Atmospheric Corrections for Areal Photographs: Effect of the Atmosphere on Radiation, Atmospheric Scattering Correction and Ignoring Atmosphere (Especially for GATE-Geospatial 2022)

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The output from the instrument on satellite depends on the intensity and spectral distribution of energy that is received at the satellite. The intensity and spectral distribution of energy/radiation have travelled some distance through the atmosphere and accordingly has suffered both attenuation and augmentation in the course of the journey. The problem comes when one is not able to regenerate the correct radiation properties of the target body on the earth surface with the data generated by the remote sensing.

Results of Image-Based Atmospheric Correction Using IKONOS I …

Effect of the Atmosphere on Radiation

Effect of the atmosphere in determining various paths for energy to illuminate a pixel and reach the sensor. The path radiation coming from the sun to the ground pixel and then being reflected the sensor. In this ongoing process, absorption by atmospheric molecules takes place that converts incoming energy into heat. In particular, molecules of oxygen, carbon-di-oxide, ozone and water attenuate the radiation very strongly in certain wavelengths. Scattering by these atmospheric particles is also the dominant mechanism that leads to radiometric distortion in image data.

Radiative Transfer theory is used to make quantitative calculations of the difference between the satellite received radiance and earth leaving radiance. Radiation travelling in a certain direction is specified by the angle between that direction and the vertical axis and a differential equation for a small horizontal element of the transmitting medium (the setting atmosphere) with thickness. The resulting differential equation is called the radiative transfer equation. The equation will, therefore, be different for different wavelengths of electromagnetic radiation because of the different relative importance of different physical process at a different wavelength.

Need for Atmospheric Correction

When an image is to be utilized, it is frequently necessary to make corrections in brightness and geometry for accuracy curing interpretation and also some of the application may require correction to evaluate the image accurately. The various reasons for which correction should be done:

  • Derive ratios in 2 bands of the multispectral image since the effect of atmospheric scattering depends on the wavelength, the two channels will be unequally affected and the computed ratio will not accurately reflect the true ratio leaving the earth՚s surface
  • When land surface reflectance or sea surface temperature is to be determined.
  • When two images taken at different times and needed to be compared or mosaic the images

Correction Methods

Rectifying the image data for the degrading effects of the atmosphere entails modelling the scattering and absorption processes that take place. There are a number of ways of correcting the image data for atmospheric correction

Ignore the Atmosphere

Collecting the ground truth measurements of target temperature, reflectance etc and calibrating these values or quantities on the ground and the radiance values by the sensor.

Modelling the absorption or scattering effects for the measurement of the composition and temperature profile of the atmosphere.

Utilizing the information about the atmosphere inherent to remotely sensed data i.e.. use the image to correct it.

Correcting for Atmospheric Scattering

This correction is done when the two bands of the image are subjected to ratio analysis. Atmospheric scattering scatters short wavelength and causes haze and reduces the contrast ratio of images. This follows two techniques for example bands 1 & 7, where band 1 has the highest component of 1 and the band 7 (infrared) has the least. Both techniques are DN value dependent as band 7 is free from scattering effect there it has DN value either 0 or (shadows) .

  • In-band 7 the shadows having DN value 0 & 1. Now for each pixel, the DN in band 7 is plotted against band 1 and a straight line is fitted through the plot using least square techniques. If there was no haze in band 1 then the line would pass through the origin. But as there is haze the intercept is offset along the band 1. Haze has an additive effect on scene brightness. Therefore, to correct the haze effect on band 1, the value of the intercept offset is subtracted from the DN of each band 1 pixel for the entire image.
  • The second technique also uses the areas with DN as 0 or 1 in band 7. The histogram of band 7 has pixels with 0 whereas the histogram of band 1 lacks the pixel in the range from 0 to 20 approximately because of light scattered into the detector by atmosphere thus this abrupt increase in pixels in band 1 is subtracted from all the DNS in band 1 to restore effects of atmospheric scattering.

The amount of atmospheric correction depends upon

  • The wavelength of the bands
  • Atmospheric conditions

Short wavelength causes more severe scattering. Humid, smoggy and dusty cause more scattering than clear and dry atmospheres.

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