One of the most simple ways to characterize vegetation from imagery is to utilize NDVI. In short, NDVI takes the difference from the spectral band with the highest EMR reflectance (nIR) and the spectral band with the lowest reflectance (red) and normalizes this value by dividing by the sum of the highest reflectance (nIR) band and the lowest reflectance band (red). To put this together, the equation looks like:
(nIR - red)/(nIR + red)
How do we put this all together using QGIS?
- First, find the appropriate imagery that has a nIR band and a red
band. Two common products are NAIP imagery and Landsat imagery
(Available from Earth Explorer). For each, band 4 = nIR and
band 3 = red.
- Next use QGIS's raster calculator to calculate the NDVI
- When you display the NDVI, make sure to stretch to MinMax in the
layer properties otherwise your image will appear to be solid gray.
Values in the NDVI range from -1 to 1. In other words, healthy green vegetation will have higher pixel values (i.e. closer to 1) and non-vegetation will have lower values (i.e. closer to -1)