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ATCOR:

The ATCOR 4 Method

The atmospheric correction of imagery acquired by airborne sensors is an essential part of preprocessing. The objective is to retrieve physical parameters of the earth's surface, e.g. surface reflectance, emissivity and temperature. This information can be used for monitoring, change detection, surface-vegetation atmosphere transfer (SVAT) modeling, and surface energy balance investigations for climatic modeling and upscaling.

The following image of a hyperspectral DAIS scene illustrates the change in information content when a combined atmospheric/topographic correction is applied.

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DAIS sub-image of Timna, Israel, RGB=2.2, 0.86, 0.5 um.
Left: original data after ortho-rectification, right: surface reflectance after atmospheric/topograpic correction


Figures 1 to 3 show a schematic sketch of the radiation components that are taken into account.

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Figure 1 : Schematic sketch of radiation components for a flat terrain (solar region).


Component 1 : path radiance; radiation scattered by the atmosphere
Component 2 : reflected radiation from the viewed pixel (reflected global flux Edir+Edif)
Component 3 : radiation reflected by the neighborhood and scattered into the view direction (adjacency effect)
Note: only component 2 contains information from the viewed pixel.

DN is the digital number recorded in a certain spectral channel, c0, c1 are the offset and gain of the radiometric calibration relating the DN to the at-sensor radiance L=c0 + c1*DN. The atmospheric correction process removes the signal components 1 and 3 from the total at-sensor radiance L, so radiance component 2 remains. This information is converted into a surface reflectance value.

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Figure 2: Radiation components: solar region 0.4 - 2.5 um, rugged terrain

Component 1 : path radiance: radiation scattered by the atmosphere (photons without ground contact).
Component 2 : reflected radiation from the viewed pixel.
Component 3 : adjacency radiation:;ground reflected from the neighborhood and scattered into the view direction.
Component 4 : terrain radiation reflected to the pixel (from opposite hills, according to the terrain view factor).
Note: Only component 2 contains information from the viewed pixel.

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Figure 3: Radiation components: thermal region 8 - 14 um, flat terrain

Component 1 : path radiance; radiation emitted by the atmosphere.
Component 2 : emitted surface radiation from the viewed pixel (emissivity e, temperature T).
Component 3 : reflected downwelling thermal atmospheric flux F, the reflected component is L3=r*F/pi, where r=1-e is the reflectance of the (opaque) surface.

Only component 2 contains the essential information from the viewed pixel.
For a rugged terrain the same three radiation components are considered, however the elevation-dependence is taken into account. The adjacency effect can usually be neglected in the thermal region, because the scattering efficiency of the atmosphere decreases strongly with wavelength.


Sequence of processing steps

The following steps are involved in the atmospheric processing:

An automatic classification of the reflectance cube is possible with the SPECL module that calculates 10-12 surface cover classes based on matching with template spectra.

The following radiative transfer effects are taken into account :

Accuracy of the Method

The accuracy of the method depends on several factors :

In the thermal region, the surface temperature retrieval additionally depends on the appropriate surface emissivity map. If the deviation of the true surface emissivity from the assumed emissivity is less than 0.02, then the temperatures will be accurate to about 1-1.5 K. Larger deviations will occur if the emissivity estimate is not close to reality. As a rule of thumb, a surface temperature error of about 0.5-0.7 K per 0.01 emissivity error is achieved as long as the surface temperature is much higher than the boundary layer air temperature.

Mountainous Terrain

In the case of mountaineous terrain, the results strongly depend on the quality of the available DEM, and the quality of the ortho-rectification (depending on roll/pitch/heading calibration and GPS flight path information). Large errors can occur in parts of the image (see Publications, Richter 1998 paper in Applied Optics for detailed examples). However, the overall impression of the processed image usually indicates a successful removal of the illumination trends caused by the topography.

Written by Rolf Richter, DLR

 
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last modified: DS, 18.10.14