Analyze Narrowband Vegetation Indices

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Lesson 1 of 1

Analyze Narrowband Vegetation Indices

Background

Narrowband vegetation indices (VIs) use discrete portions of the electromagnetic spectrum to characterize various vegetation properties. These properties include greenness, nitrogen content, water content, dry or senescent carbon, leaf pigments, and light use efficiency. Multispectral sensors (those with 10s of bands) and especially imaging spectrometers (with hundreds of bands) provide the spectral fidelity needed to compute narrowband VIs.

ENVI provides 33 different narrowband VIs. See the Vegetation Indices Background(opens in a new tab) topic in ENVI Help for details. One example is the Modified Red Edge NDVI(opens in a new tab), which is computed from the following equation:

To create this index, ENVI identifies bands whose center wavelengths are closest to 750 nm, 705 nm, and 445 nm. It allows a narrow tolerance of values for each wavelength term. If no bands fall within the allowable ranges for each wavelength, ENVI issues an error message saying the image does not contain the appropriate wavelengths for that index.

Let's look at some examples, using an AVIRIS image for demonstration.

Open an AVIRIS Image

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Select File > Open from the Menu bar. The Open dialog appears. 2. 2

Go to the "hyperspectral" directory in your ENVI installation path.

Windows: C:\Program Files\NV5\ENVIxx\data\hyperspectral (xx is the version number)
Linux: /user/local/NV5/envixx/data/hyperspectral
Mac: /Applications/NV5/envixx/data/hyperspectral 3. 3

Select the file AVIRISReflectanceSubset.dat and click Open. This is an AVIRIS hyperspectral image of an agricultural area in central California.

This image was corrected for atmospheric effects using the FLAASH® tool in ENVI. Pixel values represent surface reflectance; they range from 0.0 to 1.0. This is ideally how images should be preprocessed(opens in a new tab) before creating narrowband spectral indices. Refer to the Mitigate Atmospheric Effects and Scale and Mask Reflectance Images quick guides for more information.

Run the Spectral Indices Tool

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In the Toolbox, expand the Band Algebra folder and double-click Spectral Indices. The Data Selectiondialog appears. 2. 2

Select AVIRISReflectanceSubset.dat and click OK. The Spectral Indices dialog appears. 3. 3

Scroll through the list of indices to see which are available for this image. Or, click and drag the lower-right corner of the dialog to expand it. The list includes not only VIs but geologic indices as well. Note that many broadband indices are also available for use, such as the Normalized Difference Vegetation Index.

For this exercise, you will create three narrowband VIs.

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Vegetation greenness: Modified Chlorophyll Absorption Ratio Index - Improved (MCARI2) and Modified Red Edge NDVI (MRENDVI) * •

Moisture content: Moisture Stress Index (MSI)

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Select *Modified Chlorophyll Absorption Ratio Index - Improved*. 2. 5

Hold down the Ctrl key on your keyboard. 3. 6

Select:

Modified Red Edge Normalized Difference Vegetation Index
Moisture Stress Index 4. 7

Release the Ctrl key. Three items are selected.

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In the Output Raster field, enter a file name of NarrowbandVIs.dat. 2. 9

Click OK. When processing is complete, ENVI creates a multi-band image where each band is a different VI. A color composite is added to the Layer Manager and displayed in the Image window. This image is unnecessary; you will remove it from the view next. 3. 10

In the Layer Manager, right-click on NarrowbandVIs.dat and select Remove.

Evaluate Greenness VIs

Reflectance plot of healthy, green vegetation.

Narrowband greenness VIs use reflectance measurements in the red and near-infrared regions to sample the red edge portion of the reflectance curve. The red edge describes the steeply sloped region of the vegetation reflectance curve between 690 nm and 740 nm that is caused by the transition from chlorophyll absorption and near-infrared leaf scattering.

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Select File > Data Manager from the Menu bar. The Data Manager appears. 2. 2

Under NarrowbandVIs.dat, select the Modified Red Edge Normalized Difference Vegetation Index band, then click Load Grayscale. The MRENDVI image is displayed. The equation for MRENDVI is at the beginning of this quick guide. This index is a variation of Red Edge NDVI that corrects for leaf specular reflection. Like other "red edge" indices, it is effective at monitoring small changes in canopy foliage content, gap fraction, and senescence.

3

In the Data Manager, select the Modified Chlorophyll Absorption Ratio Index - Improved band, then click Load Grayscale. The MCARI2 image is displayed.

The MCARI2 index(opens in a new tab) is computed with the following equation:

It indicates the relative abundance of chlorophyll while also effectively predicting leaf area index (LAI). It does this by incorporating a soil adjustment factor in the equation.

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Compare the MCARI2 image with the MRENDVI image by unchecking the Modified Chlorophyll... layer on and off in the Layer Manager. Do any fields in the MCARI2 image exhibit relatively higher LAI and chlorophyll when compared to the MRENDVI image? 2. 5

In the Layer Manager, uncheck the Modified Red Edge... image to hide it. Then compare the Modified Chlorophyll... image (i.e., the MCARI2 image) with the true-color AVIRIS image. Do you notice how some of the the bright fields in the MCARI2 image correspond to bright-green fields in the AVIRIS image? 3. 6

Before proceeding with the next step, uncheck the Modified Red Edge... and Modified Chlorophyll... images to hide them.

Evaluate the Moisture Stress Index

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In the Data Manager, select the Moisture Stress Index band under NarrowbandVIs.dat. Then click the Load Grayscale button. The Moisture Stress Index (MSI)(opens in a new tab) image is displayed.

The MSI is a ratio of reflectance between 1,599 nm and 819 nm:

Unlike other canopy water indices, higher values in an MSI image indicate greater water stress and less water content. To help visualize this, you can apply a Region of Interest (ROI) threshold to the MSI image.

2

In the Layer Manager, right-click on [2] NarrowbandVIs.dat (the layer associated with the Moisture Stress Index band) and select New Region of Interest. A "Regions of Interest" folder is added under the Moisture Stress Index layer, with a new (red) "ROI 1" entry. The Region of Interest (ROI) Tool also appears.

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In the ROI Tool, select the Threshold tab. 2. 4

Click the Add New Threshold Rule button.

The Data Selection dialog appears.

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Select the Moisture Stress Index band and click OK. the Choose Threshold Parameters dialog appears. The dialog shows a histogram of MSI values. You are interested in the highest values since they represent less water content. 2. 6

On the left side of the histogram, drag the red vertical slider bar all the way to the right. The entire histogram is shaded red. 3. 7

On the left side of the histogram, drag the red vertical slider to approximately 1.4 on the X-axis. The right side of the histogram is shaded red. 4. 8

Enable the Preview option. Pixel values greater than 1.4 are colored red in the MSI image. See the following video demonstration for these steps:

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The fields that are colored red represent those with less water content than others and may aid in understanding potential drought conditions and plant health. The minimum value of 1.4 is purely arbitrary. Feel free to experiment with different minimum values by dragging the left-most slider back and forth in the Choose Threshold Parameters dialog.

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Click OK to dismiss the Choose Threshold Parameters dialog. 2. 10

This concludes the exercise. When you are finished, exit ENVI.

To summarize, narrowband VIs are useful for characterizing vegetation properties such as greenness, moisture content, leaf pigment concentration, and others. Since they use specific reflectance wavelengths to evaluate these properties, multispectral and hyperspectral sensors are best suited for creating narrowband VIs.

Ideally, images should be preprocessed prior to creating narrowband VIs. See the following quick guides for more information:

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Preprocess AVIRIS-3 Radiance Data * •

Scale and Mask Reflectance Images

Additional Resources

•

Analyze Broadband Vegetation Indices quick guide * •

Create Vegetation Products quick guide

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