A vegetation index can be defined as a parameter calculated from reflectance values at different wavelengths, and is particularly sensitive to vegetation cover, according to Gilabert et al (1997).
The use of these indices is based on the particular radiometric behavior of the vegetation. A vegetation cover in good health has a spectral signature characterized by the contrast between the red band (between 0.6 and 0.7 μm.), Which is largely absorbed by the leaves, and the Infrared (between 0.7 and 1.1 μm.), Which is mostly reflected. This quality of the vegetation allows the realization of its qualitative evaluation.
The Normalized Differential Vegetation Index is calculated by the following expression:
Where: IRC is the reflectivity in the near infrared and R is the reflectivity in the red.
The range of values of the spectral reflections is between 0 and 1; Since both near infrared and red reflectivity are ratios of the reflected radiation on incoming radiation in each spectral band. As a consequence of these values ranges, the NDVI varies its value between -1 and 1. The NDVI has a great value in ecological terms, since it is a good estimator of the fraction of photosynthetically active radiation intercepted by vegetation (fPAR) According to Monteith (1981).
In the Landsat 8 images, the band 4 (0.630 – 0.680 μm) corresponds to the red (R) and the band 5 (0.845 – 0.885 μm) to the infrared (IRC), therefore for the calculation NDVI requires to have both bands.
Calculating NDVI in ArcGIS
In order to calculate the NDVI it is simply necessary to apply its respective equation, it is possible to mention that this process can be done in different applications that have a raster image calculator (ArcGIS, Erdas, QGIS, gvSIG, Surfer, Idrisi, ENVI, etc.).
The first step after opening the application ArcMap 10 is to load the bands 4 and 5 with the Add Data tool from the directory where they are stored, then a window is displayed asking for the creation of pyramids, in this case leave the default values and Accept, it should be noted that an active license of the Spatial Analyst extension is required to run the Raster Calculator.
Once loaded the bands, from the ArcToolbox, open the following tool:
ArcToolbox > Spatial Analyst Tools > Map Algebra > Raster Calculator
To obtain the NDVI values in Landsat 8 images using ArcGIS, the following equation is applied:
NDVI = Float (band 5 – band 4) / Float (band 5 + band 4)
The result is a raster image that contains values ranging from -1 to 1 (the values closest to 1 being the most vigorous vegetation). Generally, the NDVI image is displayed on a grayscale, for a more pleasant and easy-to-interpret look, go to the properties of the raster and select a color palette from the symbology tab (right click > Properties > Symbology).