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Crop Agriculture

Atmospheric Scatter Background

Visible light striking off particles in the atmosphere prior to striking the surface is the main source of scatter (illustrated in the simplified atmospheric scatter diagram below) and is the type directly accounted for in image-based atmospheric correction, as it erroneously increases sensor values. Blue scatters the most, followed by green, red, then NIR (scatter amounts for bandwidths larger than NIR are negligible at most, and do not need to be deducted). A clearer atmosphere has less overall scatter and a higher ratio of scatter between smaller and longer wavelengths than a hazier atmosphere; a hazier atmosphere, conversely, has more overall scatter and more equal scatter between bands (Chavez, 1988). There is a negative power relationship between band center wavelength and scatter (Chavez, 1988). Scatter can also result in light being redirected to the sensor from a different path after it has reflected off the surface or being redirected back down to the surface (downwelling irradiance; neither are shown in diagram).

The theory behind removing scatter, is that in a satellite image scene containing millions of pixels, there should be reflectance values that represent zero (or very close to zero); the reason there is not is because of scatter (which shifts the low end of the histogram to higher values rather starting at zero). A reflectance amount should be deducted to account for it. The image-based method to account for the increase in values due to scatter, is called dark object subtraction (DOS) and was developed by Chavez (1988). DOS does not consider secondary scattering into shadowed areas (Chavez, 1996).

Atmospheric scatter diagram for satellite imagery atmospheric correction and surface reflectance

 


 

References

Chavez, P.S., Jr. 1996. Image-based atmospheric corrections–revisited and improved. Photogrammetric Engineering and Remote Sensing 62(9): pp.1025-1036.

Chavez, P.S., Jr. 1988. An improved dark-object subtraction technique for atmospheric scattering correction of multispectral data. Remote Sensing of Environment 24: pp.459-479.