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Spectral Angle Mapper

Spectral Angle Mapper

Spectral Angle Mapper (SAM) is a physically-based spectral classification that uses an n-D angle to match pixels to reference spectra. The algorithm determines the spectral similarity between two spectra by calculating the angle between the spectra and treating them as vectors in a space with dimensionality equal to the number of bands. This technique, when used on calibrated reflectance data, is relatively insensitive to illumination and albedo effects. Endmember spectra used by SAM can come from ASCII files or spectral libraries, or you can extract them directly from an image (as ROI average spectra). SAM compares the angle between the endmember spectrum vector and each pixel vector in n-D space. Smaller angles represent closer matches to the reference spectrum. Pixels further away than the specified maximum angle threshold in radians are not classified.

SAM classification assumes reflectance data. However, if you use radiance data, the error is generally not significant because the origin is still near zero.

Reference: Kruse, F. A., A. B. Lefkoff, J. B. Boardman, K. B. Heidebrecht, A. T. Shapiro, P. J. Barloon, and A. F. H. Goetz. "The Spectral Image Processing System (SIPS) - Interactive Visualization and Analysis of Imaging spectrometer Data." Remote Sensing of Environment 44 (1993): 145-163.

Tip: You can use the Spectral Hourglass Wizard to guide you through the spectral hourglass processing flow, which includes SAM classification, to find and map image spectral endmembers from hyperspectral or multispectral data.

  1. From the Toolbox, select Classification > Supervised Classification > Spectral Angle Mapper Classification. The Classification Input File dialog appears.
  2. Select an input file and perform optional spatial and spectral subsetting, and/or masking, then click OK. The Endmember Collection:SAM dialog appears.
  3. From the Endmember Collection:SAM dialog menu bar, select Import > spectra_source and collect endmember spectra from a variety of sources. For details, see Import Spectra and Manage Endmember Spectra.
  4. In the Endmember Collection:SAM dialog, click Apply. The Spectral Angle Mapper Parameters dialog appears.
  5. Select one of the following thresholding options from the Set Maximum Angle (radians) area:
    • None: Use no threshold.
    • Single Value: Use a single threshold for all classes. Enter a value, in radians, in the Maximum Angle (radians) field. This is the maximum acceptable angle between the endmember spectrum vector and the pixel vector (in # of bands dimensional space). The default is 0.1 radians. ENVI does not classify pixels with an angle larger than this value.
    • Multiple Values: Enter a different threshold for each class. Use this option as follows:
    1. In the list of classes, select the class or classes to which you want to assign different threshold values and click Multiple Values. The Assign Maximum Angle (radians) dialog appears.
    2. Select a class, then enter a threshold value in the field at the bottom of the dialog. Repeat for each class. Click OK when you are finished.
  6. Select classification output to File or Memory.
  7. Use the Output Rule Images? toggle button to select whether or not to create rule images. Use rule images to create intermediate classification image results before final assignment of classes. You can later use rule images in the Rule Classifier to create a new classification image without having to recalculate the entire classification.
  8. If you selected Yes to output rule images, select output to File or Memory.
  9. Click Preview to see a 256 x 256 spatial subset from the center of the output classification image. Change the parameters as needed and click Preview again to update the display.
  10. Click OK. ENVI adds the resulting output to the Layer Manager. The output from SAM is a classified image and a set of rule images (one per endmember). The pixel values of the rule images represent the spectral angle in radians from the reference spectrum for each class. Lower spectral angles represent better matches to the endmember spectra. Areas that satisfied the selected radian threshold criteria are carried over as classified areas into the classified image.



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