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SPEAR Change Detection

SPEAR Change Detection

The SPEAR Change Detection tool provides a way to compare imagery collected over the same area at different times and to highlight features that have changed. There are two forms of change detection: absolute and relative. Absolute change detection highlights specifically what has changed, e.g. forest to grassland. Relative change detection shows that something has changed but does not specify what that change is. Relative change detection provides a faster method for quickly comparing images.

The SPEAR Change Detection tool performs the following types of relative change detection:

  • Transform: Input datasets are stacked into one image cube, then an image transform (principle component analysis, minimum noise fraction, or independent components analysis) is applied in order to extract the feature correlating to change.
  • Subtractive: Normalized Difference Vegetation Index (NDVI), red/blue ratio, and man-made ratio are calculated for the input datasets. The resulting ratios and input bands are subtracted from the input images to create difference images.
  • Two Color Multi-View (2CMV): One band from the Time 1 image is displayed in the red band, and the same band in the Time 2 image is displayed in the green and blue bands. Objects that are brighter in the Time 1 image compared to the Time 2 image appear in red. Objects that are darker in the Time 1 image compared to the Time 2 image appear in cyan. The colors can then be used to indicate potential areas of change.
  • Spectral Angle: This method computes the spectral angle between corresponding pixel spectra in the Time 1 and Time 2 images. The spectral angle measures the spectral similarity between the two spectra.

For best results, use images with similar view geometries. Different view geometries may cause objects such as trees and structures to “lean” in different directions. Because these issues cannot be resolved in coregistration, they cause artifacts in the results.

Reference

Unsalan, Cem., and K. L. Boyer. (2004). A system to detect houses and residential street networks in multispectral satellite images. Proceedings of the 17th International Conference on Pattern Recognition (ICPR ‘04), Volume 3.

The SPEAR Change Detection tool is intended for multispectral images. If panchromatic images are selected, 2CMV will be the only change detection method available.

  1. From the Toolbox, select SPEAR > SPEAR Change Detection. The Change Detection Wizard displays the File Selection panel.
  2. Time #1 and Time #2 images do not need to be in chronological order. One will be used as the base image, and the other will be warped to match that image. To preserve as much information as possible, use the highest resolution image for the Time #1 image. Click Select Time #1 File, choose a file for the base image, then click OK. The Auto Tie Points Matching Band dialog appears.
  3. Select the band to use for auto tie point matching, then click OK.
  4. Click Select Time #2 File, choose a file for the warp image, then click OK. The Auto Tie Points Matching Band appears.
  5. Select the band to use for auto tie point matching, then click OK.
  6. To optionally process only a portion of the scene, click Select Subset. A small Select Spatial Subset dialog appears. Subsetting is applied to both images. When finished, click OK to return to the File Selection panel.
  7. By default, the output file is saved to the same directory and uses the same filename as the Time #2 input file, minus any extension. The filename is appended with change_pca. To change the directory and/or filename, click Select Output Root Name.
  8. Click Next. The Method Selection panel appears with coregistration parameters. Click Next. The next two dialogs: the Review Tie Points dialog and the Check Coregistration dialog allow you to check the accuracy of the coregistration and fix tie points if needed. When complete, click Next on each dialog. The Method Selection dialog appears.
  9. Enable the check boxes for each change detection method you elect to use.
  10. Click Show Advanced Options to set advanced parameters for the Image Transform and/or Subtractive methods.
  11. For the Image Transform method, select the desired transform algorithm to use: Principal Components, Minimum Noise Fraction, or Independent Components.
  12. For the Subtractive method, select whether you wish to perform dark object subtraction and/or radiometric normalization. Dark object subtraction removes haze and performs atmospheric correction. Radiometric normalization normalizes the overall brightness of the Time #2 image to the Time #1 image based on bands. If a pixel is unchanged between the two images, the digital number (DN) difference will be zero.
  13. Click Next to begin processing. Processing may take several minutes; when it is complete, the Examine Results dialog appears.

    The change detection results and the input images (in natural color) are opened in the display and are dynamically linked for comparison. For the Image Transform and Subtractive methods, bright pixels in the output image indicate changed areas. For the 2CMV method, red or cyan colors in the output image indicate changed areas.

  14. Change the display of the method results using the See results for drop-down list. Changing the method displayed will provide different visualization options in the Wizard. Each is described below.

    Optionally, click on the dotted bar in the histogram window or enter values in the fields at the top of the histogram window to aid in visualizing areas of change. Use Auto-Flicker to examine your results (for detailed information on using this tool.

    • 2CMV: The display group on the left contains an RGB image composite in which red or cyan colors indicate changed areas. The Time #1 image is displayed in the red band, and the Time #2 image in the green and blue bands. The color depends on the relative brightness of the changed object. Changed areas are highlighted as bright and/or dark pixels in one of the transformed bands.

      The 2CMV image can only display one band at a time. To select another band, use the See 2CMV for drop-down list, then click Load Image. Toggle the colors displayed using the 2CMV Colors option.

      Optionally save the results using Export to NITF and/or Save to Graphic.

      Or, click Export Image to ArcGIS Geodatabase to save the results to a geodatabase. The Select Output Geodatabase dialog appears.

    • Spectral Angle: The display group on the left contains a grayscale image of the Spectral Angle change results. The middle display group shows a color composite of the "before" image, and the last display group shows a color composite of the "after" image. The display groups are dynamically linked for ease of comparison.
    • Image Transform: The display group on the left contains one of the transformed bands in grayscale. Transform attempts to segregate different image features into different bands. One of the transform result bands highlights changes; however, it can be any one of the transformed bands. Typically, changes are in the second or third band, but it varies depending on scene content and the amount of change. The display groups are dynamically linked for ease of comparison. To explore different transformed bands, use the Transform band drop-down list, then click Load Image.

    • Subtractive: The display group on the left contains one of the difference images in grayscale. The display groups are dynamically linked for ease of comparison. To explore difference images, use the See change for drop-down list, then click Load Image.

  15. Select new color tables using the Color Tables list, then click Apply Color Table. Use the Stretch Bottom and Stretch Top slider bars to adjust how the color table is applied.

    Optionally save the file.

    The following shows the difference in NDVI between Time 1 and Time 2. Dark areas indicate areas stripped bare of vegetation (left) Time 1 Image (center) and Time 2 Image (right):

    The following shows bright pixels (left) show changed areas between Time 1 (center) and
    Time 2 (right).

  16. When you are finished examining results, click Next in the Examine Results panel, then click Finish to exit the Wizard.

Related Topics


Change Detection Analysis, Image Change, Thematic Change, THOR Change Detection



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