Spectral indices are mathematical calculations used to analyze information obtained from remote sensing sensors such as Sentinel and Landsat satellites. These calculations are based on the relationship between the reflectance of the Earth’s surface at different wavelengths of the electromagnetic spectrum.
Each object on Earth reflects electromagnetic radiation differently depending on its composition and structure. Spectral indices enable the measurement of these differences to identify specific characteristics of the Earth’s surface, such as vegetation, soil moisture, the presence of bodies of water, air quality, among others.
Spectral indices are calculated using a combination of reflectance values in different spectral bands, which are selected according to the physical properties of the Earth’s surface being measured. These calculations can be performed automatically using computer programs, which allows for the processing of large amounts of information quickly and efficiently.
In summary, spectral indices are an important tool for the analysis of remote sensing data, as they enable the measurement and quantification of specific characteristics of the Earth’s surface in an objective and quantitative manner.
Some common spectral indices for Sentinel and Landsat:
- Normalized Difference Vegetation Index (NDVI): This index is commonly used to measure vegetation. It is calculated by subtracting the near-infrared (NIR) reflectance value from the red (RED) reflectance value and dividing the result by the sum of both values. Both Sentinel and Landsat provide information in the necessary bands to calculate the NDVI.
- Normalized Difference Water Index (NDWI): This index is used to identify bodies of water. It is calculated by subtracting the NIR reflectance value from the green (GREEN) reflectance value and dividing the result by the sum of both values. Both Sentinel and Landsat provide information in the necessary bands to calculate the NDWI.
- Enhanced Vegetation Index (EVI): This index is similar to the NDVI, but it is used to correct for some limitations of the NDVI. The EVI is calculated using reflectance values in the red, blue, and NIR bands. Both Sentinel and Landsat provide information in these bands.
- Water Difference Index (WDI): This index is used to measure soil moisture. It is calculated by subtracting the short-wave infrared 1 (SWIR1) reflectance value from the NIR reflectance value and dividing the result by the sum of both values. Both Sentinel and Landsat provide information in the necessary bands to calculate the WDI.
These are just a few of the spectral indices that can be used with Sentinel and Landsat. There are many others that can be used to measure different characteristics of the Earth’s surface, such as surface temperature, soil salinity, air quality, and more.
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