Vegetation Indices and Remote Sensing: Infrared Influenced
The following is an extract and summation of a research article published by Dr. Mohd. Ibrahim Seeni Mohd, Azhar Jj. Salleh titled, “Detecting forest areas and crops using vegetation indices”. The analysis performed in the study utilized landsat-5 Thematic Mapper satellite data which has a 30m raster pixel resolution.
Vegetation Indices (VI) represent a linear transformation of spectral bands that express the spectral behavior of crops and natural vegetation. VI emphasizes spectral contrast between different surface, and therefore have been widely used for enhancing vegetation and classifying agricultural areas.
The Perpendicular Vegetation Index ( PVI), Normalized Difference Vegetation Index (NDVI), and Ratio Vegetation Index (RVI) have been evaluated on the basis of their spectral band combinations. The combination of Mid-Infrared (MIR) band 5 and Visible (VIS) band 2 in the PVI provide the most difference among crops whereas the NDVI and RVI resulted in similar values. Significant differences were found between the forest areas and the other crops thus enhancing the forest areas. Better classification accuracies were achieved by using the PVI compared with other Vegetation Indices.
Several empirical indices have been used as quantitative indicators to reduce the multidimensional spectral space of vegetated scene to one dimension in order to sense variability in such properties as biomass, leaf areas index, fractional cover and types ( jasinski, 1990). During the past decade the wavelengths used have been restricted to VIS and Near-Infrared (NIR) spectral region. By combining differences and ratios of red ( VIS) and NIR, the vegetation indices respond to
- the relatively high radiation absorption of red light by leaves due to the presence of chlorophyll and
- the high reflectance of NIR light due to scattering in the leaf internal structure ( Curran, 1980).
Common ratio vegetation indices include the NIR/VIS index, and Normalized Difference Vegetation Index [NDVI= (NIR-VIS)/(NIR + VIS)] ( Richardson et al. ( `1991)). NDVI is commonly preferred because undesirable aspects on recorded radiance such as effect of variable illumination resulting from variation in topography can be reduced. Another VI that distinguishes the spectral response contributed by the soil background is the Perpendicular Vegetation Index which is described by Richardson et al. ( 1991) in the form.