The Image Registration Process

(This functionality is available with the Background Images module)

Image registration and checking is performed in the Image Registration window. When a raster image is imported or has its registration checked it is opened in the Image Registration window. The image is displayed at 100% scale i.e. one pixel in the image corresponds to one pixel on the screen. The following steps need to be undertaken before the image can be used within LISCAD.

Note that if the image is already georeferenced via a World file, or a geotiff file, skip steps 1 to 3 and proceed to step 4.

  1. Select the type of transformation to be used on the image.

    Four transformations are available:
    • Conformal (Minimum of 2 control points);
    • Affine (Minimum of 3 control points);
    • 2nd Order Polynomial (Minimum of 6 control points);
    • 3rd Order Polynomial (Minimum of 11 control points).

    Refer to Background Image Concepts as to the type of transformation to use in which circumstances.

  2. Establish the control points.
    In the grid control, click on the next available row to enter a new control point, or click on an existing row to check or modify the co-ordinates.

    By use of the Fit, 1:1, Zoom and Centre buttons on the Image Registrar window, navigate to the location where you want to create or edit the control points so that you can see the location you want to place the control point clearly.

    Tip: Use the snap (right) mouse button to break out of the 1:1, Zoom and Centre modes.

    To create a control point use the mouse to select the point in the image that you know the real world co-ordinate of. This will cause the image pixel co-ordinates to be entered into the grid control.

    Now enter the real world co-ordinates next to the pixel co-ordinates. This can be achieved either by typing them in directly, clicking anywhere in the plan view or snapping to points in the plan view.

    Tip: When snapping to control points in the plan view you must not have any other function active that also accepts snapping input from the plan window. If you do, this function will intercept the input rather than the background images registration dialog.

    Select the control points so that they are evenly spread out over the image.

    Note: To quickly locate a control point on the image, hold the Ctrl key down and click on the point number in the grid control of the point you want to find. The image display will be updated to place the control point as close as possible to the centre of the screen.

  3. After entering at least the minimum number of control points, press the "Compute Residuals" button. This will compute the error at each of the control points in map units. Review all residuals to determine if they are all within the acceptable range of tolerance for the resolution of the image being used. If the residuals are not within an acceptable tolerance it may be due to one or both of the following:

    One or more of the co-ordinates entered is incorrect. To track this down start by checking the control points that has the largest residual and make sure that the point in the image you selected does in fact represent the known control point and that the map co-ordinate entered is correct. After making a change press the "Compute Residuals" button again to review the transformation.

    The transformation technique selected is inappropriate for the image type used. Select one of the other transformation methods and ensure that the minimum number of control points have been entered.

    Note: Because you have good residual values does not necessarily mean that the transformation is good or appropriate for the type of image you are working with. E.g. If you selected Conformal as the transformation method and entered only 2 control points and the image was an unrectified aerial photograph you would find that the residuals computed would be 0.0. This would appear to be very good and indeed at the 2 control points it would be the case. However everywhere else in the image is very likely to contain large errors as a conformal transformation is inappropriate for this type of image.

  4. The image registration also allows you to clip information from around the outside of the image. This is typically used to remove the frame from around the edge of a map image. By default everything you see on the original image will be displayed in the transformed image. If you wish to clip the image select the Clip radio button to make clip active and you can then position the four corner points defining the clip region by dragging them where you want.

  5. To register the image press the "Register Image" button and the original image will be transformed into a new image that can be viewed as a registered image in the LISCAD plan view.