The Apparent Reflectance raster function calibrates image brightness values (DN) for some satellite sensors. The main advantage of apparent reflectance function is to adjust the images to a theoretically common illumination condition, so there should be less variation between scenes from different dates and from different sensors. This can be useful for image classification, color balancing, and mosaicking.
The function performs two calibrations. The first calibration is to convert the DN value to the top of atmosphere (TOA) radiance based on the sensor properties (i.e. gain/bias or LMAX/LMIN). The second calibration is to convert the TOA radiance to apparent reflectance, based on sun elevation and acquisition date. The formulas used in the two conversions can be found in section 11.3.1 and 11.3.2 of the Landsat 7 Science Data User Handbook.
Apparent reflectance is a ratio and its native output range is 0-1. For display purposes, in this function, the ratio is multiplied by 255, and the output is therefore stretched from 0-1 to 0-255. When using default 8-bit unsigned integer output data type, the value is rounded down to integers.
Some scientific research (e.g. serving as input of atmospheric model to achieve surface reflectance) requires the original ratio. This can be achieved by 1) selecting apparent reflectance raster function output type to be 32 Bit float, and 2) adding an arithmetic raster function immediately after this function to divide the value by 255.
This function can only be used with specific imagery and may be automatically applied when adding data to a mosaic dataset using the appropriate raster type. The applicable sensors are Landsat, IKONOS, and QuickBird. If this function is applied to a dataset that is invalid it will slow the function chain but make no adjustments.
This function modifies the image values, so previous statistics and histograms are no longer valid. This function should be applied early in the function chain, after band extraction (reordering) and prior to any stretching or other radiometric function.
Written by: Hua Wei
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