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Title:   Author  item
                                            Gao, Fengitem
                                            HE, TAO - University Of Marylanditem
                                            MASEK, JEFFREY - National Aeronautics And Space Administration (NASA)item
                                            SHUAI, YAMIN - National Aeronautics And Space Administration (NASA)item
                                            SCHAAF, CRYSTAL - University Of Massachusettsitem
                                            WANG, ZHUOSEN - University Of Massachusetts
  Submitted to: IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 4/17/2014
                                    Publication  Date: 8/22/2014
                                    Publication URL: https://handle.nal.usda.gov/10113/59903Citation: Gao, F.N., He, T., Masek, J., Shuai, Y., Schaaf, C., Wang, Z. 2014. Angular effects and correction on medium resolution sensors for crop monitoring. IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing. 99:1-10.Interpretive Summary: Angular effects arising from variable viewing and solar geometry can cause significant variation in reflectance observed from remote sensing imagery. Many agricultural applications such as crop type mapping and crop conditions monitoring require consistent reflectance across the image frame and through time. This paper examined the influence of viewing and illumination angular effects on surface reflectance of typical surface and crop types for both narrow swath (e.g. Landsat) and wide swath (e.g.  AWiFS) sensors. In order to correct these angular effects, a Bidrectional Reflectance Distribution Function (BRDF) look-up map (LUM) for major cover types was built using the Cropland Data Layer (CDL) and the Moderate-resolution Imaging Spectroradiometer (MODIS) BRDF product. The BRDF LUM was applied to an AWiFS image to correct view angle effects in an agricultural area in central Illinois. The resulted nadir BRDF-adjusted reflectance (NBAR) provides a consistent data product to use in mapping crop type and monitoring crop conditions required by the National Agricultural Statistics Service and Foreign Agricultural Service.
                                Technical Abstract: Remote sensing imagery at medium spatial resolutions (20-60m) such as Landsat, the Advanced Wide Field Sensor (AWiFS) and the Disaster Monitoring Constellation (DMC) have been broadly used in mapping crop types and monitoring crop conditions. This paper examined the influence of viewing and illumination angular effects on surface reflectance of typical surface and crop types for both narrow swath (e.g. Landsat) and wide swath (e.g.  AWiFS) sensors. Three types of angular effects (view angle effect, day of year effect and mean local time drift effect) were analyzed based on both field and satellite bi-directional reflectance distribution function (BRDF) measurements. In order to correct these angular effects, a BRDF look-up map (LUM) for major cover types was built using the Cropland Data Layer (CDL) and the Moderate-resolution Imaging Spectroradiometer (MODIS) BRDF products. The BRDF LUM was applied to an AWiFS image to correct view angle effects in an agricultural area in central Illinois. The nadir BRDF-adjusted reflectance (NBAR) provide consistent data source for the intra-annual crop condition monitoring and the inter-annual long-term time-series analysis.


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