The technique of ratioing bands involves, at its most basic form, dividing the spectral response value of a pixel in one image (or band) with the spectral value of the corresponding pixel in another image (or band). This is done in order to suppress, or normalize, varying effects such as viewing angles, sun shading, atmospheric effects, soil difference, and so on. It is also applied to maximize sensitivity to the feature of interest, such as the relative health of vegetation. To achieve this most Indices go beyond simple band division to include differencing, weighting, and the introduction of other variables.
Band ratioing (also known as Indices), is used extensively in mineral exploration and vegetation analyses to bring out small differences between various rock types and vegetation classes. In many cases, judiciously chosen indices can highlight and enhance differences which cannot be observed in the display of the original color bands.
Indices can also be used to minimize shadow effects in satellite and aircraft multispectral images. Grayscale images of individual indices or a color combination of three ratios may be generated.
This dialog provides some commonly used mineral and vegetation ratios and indices.
To learn how to identify minerals using Indices, see Enhance Images using Spectral Enhancement workflow.
This dialog opens when you:
- Click Raster tab > Unsupervised > Indices
- Click Raster tab > Unsupervised > NDVI
[file name] Enter the name of the input file, or click the arrow to open Recent Files menu.
[file name] Enter the name of the output file.
Navigate to the directory to store the output file and choose the format type for the output file.
Files of type In the File Selector, select the output file format.
Sensor Select the appropriate sensor that collected the input file data. The sensor names that are presented depend upon the input file that is selected. It is useful to identify the source sensor so that initial defaults can be provided to map the bands of the input file to the wavelengths defined for processing in any given Index. You can also use the Band Selection CellArray to select which input image band is used for a specific Index variable.
Category Select the category of indices you want. The default is All, which includes indices from all categories. The categories include Vegetation, Water / Snow / Ice, Iron, Carbonates / Mafic, Silica / Silicates, General Soil / Mineral, Sulphates, and Other.
Index Select the index method to apply to the data. Different indices are listed depending on the Category and data type. The contents of this list depend upon the sensor that is selected. If no indices are shown, then all the indices in the selected Category have at least one required band that does not overlap any of the bands in the selected Sensor.
See Vegetation, Mineral, Hydrological Indices Formulas and Uses for descriptions.
Formula Shows the formula of the selected Index.
Set the current option as the default option type. Restarting this dialog from Unsupervised > Indices shows this Index as the default. Starting this dialog from Unsupervised > NDVI shows the Normalized Difference Vegetation Index (NDVI) as the default.
Show All If this checkbox is not checked, hide the Indices that require a band that does not overlap any existing sensor band. Check this box to show all Indices in the selected Category, including the ones that require a band that does not overlap any band of the selected sensor.
For the selected image, the corresponding number of bands display in the CellArray. Values for each band are listed.
Band Band number.
Wavelength Electromagnetic (EM) wavelength center corresponding to the band of the sensor in units expressed in Wavelength units.
Width EM wavelength width for the band of the sensor. The range of the range is from Wavelength – ½ Width to Wavelength + ½ Width
Each index requires a set of bands such as NIR & Red to be entered into its formula. Some indices require additional scalar parameters.
Columns 3 to N correspond to the required bands from the index formula. Some indices require NIR & Red, while other indices have different band requirements.
NIR Near Infrared band
Red Red band
Checkmarks match up bands of the input sensor to bands required by the index formula. The default checks indicate the optimum sensor band to index band mapping. If there is no checkmark in a column there is no overlap between the wavelength range of the band required by the index and the wavelength range of any of the sensor’s bands. You may place a checkmark to override the default sensor band to index band mappings. All index bands must be mapped to a sensor band to make the OK button available.
Wavelength units Unit of measure for Wavelength
For Formulas containing a variable:
Param Parameter of the variable listed in the Formula.
Value Value of parameter. Default is shown, or change the value if needed.
Description Description of parameter.
Coordinate Type Click the appropriate radio button to select the type of coordinates to use. If the input file does not have map coordinates, the coordinate type defaults to File.
Map Click to use map coordinates, if the data are rectified.
File Click to use file coordinates, which are pixels, starting with 0,0 in the upper-left corner.
Subset Definition Define a rectangular area of the data to be used as the output file. The coordinates of the subset may be derived from an Inquire Box in the View or by entering explicit X,Y values for the upper-left and lower-right corners of the subset. The default coordinates are the entire input file.
UL X Upper-left X coordinate
UL Y Upper-left Y coordinate
LR X Lower-right X coordinate
LR Y Lower-right Y coordinate
From Inquire Box Click to define a subset area of the data by using the Inquire Box. When you click this button, the coordinates are replaced with the coordinates of the Inquire Box in the View.
To change these coordinates, you can move or resize the Inquire Box in the View, then click this button again.
The image you are using and the Inquire Box must already be displayed in a View in order to use the From Inquire Box option. Otherwise, you may manually enter coordinates.
Input Shows the input raster data type.
Output Set the raster data type for the output file.
Stretch to Unsigned 8 bit Check the checkbox to apply a stretch to the range of data values in the output file to 0-255.
Interactive Viewing Buttons
The interactive connections between the Indices dialog and Spatial Model Editor and Preview windows remain open while you change Band Selection or Parameter values. Changes in these CellArrays are reflected in the connected Spatial Model Editor or the connected Preview window. Other changes in the dialog, such as Input File, Sensor, or Index, will break the connection. When the connection is broken, the Connected to message disappears. Clicking View or Preview again during an active connection breaks the current connection, and opens a new Spatial Model Editor or Preview window. The new window now takes over the active connection.
Note that connections are one-way only. Changes in the Indices dialog affect the Spatial Model Editor and Preview, but changes made in the Spatial Model Editor are not reflected in the Indices dialog. Also, closing a Spatial Model Editor or Preview does not update the Indices dialog, so you may still see the Connected to message in the dialog after closing the connected window.
When the connection to a Spatial Model Editor is broken, changes in the dialog no longer update the Editor, however, you can still edit the model and run it from the Spatial Model Editor.
ViewOpen the Spatial Model Editor showing a spatial model with your input from this dialog. The message "Connected to Spatial Model Editor" appears next to the View button, indicating an interactive connection between the Indices dialog and the Spatial Model Editor.
Preview Open a Preview window showing the output of the Index using your input from this dialog. The message Connected to 2D Preview window appears next to the Preview button, indicating an interactive connection between the Indices dialog and the Preview window.
OK Run this program with the options selected and close this dialog.
Batch Open Batch Command Editor to schedule one or multiple processing jobs.
View Open the Spatial Model Editor showing a spatial model with your input from this dialog. You can save the model, but you must use a unique name. The name cannot be the standard Indices model name defined in <IMAGINE_HOME>/etc/models.
AOI Select an AOI (area of interest) in the image to which to apply this function. The Choose AOI dialog opens.
Cancel Cancel this process and close this dialog.
Help Open this Help document.