What is an Atmospheric Correction?
As name suggest, atmospheric correction is the process of removing the effects of the atmosphere on the reflectance values of remotely sensed images. The atmospheric effects may be referred to as presence of gas absorption, molecule and aerosol scattering that can influence incident and reflected radiation the atmosphere can have high impact on the reflectance values of images (especially those are taken from space). Atmospheric correction is likely to give better results while using multiple images from different dates, while determining biophysical parameters using imagery, or while creating derivative bands (ratios) such as vegetation indices.
“While atmospheric correction may not be important for certain applications (e.g., when conducting land cover classification for a single year), it is absolutely necessary when performing a time-series analysis in crop growth. For example, in comparing spectral characteristics of a pixel or group of pixels (an object) acquired on different dates, removal of the influence of atmospheric conditions prior to comparison is crucial.”stars-projects.org
Phases of Atmospheric Correction
Before diving into it, let’s recap on how sensors work. A sensor records radiance. Radiance is the combination of the electromagnetic energy being reflected by the objects minus the energy absorbed by the atmosphere plus the energy scattered into the path of the sensor by the atmosphere. Reflectance is the proportion of the amount of radiation hitting an object often referred to as ratio value and gives better representation of physical properties of the objects because it has been corrected to compensate the atmospheric impacts.
Atmospheric correction is done in 2 phases/steps. In the first phase, the Digital Numbers (DNs) are converted to radiance, and then to top of atmosphere radiance. In a next phase, the top-of-atmosphere reflectance is converted to surface reflectance (also known as bottom-of-atmosphere reflectance, or top-of-canopy reflectance or in vegetation studies). The resulting image is called atmospherically corrected. Some image provider agencies (like the United States Geological Survey) now deliver atmospherically corrected images free of charge on their EarthExplorer website.
Types of Atmospheric Correction
Atmospheric correction is of 2 types: 1. Relative correction, and 2. Absolute correction.
Relative Atmospheric Correction
Relative correction normalizes the images and is easier to achieve because normalization makes the images directly comparable to other images rather than removing the atmospheric effects. One of the most common relative correction is Dark Object Subtraction (DOS) where the dark objects (object with very low reflectance) is subtracted from image which not only normalize the image but also removes the atmospheric effect. Another process could be multiplying the images to normalize each other so that then can be directly compared without the impact of the atmosphere. In this process, we use regression model to transform the spectral characteristics of the other images to the base image (images which has the least atmospheric effect or one that has been previously atmospherically corrected).
Absolute Atmospheric Correction
In absolute atmospheric correction, the atmospheric profile obtained on the same day at the dame location are used in conjunction with an algorithm to compensate for the atmosphere. Unavailability of this information makes it difficult to perform absolute atmospheric correction. This information is usually obtained from one of the two very robust and time-tested radiative transfer models: MODTRAN (MODerate resolution atmospheric TRNAsmission) or 6s (Second Simulation of the Satellite Signal in the Solar Spectrum). These models store information of the location, time, average ground elevation, altitude of the sensor, and band wavelength ranges. These characteristics are used for absolute atmospheric correction. The most common algorithms used to perform this correction are Atmospheric CORrection Now (ACORN), Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes (FLAASH), and Atmospheric CORrection (ATCOR).
Importance of Atmospheric Correction
It is with no doubt that the importance of atmospheric correction is gain the better results—in the sense that atmospheric correction removes the atmospheric noises and provides the true surface reflectance. The presence of aerosols makes it difficult to read the true reflectance values which hindrances the accurate study of, for example, water depth. Atmospheric correction gives better results while using multiple images from different dates, while determining biophysical parameters using imagery, or while creating derivative bands (ratios) such as vegetation indices.