I am trying to compare the Remote sensing reflectance (Rrs) and top of the atmosphere (TOA) Rrs of SeaHawk with other ocean-color missions like MODIS-Aqua, Terra, and VIIRS for different matching bands. However, I found many Rrs values of SeaHawk and other sensors are negative. My questions are the following:
Q1. AFAIK, the negative Rrs values in L2 products might be a reason for the atmospheric correction algorithm or over-correction. But I am still wondering why the TOA reflectance has some negative values too. Do you have any insights on the reasons why should Rrs and TOA have negative values?
Q2. Would I exclude the negative numbers of Rrs and TOA from my comparison? Should those negative values be flagged as bad or outliers for better comparison even though the image is not flagged as bad data? Does anyone have any experience with dealing with the negative Rrs in NASA Ocean color data?
I appreciate any help/comments or even an educated guess regarding this.
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How you handle negative Rrs in your analysis depends on how negative it is, which bands are negative and what your goal is. Since it is typically an atmospheric correction issue, you may try alternate correction methods to see if a more realistic result is possible. Negative Rrs is not a realistic result unless it is borderline negative, which could simply be noise on a signal that is close to zero.