The Epipolar 3D Cursor tool allows you to perform 3D measurements in a 3D stereo viewing environment based on an existing epipolar stereo pair of images. You can view an anaglyph of epipolar stereo imagery and adjust the apparent height of the cursor to extract elevation data. Elevation data obtained this way can be exported to an ASCII file, ENVI vector file (EVF), or ArcView 3D shapefile.

Note: The epipolar stereo image pair can be created using the DEM Extraction Wizard (see Generate Epipolar Images) or by selecting Terrain > DEM Extraction> Build Epipolar Images from the Toolbox (see Build Epipolar Images).

Currently, the Epipolar 3D Cursor tool works with any stereo image pairs that have rational polynomial coefficient (RPC) data, either included in the original file or calculated by ENVI.

Open the Epipolar 3D Cursor

To use this tool, you need an existing epipolar stereo pair.

  1. From the Toolbox, select Terrain > DEM Extraction > Epipolar 3D Cursor. The Select Left Epipolar Image file selection dialog opens.
  2. In the Select Left Epipolar Image dialog:
    • Select the left epipolar image from the Select Input Band list, or by opening an epipolar image file using the Open drop-down button and selecting the file.
    • Click OK. The Select Right Epipolar Image dialog opens.
  3. In the Select Right Epipolar Image dialog:
    • Select the right epipolar image from the Select Input Band list, or by opening an epipolar image file using the Open drop-down button and selecting the file.
    • Click OK. The epipolar stereo image opens in a new display group, with the left epipolar image as the red band, and the right epipolar image as the blue band. The Epipolar 3D Cursor dialog opens.

    4. The cursor in the Image window view is now a double red and blue cursor which, when viewed using anaglyph glasses, merges into a single cursor. You can control the 3D cursor using the specific mouse and keyboard controls shown in the following table:

    Keyboard/Mouse

    Action

    Mouse

    Moves the 3D cursor around the epipolar stereo image.

    Left Mouse Button

    Causes the 3D cursor to snap to ground.

    Middle Mouse Button

    Exports the map and elevation for the point to an ENVI Point Collection table. See Collect Elevation Points.

    Right arrow

    Moves the 3D cursor one pixel to the right.

    Left arrow

    Moves the 3D cursor one pixel to the left.

    Up arrow

    Moves the 3D cursor one pixel toward the top of the image.

    Down arrow

    Moves the 3D cursor one pixel toward the bottom of the image.

    Num Keypad + (plus)

    Keyboard Key = (equal)

    Raises the apparent elevation of the 3D cursor.

    Num Keybad - (minus)

    Keyboard Key - (hyphen)

    Lowers the apparent elevation of the 3D cursor.
  4. Select which window to use for viewing the 3D cursor: Image (default), Scroll, Zoom, or Off.
  5. Set the following 3D Cursor Parameters according to your preferences.
    • Size: Size of the cursor. The default value is 2.
    • Brightness: Brightness of the cursor. The maximum value is 255, and the default value is 180.
  6. Click Snap to Ground Parameters to open the Snap to Ground Parameters dialog.
  7. In the Snap to Ground Parameters dialog, you can adjust the Search Window Size and Moving Window Size. See Generate Tie Points Automatically for a description of these parameters. If the image is a high resolution image, increase the Moving Window Size.
  8. If the topography appears to be inverted in the stereo anaglyph, use the Swap Left/Right in Display button to swap the red and blue channels in the display.

Collecting Elevation Points

Find a topographic feature whose elevation you want to know. When the cursor is located on the feature, click the left mouse button to perform a snap to ground. Snap to ground typically obtains better results when the current cursor is on a ground feature with distinctive spectral patterns. ENVI uses an image correlation method to find the conjugate point in the right image for the current left cursor location. When a snap to ground operation finds correct matches, the 3D cursor appears to snap to, or touch the ground point. Some fine tuning may be necessary to adjust the 3D cursor to obtain better results; the fine tuning keyboard and mouse controls are listed in Open the Epipolar 3D Cursor.

If you are satisfied with the location of the cursor, click the middle mouse button to export the map and elevation information for that point to an ENVI Point Collection table. Each time you select a point and export the values using the mouse buttons, the point is added to the ENVI Point Collection Table. See Collect Elevation Points for more information.