Use Band Ratios to enhance the spectral differences between bands and to reduce the effects of topography. Dividing one spectral band by another produces an image that provides relative band intensities. The image enhances the spectral differences between bands. ENVI can output band ratio images in either floating-point (decimal) format or byte data format. Floating-point format is the default. You may combine three ratios into a color-ratio-composite (CRC) image to determine the approximate spectral shape for each pixel’s spectrum.

To calculate band ratios, you specify a numerator band and a denominator band. The band ratio is the numerator divided by the denominator. ENVI checks for division-by-zero errors and sets them to 0. You can also calculate multiple ratios and output them as multiple bands in one file.

To calculate band ratios:

  1. Display a multispectral image.
  2. From the Toolbox, select Band Algebra > Band Ratios. The Band Ratio Input Bands dialog appears.

  3. Select the Numerator band (the first band) and the Denominator band (the second band) from the list of available bands.

    To change the bands you have selected click Clear.

  4. Click Enter Pair to create a new band pair listed in the Selected Ratio Pairs list. When you select the first pair, only bands with the same spatial size appear in the Available Bands List. The Numerator and Denominator fields clear.

    Create as many ratio combinations as needed by entering additional band pairs. All ratios in the Selected Ratio Pairs list are output as multiple bands in a single file.

  5. Click OK. The Band Ratios Parameters dialog appears.
  6. To select an optional spatial subset, click Spatial Subset.
  7. Select output to File or Memory.
  8. To output the ratio values as byte or floating-point data, select from the Output Data Type drop-down list. If you select Byte, ENVI stretches the output ratio values by mapping the values entered in the Min and Max fields to 0 to 255.
  9. If you selected an output ratio of Byte, change the byte stretching ratio data range in the Min and Max values.
  10. Click OK. ENVI adds the resulting output to the Layer Manager.

Creating Complex Band Ratios

To use band ratios in second-tier band ratio calculations:

  1. Make the single band ratios as described above.

  2. From the Toolbox, select Band Algebra > Band Ratios. You use the previously-computed set of ratios in a new band ratio.

    For example, if you are using Landsat TM data, you could first calculate the band 5/band 7 ratio and the band 3/band 1 ratio. You can calculate a complex ratio of 5/7 divided by 3/1, by selecting the 5/7 ratio from the Band Ratio Input Bands dialog as the numerator and the 3/1 ratio as the denominator.

  3. Select a new Numerator and Denominator.
  4. Click Enter Pair.
  5. Click OK. ENVI adds the resulting output to the Layer Manager.

Tip: You can also use the Band Math tool to calculate these kinds of ratios.

See Also

Band Math, Spectral Indices