Atmospheric Correction

From EOpedia

Atmospheric correction is a critical step in the processing of Earth observation data. This process involves removing the effects of the Earth's atmosphere from remote sensing data to obtain accurate measurements of the surface features being observed. The correction usually turns top of atmosphere (TOA) radiances into bottom of atmosphere (BOA) reflectance.

Background

The Earth's atmosphere is a complex and dynamic system that can interfere with the measurements made by remote sensing instruments. Light from the sun and other sources passes through the atmosphere before reaching the Earth's surface, and is scattered, absorbed, and reflected by various atmospheric components such as molecules, aerosols, and clouds. This can result in distortions, noise, and errors in remote sensing data, which can in turn affect the accuracy of the information extracted from the data.

Types of Reflectance

TOA (Top of Atmosphere) Reflectance

  • TOA reflectance values represent the “raw” reflectance of the Earth as measured from space. It’s a mix of light reflected off the Earth’s surface and off the atmosphere.
  • These values are obtained directly from satellite sensors and are not corrected for atmospheric effects.
  • TOA reflectance is useful for initial analysis but may not accurately represent the true surface properties.

BOA (Bottom of Atmosphere) Reflectance

  • BOA represents the actual reflectance of areas on the Earth’s surface.
  • It corrects for atmospheric effects, removing the influence of scattering and absorption by the atmosphere.
  • BOA values are more accurate for applications such as land cover classification, vegetation monitoring, and change detection.

Remote Sensing Reflectance (Rrs)

  • Rrs is the ratio of upwelling “water-leaving” radiance to downwelling irradiance, just above the sea surface.
  • Rrs(λ; sr⁻¹) represents the spectral distribution of reflected visible solar radiation that emerges from below the ocean surface and passes through the sea-air interface. It is corrected for bidirectional effects of the air-sea interface and sub-surface light field.
  • Rrs is normalized by the downwelling solar irradiance, Ed(λ), just above the sea surface.
  • Rrs is the fundamental remote sensing quantity from which most ocean color products are derived (e.g., chlorophyll, particulate organic and inorganic carbon, inherent optical properties)

Surface Reflectance

  • Land surface reflectance refers to the amount of sunlight reflected by the Earth’s surface. It plays a crucial role in various applications, including ecology, agriculture, and environmental studies.
  • Surface reflectance is the foundation for many quantitative remote sensing products. These products include parameters like leaf area index, vegetation cover, and surface biomass.

Atmospheric Correction Techniques

To correct for these atmospheric effects, various techniques have been developed over the years, ranging from simple empirical models to sophisticated physical models based on radiative transfer theory. Some common techniques for atmospheric correction include:

Dark object subtraction: This technique involves using the darkest pixels in an image, such as shadowed areas or water bodies, as a reference for determining the atmospheric effects on the rest of the image. The dark object is assumed to have no reflectance in the visible or near-infrared regions, and the atmospheric effects are calculated by comparing the radiance of the dark object with the radiance of other pixels in the image.

Look-up table (LUT) method: This method involves creating a pre-computed table of atmospheric radiative transfer calculations for different atmospheric conditions and surface types. The LUT is then used to correct the remote sensing data by interpolating between the values in the table that correspond to the atmospheric conditions and surface types observed in the image.

Physical models: These models use radiative transfer theory to simulate the interaction of light with the atmosphere and the surface. They consider factors such as the angle of the sun, the altitude of the sensor, and the composition and distribution of atmospheric components. Physical models can be quite accurate but also computationally intensive and require detailed knowledge of the atmospheric conditions.

Applications of Atmospheric Correction

Atmospheric correction is an essential step in many Earth observation applications, including land cover mapping, vegetation monitoring, and atmospheric studies. Accurate atmospheric correction can improve the quality and reliability of remote sensing data, leading to better decision-making and policy development in areas such as agriculture, forestry, and disaster management.

Useful Resources

General

Physical Models

Tools

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