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Last Post 12 Oct 2017 12:26 PM by  MariM
Using FLAASH with RapidEye Level 3A image
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André Baldauf



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12 Oct 2017 09:30 AM
    Hello,

    I am trying to perform an atmospheric correction on RapidEye Level 3A data. From the product specification, I could derive the delivered product DN represent radiance in 1/100th of a W/m² sr ym ("This scaling converts the (relative) pixel DNs coming directly from the sensor into values directly related to absolute at sensor radiances. The scaling factor is applied so that the resultant single DN values correspond to 1/100th of a W/m2 sr µm.").
    With that information, I choose 0.01 as the scaling factor when defining the input radiance image. As the "Output Reflectance Scaling Factor" in the "Advanced Settings", I select 1000. The resulting image contains pixels with DNs > 10000 in all 5 bands. In my interpretation, this leads to a reflectance > 100%. How is that possible?

    The band statistics for the 5 bands are as follows (Band MIN MAX MEAN STDEV):
    Band 1 210 5633 559 146
    Band 2 245 8351 796 245
    Band 3 191 11397 1121 429
    Band 4 231 11983 1274 342
    Band 5 95 11864 1572 336

    Aside there are only a few pixels with values > 10000, are my input parameters correct? I appreciate any given help.
    Thank You!!

    MariM



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    12 Oct 2017 12:26 PM
    It is not uncommon to have some pixel values outside of the range of 0-10000 (when using an output scale factor of 10000). This can happen in some areas of very low or high radiance due to water, shadow to clouds and corner reflectors. These areas are poorly modeled and I often suggest they be masked in further processes. However, if there are significant numbers of pixels or areas of pixels that are outside of the expected range, then it is often a sign that a parameter is off or most commonly, the scale factor is not correct for the input data.

    The input scale factor is divided into the data to get to the expected unit of microW/(cm^2*sr*nm). I suggest that the data be processed using the Radiometric Calibration tool using 'FLAASH Settings' to ensure the input data are in the correct units. If you use FLAASH Settings in this tool, the input scale factor will be 1 for FLAASH. The typical radiance cube in the correct units will range from about 2-50.

    I would also suggest you leave the output scale factor as 10000 to ensure that you have an appropriate dynamic range of output integers for the reflectance cube. Then, if you want the output cube in 'percent' reflectance, you can use Band Math to divide the data by 10000 to get to values of approximately 0-1 (floating point output). Then I would mask pixels that are outside of this range when processing further.

    It can also be useful to run Quac on the radiance data for comparison. Quac is developed by the same scientific group that developed FLAASH but uses a different method to determine surface reflectance that is typically within +/-15% of the accuracy of the FLAASH model. It is far easier to use and can be a better option if you are unsure of the parameters to use for FLAASH. Using the wrong parameters in FLAASH can produce very bad results. Quac also outputs a reflectance cube scaled by 10000 to integers. When using this tool, be sure to include a mask that excludes background border pixels for the calculation.
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