Spectral indices are calculated using a mathematical equation that is applied on two or more spectral reflectance bands of the image. The calculated spectral index is a ‘new’ image that highlights particular land surface features or properties e.g. vegetation, soil, water, better than the original input bands. The spectral indices vary from simple spectral ratioing of two bands to more complex combinations of multiple bands. Spectral indexes are developed based on the spectral properties of the object of interest. For example, spectral indices dedicated to the vegetation condition are developed based on the principle that the healthy vegetation reflects strongly in the near-infrared spectrum while absorbing strongly in the visible red. These properties are used to develop more complex spectral indexes for monitoring vegetation condition, phenology parameters, i.e. Normalised Difference Vegetation Index (NDVI), Advanced Vegetation Index (AVI). The spectral indices calculated using the short wave infrared spectral bands are more sensitive to vegetation water content and spongy mesophyll structure in the vegetation canopy thus are used to assess the vegetation decline, moisture that is particularly useful for drought monitoring (e.g. Normalized Difference Water Index (NDWI) or Normalized Difference Moisture Index – NDMI). The water-related spectral indices are widely applied in agricultural and ecological applications including surface water body characteristics, vegetation water stress, soil water content assessment and wetlands monitoring. The combination of near infrared and short wave infrared spectral bands is also used to detect burned area and to monitor the vegetation recovery (e.g. Normalised Burned Ratio – NBR). There are other spectral indices dedicated to snow cover and glacier monitoring, which are developed based on visual green and short wave infrared spectral bands. Snow reflects most of the radiation in the visible bands whiles absorbing in the short wave infrared.
Explain an application example where the spectral indices are used for vegetation, water or snow monitoring
Examine how the vegetation indices relates to the vegetation dynamics and health
Examine how the water-related spectral indices relates to changes in the vegetation and soil water content
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