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Graphs of spectral signatures of water, soil, and vegetation
When a satellite distinguishes between different surfaces it senses radiation or reflection within specific wavelengths, also called channels, which are typical of the spectral signatures of these surfaces. The illustration on the left shows, for example, that if you want to distinguish between bare ground and vegetation you should scan in the areas of 0.6 - 0.7 micrometres and 0.7 - 0.9 micrometres.
Vegetation will give a strong reflection in the 0.7 - 0.9 micrometres area, whereas it will give a weak reflection in the 0.6 - 0.7 area. Because the spectral signature of vegetation is so
characteristic, the distinction between bare ground and green vegetation normally offers no problems. The difference between the reflection in the visible and the near infrared ranges can, as already mentioned, be used to determine the photosynthesis and the growth of the plants.
The Normalised Difference Vegetation Index (NDVI) is usually calculated as follows:
||near infrared - red
|near infrared + red
On the basis of this simple formula the global distribution of vegetation is currently mapped.
Global vegetation map for July 1992 based on a mosaic of NOAA data
The image on the left shows a global vegetation map for July 1992 based on a mosaic of NOAA data. Compare maps of climate and vegetation in the atlas.
Due to recurrent drought problems in the Sahel area south of the Sahara, special attempts have been made to map its vegetation in greater detail. A series of vegetation maps covering the whole season will give an impression of the total biomass production in the growth period. Satellite data can be transformed into kilogramme (kg) biomass per hectare (ha) with great accuracy by measuring selected control areas and adjusting the remote sensing results. In this way, large geographical areas can be mapped at short intervals and drought problems can be detected at an early stage. See, for example, the HAPEX SAHEL Information System.
See the QuickTime movie of monthly variations (342Kb)
Vegetation map of Europe and Africa in July and January 1992
Remote sensing in depthIntroductionRadar technologyLandsat MSS channelsSpectral signaturesArea classificationAtmospheric interference
LinksHAPEX SAHEL Information System
AnimationQuickTime movie of monthly variations