How to calculate and overlay contours from a non-displayed image

This help article provides instructions for drawing contours based on a non-displayed image in Classic ENVI. For example, it may be useful to display an image and then have the contours for an associated DEM drawn in the image display. Many Classic ENVI users do not realize that when calculating image contours to overlay onto a display using Overlay -> Contour Lines... they could base the contours on a completely different image than the one in the display.

The following steps assume that an image will be displayed, and that contours based on an associated DEM will be overlaid onto the image display. These steps can also be used to calculate contours from an image other than a DEM.

Note:This procedure will only work if the two images have the same x and y dimensions:

1. Open both the image and the DEM in ENVI. Make sure that both are listed in the Available Bands List.
2. Display the image.
   From the display menu, choose Overlay -> Contour Lines....
   
3. In the resulting Input Contour Band dialog, choose the DEM.
4. In the resulting Contour Plot window, choose the Apply button to apply the default contours calculated for the DEM.

Note: If you would like to change the data on which the contours are based, from the Contour Plot dialog window, choose Options -> New Contour Band. The Input Contour Band dialog will reappear, and you can pick a different image.