From UPSC perspective, the following things are important :
Prelims level : NDVI
Mains level : Utility of NDVI
- A field study by researchers from Bengaluru shows that a popularly used index that remotely estimates density of vegetation does not yield a reliable estimate of food abundance for elephants in tropical forests.
- In fact, researchers show that this index has a negative correlation with graminoids (grassy food – grasses, sedges and rushes – preferentially consumed by elephants) in tropical forests.
Normalized Difference Vegetation Index (NDVI)
- For both academic and practical purposes, there is the practice of remotely monitoring vegetation in an area and representing it in terms of maps and parameters.
- One such parameter used is the normalized difference vegetation index (NDVI) which is measured remotely from satellite data.
- The NDVI is a simple indicator which tells how much of the ground is covered with vegetation.
- It basically calculates the difference between the red and near infrared (NIR) components of light reflected by objects, from, say, a satellite.
- Since healthy vegetation strongly absorbs red and reflects near infrared light, this difference can indicate the presence of healthy vegetation and map it into a colour code.
- NDVI always ranges from -1 to +1. But there isn’t a distinct boundary for each type of land cover.
- For example, when you have negative values, it’s highly likely that it’s water. On the other hand, if you have a NDVI value close to +1, there’s a high possibility that it’s dense green leaves.
- But when NDVI is close to zero, there aren’t green leaves and it could even be an urbanized area.
Why NDVI isn’t a good measure of vegetation cover?
- NDVI was negatively correlated to grasses. This means grass abundance tends to be low in locations where NDVI is high and vice-versa.
- While canopy cover and shrub abundance contribute positively to NDVI, they negatively affect grass abundance.
- Because of the poor correlation, NDVI cannot be reliably used as a measure of forage abundance in a multi-storeyed forest with a low proportional abundance of food species.
- Grasses form a large component of food of elephants and also ungulates (hoofed animals) like deer, sambar and gaur.
Misleading Elephants data
- This has been used to estimate the amount of food abundance available to herbivorous animals, for example, elephants.
- The NDVI is used, for instance, in attempts to track the presence of elephants using the vegetation they consume.
- However, this work clearly establishes that this can be misleading, and field-based studies are the ones which can yield definitive results.
- Researchers in India have found that the abundance of food plants is not correlated with NDVI.