in which they were selected. RMS error order lists the GCP with the highest error first.

Clearing the Image to Image GCP List

To delete all of the GCPs listed in the Image to Image GCP List, select Options  Clear All Points.

Automatically Generate Tie Points

Image coregistration is the process of geometrically aligning two or more images so that corresponding pixels representing the same objects may be integrated or fused. Automatically Coregister Images describes how to perform image coregistration.

Managing GCPs

Use the File menu in the Ground Control Points Selection dialog to save and restore ground control points files.

Saving GCPs to ASCII Files
  1. In the Ground Control Points Selection dialog or in the Image to Image GCP List, select File  Save GCPs to ASCII.

    2.  

  2. Enter an output filename with the .pts extension or use the Choose button to select an output filename.

    3.  

  3. Click OK. See GCP File Format (.pts) for information on the format.
Saving Coefficients to ASCII Files

To save the polynomial coefficients to an ASCII file:

  1. In the Ground Control Points Selection dialog, select File  Save Coefficients to ASCII.

    2.  

  2. In the Output Polynomial Coefficients dialog, enter or choose an output filename and click OK.

The ASCII file contains the polynomial degree and polynomial coefficients P and Q, where:

Where x¢and y¢ are the locations in the base image, x and y are the locations in the warp image, N is the polynomial degree, and P and Q are the polynomial coefficients. The P and Q polynomial coefficients matrices are written to the file by rows, one element per line. For example, a 2 x 2 matrix of P and Q would be written in the following format:

P[0, 0]  Q[0, 0]

P[0, 1]  Q[0, 1]

P[1, 0]  Q[1, 0]

P[1, 1]  Q[1, 1]

Restoring Saved GCPs
  1. In the Ground Control Points Selection dialog, select File  Restore GCPs from ASCII.

    2.  

  2. In the Enter the Ground Control Points Filename dialog, choose the desired GCP .pts filename.
Closing the Dialog

To exit the Ground Control Points Selection dialog, select File  Cancel.


Note
If the current points have not been saved to a file, a prompt appears.

Image-to-Map Ground Control Points

Use Select GCPs: Image-to-Map to select ground control points (GCPs) for image-to-map registration. Use the Zoom window (image-to-map registration requires that at least one image displays) to select GCPs. When selecting GCPs, you can select subpixel coordinates. Choose to enter corresponding map coordinates manually, from a vector window, from a base image, or from GPS link. When enough points to define a warp polynomial are selected, GCP locations in the warp image can be predicted. GCPs can be saved and restored to/from files and the color labels and ordering of the GCP markers can be changed. See Warping and Resampling for descriptions of the warping methods.

Opening and Displaying Images

  1. Open the image file and display the desired band.

    2.  

  2. Choose Map  Registration  Select GCPs: Image to Map. The Image to Map Registration dialog appears.

    3.  

  3. If more than one image displays, select the display to use from the Input Display drop-down list. If only one image displays, it is automatically selected.

    4.  

  4. In the Select Registration Projection list, select the output projection by clicking on it (see Selecting Map Projection Types). To enter a custom projection type, click New (see Building Customized Map Projections).

    5.  

  5. Click OK. The Ground Control Points Selection dialog appears.

    6.  


    Tip
    See the "Image Georeferencing and Registration" ENVI Tutorial for additional step-by-step descriptions of image registration.

Collecting Ground Control Points (Image-to-Map)

The procedure for selecting Ground Control Points (or Tie Points) is similar to that for image-to-image registration (see Image-to-Image Ground Control Points).

  1. In the Image window, position the Zoom box over the desired GCP area (see Controlling the Zoom Window from the Image Window).

    2.  

  2. In the Zoom window, click the left mouse button on a specific pixel to position the cursor over that pixel or portion of that pixel (see The Zoom Window).

    3.  

    The coordinates of the selected location appear in the Ground Control Points Selection dialog in the fields labeled Image X and Image Y. Subpixel coordinates are supplied to provide higher accuracy in selecting GCPs (see Collecting Ground Control Points (Image-to-Image)).

     


    Note
    Select Tools  Pixel Locator from the Image window menu bar to select the pixel positions and provide whole number GCP coordinates by clicking Export (see Using the Pixel Locator).

     

  3. Select from the following options:
    • To enter GCPs from a base image, see Entering GCPs from a Base Image.

      •  

    • To automatically enter map coordinates from vector data, see Entering Map GCPs from Vector Windows.

      •  

    • To automatically enter map coordinates from GPS locations, see Using GPS-Link.

      •  

    • To enter the map coordinates from a paper map, enter the easting and northing (or lat/long) of the selected GCPs into E and N text boxes, respectively.

      •  

    • To enter the map GCP location from a paper map in latitude and longitude, click the arrow toggle button next to the map projection name and enter the values into the Lat and Long text boxes.

      •  


      Note
      Use negative (-) longitude for the western hemisphere and negative (-) latitude for the southern hemisphere.

       

    • To change the latitude and longitude values to degrees, minutes, seconds, click DMS. To use decimal degrees, click DDEG.

      •  

    • To return to map projection coordinates, click the projection arrow toggle button.

      •  

      The corresponding map projection coordinates are calculated automatically.

     

  4. When the desired pixel is selected in the image, and the map coordinates are entered, click Add Point in the Ground Control Points Selection dialog to add the point to the list of GCPs.

    5.  

    To view the Image to Map GCP List, click Show List.

     

    The Image to Map GCP List appears with the GCPs listed in a table. For a description of the Image to Map GCP List, see Using the Image to Map GCP List.

     

    When a GCP is added, a marker is placed in the image. The GCP marker in the image is the identifying number of the GCP displayed next to an encircled crosshair. The marker indicates the selected pixel (or subpixel location); the center of the marker (located under the crosshair) indicates the actual GCP location.

     


    Note
    When GCPs are selected from a vector window, a marker is also placed in the vector window. The marker is the identifying number of the GCP displayed with a +.

     

  5. Add additional GCPs using the same procedure.

    6.  

    When enough points are entered, the predicted X,Y coordinates for the selected warp, the X, Y error, and the RMS error are listed in the Image to Map GCP List.

The number of GCPs selected displays in the Ground Control Points Selection dialog. When a sufficient number of GCPs are selected to conduct a 1st degree polynomial warp, the total RMS error is also displayed.


Tip
For the best registration, try to minimize the RMS error by refining the positions of the pixels with the largest errors or by removing them (see Minimizing RMS Error). Errors can be reduced by using more points. If you only have a few points, place them near the image corners or widely scatter them in the image.

For details about editing, predicting, and positioning GCPs, see GCP Options. For details about managing GCPs, see Managing GCPs.

Entering GCPs from a Base Image

If you have two files containing the same scene and one of the files contains map coordinate data, you can use that file as a base image and add Ground Control Points to the image that is not georereferenced.

  1. Open and display a georeferenced image. This is the base image.

    2.  

  2. Open and display a like image that is not georeferenced.

    3.  

  3. Select Map  Registration  Select GCPs: Image to Map. The Image to Map Registration dialog appears.

    4.  

  4. In the Image to Map Registration dialog, select the non-georeferenced image as the Input Display and enter projection information. Click OK. The Ground Control Points Selection dialog appears.

    5.  

  5. In the Image window of the georeferenced image, right-click and select Pixel Locator to open the Pixel Locator dialog.

    6.  

  6. In the Zoom window of the georeferenced image, left-click on a specific pixel to position the cursor over that pixel or portion of that pixel (see The Zoom Window).

    7.  

    The coordinates of the selected location display in the Pixel Locator dialog, in sample and line order. Positioning in the Zoom window provides subpixel coordinates so floating point coordinate values are shown in the dialog.

     

  7. In Zoom window of the non-georeferenced image, select the desired pixel.

    8.  

  8. When the desired pixel is selected in both images, click Export in the Pixel Locator dialog. The pixel information is exported to the Ground Control Points Selection dialog.

    9.  

  9. In the Ground Control Points Selection dialog, click Add Point. The GCP is added to the non-georeferenced image.

    10.  

  10. Add additional GCPs by following the same procedure.

Entering Map GCPs from Vector Windows

If you have a vector file for the area to map-register, you can extract the map coordinates directly from the vector data and load them into the Ground Control Points Selection dialog.

  1. Select GCPs from the displayed image as described in Collecting Ground Control Points (Image-to-Map).

    2.  

  2. Load the vectors into a Vector window and use the vector cursor positioning capabilities (left mouse button) to locate a feature corresponding to the feature selected on the image (See Cursor Functions in Vector Windows).

    3.  

  3. At the bottom of the Vector Window Parameters dialog, click Export to enter the map coordinates of the vector cursor directly into the Ground Control Points Selection dialog, or select Export Map Location from the Vector window right-click menu.

    4.  

  4. In the Ground Control Points Selection dialog, click Add Point to enter the selected point into the registration.
Saving GCPs as Vector Files

To save the GCP points and all the associated information and attributes as an ENVI vector file, in the Available Vectors List, select the new registration's GCP vector layer, and select File  Save Memory Layers to File.

Warping Options

To use the GCPs to perform a standard registration, select either Options  Warp Displayed Band or Options  Warp File in the Ground Control Points selection dialog. The Registration Parameters dialog appears. For details, see Warping and Resampling Image-to-Map.

Using the Image to Map GCP List

Using the Image to Map GCP List is similar to using the Image to Image GCP List. See Using the Image to Image GCP List

GCP Options

See GCP Options for details on the Ground Control Points Selection dialog options.

  ENVI Online Help (August 12, 2005)