Use Continuum Removal to normalize reflectance spectra so you can compare individual absorption features from a common baseline. The continuum is a convex hull fit over the top of a spectrum using straight-line segments that connect local spectra maxima. The first and last spectral data values are on the hull; therefore, the first and last bands in the output continuum-removed data file are equal to 1.0.

Note: Using different spectral subsets gives different results, so you should spectrally subset the data to the region containing the absorption features of interest.

The continuum is removed by dividing it into the actual spectrum for each pixel in the image:

Scr = (S / C)


Scr = Continuum-removed spectra

S = Original spectrum

C = Continuum curve

The resulting image spectra are equal to 1.0 where the continuum and the spectra match, and less than 1.0 where absorption features occur. You can perform continuum removal on data files or on individual spectra in a plot window. See Spectral Tools References.

  1. From the Toolbox, select Spectral > Mapping Methods > Continuum Removal. The Continuum Removal Input File dialog appears.
  2. Select the input file and perform optional spatial and spectral subsetting, and/or masking, then click OK. For the best results, spectrally subset around the region containing the absorption features of interest.
  3. Click OK. The Continuum Removal Parameters dialog appears.
  4. Select output to File or Memory.
  5. Click OK. ENVI adds the resulting output to the Layer Manager.