Using the tie points, ENVI will calculate the epipolar geometry and epipolar images that are used to extract the DEM. These epipolar images describe the relationship between the pixels in the stereo pair and they can be viewed in 3D using anaglyph glasses.

Epipolar images are stereo pairs in which the left and right images are oriented in such a way that ground feature points have the same y-coordinates on both images. Using epipolar images removes one dimension of variability, thus greatly increasing the speed of image-matching processing as well as the reliability of the matching results.

In Step 6 of 9 of the DEM Extraction Wizard, the left and right epipolar images are created and saved. You have the option of viewing these epipolar images at this time, or you can continue with the DEM extraction and examine the epipolar images at a later time. If you decide to view the epipolar images now using anaglyph glasses, you can use the Epipolar 3D Cursor Tool to collect map and elevation points. You can export these to an ASCII file, an EVF file, or an ArcView 3D shapefile. See The Epipolar 3D Cursor Tool for more information about this DEM Extraction tool.

Note: If the y-parallax error of any of the tie points exceeds 10 pixels, an ENVI Error dialog appears with the maximum y-parallax value. You should use the Prev button in Step 6 of the Wizard to return to Step 5 (see Viewing, Adding, Editing Tie Points), and make sure all of the tie points are correct before proceeding.

The Epipolar Reduction Factor controls the down-sampling of the output epipolar image size. This factor is a floating-point value with a minimum value of 1.00 (the default), which determines the lowest image pyramid level on which image matching can be performed. A value of 1.00 means the epipolar images have the same resolution as the input images. A value of 2.00 means the epipolar images have half the spatial resolution as the input images. In this case, the processing takes less time, requires less disk space, but limits the highest resolution of the DEM output.

The buttons in the Examine Epipolar Results section enable you to generate and examine the epipolar images immediately. Your choice for the left and right images when you started the Wizard determines whether the epipolar image viewed is thetrue stereo 3D epipolar image or the inverse stereo 3D epipolar image. You can view the resulting image and use the Epipolar 3D Cursor to adjust the apparent height of the cursor in order to extract elevation data. See The Epipolar 3D Cursor Tool for detailed information.

• Click RGB=Left, Right, Right to create an epipolar image pair and to load the stereo epipolar image pair in RBG color mode. In this case, the red channel is the left epipolar image, and the blue and green channels are the right epipolar image. When the RGB epipolar image is created this way, you can view the result in 3D using anaglyph glasses.

If your image appears inverted (where high elevation features appear to go into the screen), the image is in the inverse stereo 3D epipolar view.

• Click RGB=Right, Left, Left to create an epipolar image pair and to load the stereo epipolar image pair in RBG color mode. In this case, the red channel is the right epipolar image, and the blue and green channels are the left epipolar image. When the RGB epipolar image is created this way, you can view the result in 3D using anaglyph glasses.

Click Next to continue.