Vegetation Indices (VIs) are combinations of DNs or surface reflectances at two or more wavelengths designed to highlight a particular property of vegetation. Analyzing vegetation using remotely sensed data requires knowledge of the structure and function of vegetation and its reflectance properties. This knowledge enables the linking of vegetative structures and their condition to their reflectance behavior in an ecological system of interest.
The solar-reflected optical spectrum spans a wavelength range of 400 nm to 3000 nm. Of this range, the 400 nm to 2500 nm region is routinely measured using a variety of optical sensors, including Landsat. Vegetation interacts with solar radiation differently from other natural materials, such as soils and water bodies. The absorption and reflection of solar radiation varies depending on the characteristics of the vegetation and the incident wavelength. Water, pigments, nitrogen, and carbon are each expressed in the reflected optical spectrum from 400 nm to 2500 nm, with often overlapping, but spectrally distinct, reflectance behaviors. The known signatures allow scientists to combine reflectance measurements at different wavelengths to enhance specific vegetation characteristics.
PANCROMA currently computes six VIs: Simple Vegetation Index (VI), Normalized Difference Vegetation Index (NDVI), the Modified Soil Adjusted Vegetation Index (MSAVI2), the Enhanced Green (ENHG) index, the Moisture Stress Index (MSI), the Normalized Burn Ratio (NBR) and the Normalized Difference Water Index (NDWI). These indices are defined as follows:
VI = ρNIR / ρRED
NDVI = ρNIR - ρRED / ρNIR + ρRED
MSAVI2 = (1/2)*(2*(ρNIR +1)-sqrt((2*ρNIR +1)^2-8*ρNIR -ρRED)))
MSI = ρMIR/ρNIR
NDWI = ρNIR - ρMIR / ρNIR + ρMIR
NBR = ρNIR - ρband7 / ρNIR + ρband7
This Vegetation Indices reference provides a good introduction to VIs, as does this Yale University reference. The VI, NDVI, MSAVI2 and ENHG all use the Landsat near infrared (NIR), green and red bands. The same procedure is used to compute all three. Open three band files: NIR band, band 2 (green) and band 3 (red) in this order. Select 'Preprocessing' | 'Vegetation Index' and the desired VI from the menu. If you check the 'Activate Image Processing Routines' you can adjust the VI Scale Factor. This will decrease or enhance the effect. After you compute the index, a grayscale image will be displayed. This is the Vegetation Index image. It will be used to replace the green band for subsequent display as an RGB color composite. Save it in your desired format, naming it 'VI.tif' or the like.
You can also save the raw index data as well. PANCROMA will output the data in flat binary form using IEEE 32-bit FLOAT (little endian) format. The output file will be ((ROWS by COLUMNS * 4) ) + 8) bytes in size. The extra 8 bytes result from the number of rows and number of columns that precede the FLOAT data. These are output as 4 byte integers, again in little endian format. The file structure is as shown in the previous section.
In order to save the binary data, check 'Activate Image Processing Routines' on the Main Window and then check 'Save VI Data to File' on the Image Processing Data Input box.
Now create an RGB color composite as usual by opening band1 (blue), the vegetation index band, and band3 (red), in this order. The resulting RGB image should have greener vegetation than the corresponding RGB color composite image using bands 1, 2, and 3. (PANCROMA displays the vegetation enhancement by accentuating the green spectral band.) If the vegetation is too green or not green enough, run another iteration except this time check the 'Activate Image Processing Routines'. Adjust the VI scale factor using the slider bar at the lower right of the image processing data form. The default value is 0.05. An example NDVI image is shown below.
These images are often rendered with the green and red channels interchanged, to highlight the vegetation in red, as shown below.
The MSI and the NDWI use the mid infrared (MIR band 5) instead of the visible red band, in addition to near infrared (NIR band 4) and green (band 2) bands. To compute these indices, open the NIR, green and MIR bands in this order. Select 'Preprocessing' | 'Vegetation Index' and either 'NDWI' or 'MSI' from the menu. If you check the 'Activate Image Processing Routines' you can adjust the VI Scale Factor as before. This will decrease or enhance the effect. After you compute the index, a grayscale image will be displayed. This is the Vegetation Index image. It will be used to replace the green band for subsequent display as an RGB color composite. Save it in your desired format, naming it 'VI.tif' or the like.