|
The introduction to Unit 3 - Drought in the Mediterranean shows a satellite picture of the Normalised Difference Vegetation Index (NDVI) for the Mediterranean Region.
Two explanations of the NDVI are:
a) Normalised Difference Vegetation Index (NDVI) imagery is calculated from the red and near infra-red reflectance observed by the AVHRR (Advanced Very High Resolution Radiometer) sensor on NOAA meteorological satellites.
The NDVI image provides an indication of the growth and density of vegetation at the surface. Images of NDVI are sometimes referred to as "greenness maps" since they represent the vegetative vigour of plants.
The time series of NDVI data (from 1982 to the present) allows analysis of changes in vegetation growth and density in response to bio-physical conditions (including plant types, weather and soil).
Processed by NASA and the U.S. Geological Survey, the data are represented as pixels (cells), with each pixel representing an area of 8.0 x 8.0 km.
NDVI values range between -1 and +1, with dense vegetation having higher values (e.g., 0.4 - 0.7), and lightly vegetated regions having lower values (e.g., 0.1 - 0.2).
The primary use of these images is to compare the current state of vegetation with previous time periods, for example the same time in an average year or a reference year (a particularly good or bad year) to detect anomalous conditions.
b) To measure and map the density of green vegetation across the Earth's landscapes, scientists use satellite sensors that observe the distinct wave lengths of visible and near-infrared sunlight that is absorbed and reflected by the plants.
Calculating the ratio of the visible and near-infrared light reflected back up to the sensor yields a number from minus one (-1) to plus one (+1).
The result of this calculation is called the Normalised Difference Vegetation Index, or NDVI.
An NDVI value of zero means no green vegetation and close to +1 (0.8 - 0.9) indicates the highest possible density of green leaves.
(Source: http://earthobservatory.nasa.gov/Observatory/Datasets/ndvi.fasir.html)
Exercise 1:
The following satellite pictures show the NDVI (Normalised Difference Vegetation Index) for Spain in 1998. Look at the sequence of images and describe the changes in vegetation throughout the year!
|
