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FlexBRDF: A Flexible BRDF Correction for Grouped Processing of Airborne Imaging Spectroscopy Flightlines.

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Title: FlexBRDF: A Flexible BRDF Correction for Grouped Processing of Airborne Imaging Spectroscopy Flightlines.
Authors: Queally, Natalie1 queally@wisc.edu, Ye, Zhiwei1, Zheng, Ting1, Chlus, Adam1, Schneider, Fabian2, Pavlick, Ryan P.2, Townsend, Philip A.1
Source: Journal of Geophysical Research. Biogeosciences. Jan2022, Vol. 127 Issue 1, p1-16. 16p.
Subject Terms: *Vegetation & climate, Distribution (Probability theory), Spectral imaging, Space telescopes, Open source software
Abstract: Bidirectional reflectance distribution function (BRDF) effects are a persistent issue for the analysis of vegetation in airborne imaging spectroscopy data, especially when mosaicking results from adjacent flightlines. With the advent of large airborne imaging efforts from NASA and the U.S. National Ecological Observatory Network (NEON), there is increasing need for methods that are flexible and automatable across images with diverse land cover. Flexible bidirectional reflectance distribution function (FlexBRDF) is built upon the widely used kernel method, with additional features including stratified random sampling across flightline groups, dynamic land cover stratification by normalized difference vegetation index (NDVI), interpolation of correction coefficients across NDVI bins, and the use of a reference solar zenith angle. We demonstrate FlexBRDF using nine long (150–400 km) airborne visible/infrared imaging spectrometer (AVIRIS)‐Classic flightlines collected on 22 May 2013 over Southern California, where diverse land cover and a wide range of solar illumination yield significant BRDF effects. We further test the approach on additional AVIRIS‐Classic data from California, AVIRIS‐Next Generation data from the Arctic and India, and NEON imagery from Wisconsin. Comparison of overlapping areas of flightlines show that models built from multiple flightlines performed better than those built for single images (root mean square error improved up to 2.3% and mean absolute deviation 2.5%). Standardization to a common solar zenith angle among a flightline group improved performance, and interpolation across bins minimized between‐bin boundaries. While BRDF corrections for individual sites suffice for local studies, FlexBRDF is an open source option that is compatible with bulk processing of large airborne data sets covering diverse land cover needed for calibration/validation of forthcoming spaceborne imaging spectroscopy missions. Plain Language Summary: Airborne imaging spectroscopy data are used to map a suite of canopy functional traits, their functional diversity, and species composition. However, variation in solar and view geometry, and land cover type can cause unwanted brightness gradients across an image. In studies, where multiple images are mosaicked or where reflectances from multiple images will be compared, image brightness gradients may significantly confound analyses. Here, we present a flexible bidirectional reflectance distribution function, a flexible correction technique that concurrently removes brightness artifacts in groups of flightlines. We tested our method across a diverse set of ecosystems and provide a generalized set of model parameters, which can be easily customized to fit the user's needs. The method is suitable for application to large airborne campaigns in which site‐specific corrections are infeasible, and which provide baseline data for current and future satellite missions. Key Points: A flexible bidirectional reflectance distribution function (FlexBRDF) corrects BRDF effects in groups of adjacent flightlines to minimize between‐image spectral differencesInterpolation of normalized difference vegetation index bins prevents between‐bin edgesFlexBRDF is highly customizable and performed well for a variety of sites and sensors [ABSTRACT FROM AUTHOR]
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Abstract:Bidirectional reflectance distribution function (BRDF) effects are a persistent issue for the analysis of vegetation in airborne imaging spectroscopy data, especially when mosaicking results from adjacent flightlines. With the advent of large airborne imaging efforts from NASA and the U.S. National Ecological Observatory Network (NEON), there is increasing need for methods that are flexible and automatable across images with diverse land cover. Flexible bidirectional reflectance distribution function (FlexBRDF) is built upon the widely used kernel method, with additional features including stratified random sampling across flightline groups, dynamic land cover stratification by normalized difference vegetation index (NDVI), interpolation of correction coefficients across NDVI bins, and the use of a reference solar zenith angle. We demonstrate FlexBRDF using nine long (150–400 km) airborne visible/infrared imaging spectrometer (AVIRIS)‐Classic flightlines collected on 22 May 2013 over Southern California, where diverse land cover and a wide range of solar illumination yield significant BRDF effects. We further test the approach on additional AVIRIS‐Classic data from California, AVIRIS‐Next Generation data from the Arctic and India, and NEON imagery from Wisconsin. Comparison of overlapping areas of flightlines show that models built from multiple flightlines performed better than those built for single images (root mean square error improved up to 2.3% and mean absolute deviation 2.5%). Standardization to a common solar zenith angle among a flightline group improved performance, and interpolation across bins minimized between‐bin boundaries. While BRDF corrections for individual sites suffice for local studies, FlexBRDF is an open source option that is compatible with bulk processing of large airborne data sets covering diverse land cover needed for calibration/validation of forthcoming spaceborne imaging spectroscopy missions. Plain Language Summary: Airborne imaging spectroscopy data are used to map a suite of canopy functional traits, their functional diversity, and species composition. However, variation in solar and view geometry, and land cover type can cause unwanted brightness gradients across an image. In studies, where multiple images are mosaicked or where reflectances from multiple images will be compared, image brightness gradients may significantly confound analyses. Here, we present a flexible bidirectional reflectance distribution function, a flexible correction technique that concurrently removes brightness artifacts in groups of flightlines. We tested our method across a diverse set of ecosystems and provide a generalized set of model parameters, which can be easily customized to fit the user's needs. The method is suitable for application to large airborne campaigns in which site‐specific corrections are infeasible, and which provide baseline data for current and future satellite missions. Key Points: A flexible bidirectional reflectance distribution function (FlexBRDF) corrects BRDF effects in groups of adjacent flightlines to minimize between‐image spectral differencesInterpolation of normalized difference vegetation index bins prevents between‐bin edgesFlexBRDF is highly customizable and performed well for a variety of sites and sensors [ABSTRACT FROM AUTHOR]
ISSN:21698953
DOI:10.1029/2021JG006622