The Change Detection workflow is used to identify features that have changed between two images collected over the same area at different times. Input files must have the same number of spectral bands. THOR is intended for use with hyperspectral data, but input files are only required to have at least two bands. The input files must at least partially cover the same area on the ground, but they do not need to be georeferenced. The two images will be co-registered later in the Change Detection workflow.

  1. From the Toolbox, select THOR > THOR Change Detection. The Change Detection dialog appears.
  2. Click Select Time #1 File. A file selection dialog appears.
  3. Select a multi-band image and perform optional spatial subsetting.
  4. Click Select Time #2 File. A file selection dialog appears.
  5. Select a multi-band image to be compared with the Time #1 file. The Time #1 and #2 images do not necessarily need to be in chronological order. The Time #1 file will serve as the base image that the Time #2 image is warped to for purposes of co-registration. The desired spatial subset for change detection is based on the Time #1 image.
  6. You can choose to process a subset of spectral bands by clicking one of the following buttons:
    • Graphically: The THOR Spectral Subsetting dialog appears.
    • By List: The File Spectral Subset dialog appears.
  7. By default, the output rootname will be change. Miscellaneous output files produced during the workflow will use this rootname, plus other words as needed, such as _subset. To change the rootname to something else, click Select Output Root Name.
  8. By default, the output file is written to the same directory as the input file. You can choose another output directory, but if it is a read-only directory, ENVI will default to the Temp Directory. See ENVI Classic Help for details on this preference.

  9. Click Next.
  10. Select tie points for the images.
  11. Perform atmospheric correction on the Time #1 and Time #2 images.
  12. In the Change Detection Methods panel, select Spectral Angle. This method computes the spectral angle on a pixel-by-pixel basis from the two co-registered input images. The larger the spectral angle, the greater the change.
  13. Perform rule thresholding.
  14. Perform spatial filtering.
  15. Export targets.

Related Topics

Change Detection Analysis, Image Change, Thematic Change, SPEAR Change Detection