The Spectral Analogues tool maps the occurrence throughout an image of some desired material. Spectral Analogues uses spectral matching algorithms that compare the spectrum of each pixel to the mean spectrum of user-specified training pixels. You must know at least one location in the image where the desired material exists, usually through visual observation or prior knowledge, so that it can be used for training. The Spectral Analogues tool is designed for use with multispectral data.

  1. From the Toolbox, select SPEAR > SPEAR Spectral Analogues. The ENVI Spectral Analogues Wizard displays the File Selection panel.
  2. Click Select Input File, choose a file, then click OK. The input image should be a multispectral file in any format readable by ENVI.
  3. To optionally process only a portion of the scene, click Select Subset. A small Select Spatial Subset dialog appears.
  4. Click Spatial Subset. The standard Select Spatial Subset dialog appears. When finished, click OK to return to the File Selection panel.
  5. By default, output files are saved to the same directory and use the same rootname as the input file, minus any extension. Output files are appended with a unique suffix. To change the directory and/or root filename, click Select Output Root Name.
  6. Click Next. The Atmospheric Correction panel appears.
  7. Perform atmospheric correction. Dark object subtraction provides good results with minimal user input.
  8. When atmospheric correction is complete, click Next. A natural color composite image is loaded into a display group, the ROI dialog appears, and the Select Training Pixels SPEAR dialog appears.
  9. Use the ROI Tools dialog to add pixels to the ROIs. You can create new ROIs as needed through the ROI Tool dialog, and populate them with pixels that represent some other class. Choose multiple pixels for each ROI to represent variability within that type.
  10. Click Next. The ROI Selection dialog appears.
  11. Select one or more ROIs to map, then click OK. The Spectral Processing panel appears.
  12. Select one or more Spectral Processing Parameters:
    • Spectral Angle Mapper
    • Matched Filter
    • MF/SAM Ratio: Suppresses false positives that may be present in one method, but not the other, while enhancing true positives. For example, a pixel containing water has a high MF value and low SAM value. A high value divided by a low value results in a very large value, therefore enhancing the positive result. Conversely, if the MF product has a high value for a false positive, but the SAM correctly maps it as non-water so it has a high value, the high value divided by high value results in a smaller value, suppressing the false positive.
    • Normalized Euclidean Distance: Calculates the distance between two vectors in the same manner as Euclidean Distance, but normalizes the vectors first. Normalization is performed by dividing each vector by its mean.

      Note: You cannot disable the Spectral Angle Mapper and Matched Filter options when MF/SAM Ratio is enabled.

  13. Click Next. The Examine Results panel appears.
  14. Select the image within the processing method to display from the Examine Result for drop-down list. For example, if four different ROIs were used as input for spectral processing, the list contains four separate rule images.
  15. Click Load image type to load the image. The following appear:
    • One display group contains a natural color composite of the input image, to use as reference.
    • One display group contains the selected spectral processing result. This display group is dynamically linked to the natural color display group. Use the dynamic overlay tool to see how the spectral processing result relates to the natural color reference image.
    • A histogram window appears for the spectral processing result display group to allow for adjusting the stretch to highlight positive matches. For MF/SAM Ratio and MF results, positive matches are always bright. For SAM, positive matches are dark. For Normalized Euclidean Distance, bright pixels indicate better matches.
  16. When you are finished examining results, click Next in the Examine Results panel, then click Finish to exit the Wizard. The following image shows an unstretched rule image (left), and a stretched rule image (right) for SAM results, highlighting aircraft and spectrally similar materials