SR: GDA True Surface Reflectance Product
GDA’s true Surface Reflectance (SR) product is a per-pixel, per-band estimate of the surface spectral reflectance as it would have been measured at ground level as if there were no atmospheric or topographic contributions and deviations from satellite nadir view or solar zenith position.
GDA calibration methodology relies on physical models that employ (i) sensor specific orbital and imaging information, (ii) sensor and band specific atmospheric radiative transfer calculations, (iii) sensor, location, and time dependent estimations of surface radiative budget, and (iv) modeling of surface topographic and land cover anisotropy. Reliance on physics based models ensures the predictability, transparency, and accuracy of the results, allows for the simulation of data acquisition, and enhances the comparison and analysis of data from various sensors and/or acquisitions.
GDA SR calibration approach is similar to NASA’s Landsat Ecosystem Disturbance Adaptive Processing System (LEDAPS) and USGS’ Landsat SR calibration approach. However, GDA SR models are also applicable to a range of other sensors, account for per pixel variation in solar and view angles within the scene, use MODIS based coincident atmospheric properties and BDRF coefficients, and cross-calibrate imagery to the MODIS ‘gold standard’ SR products from NASA. Cross-calibration with MODIS further improves calibration of Landsat-class imagery and ensures cross-compatibility of Landsat-class data as well as of Landsat-class data and MODIS.
The product is a 16-bit raster image, linearly scaled between 0 (representing 0% reflectance) to 10,000 (representing 100% reflectance). The SR calibration improves radiometric properties of the imagery and allows for superior land cover and crop mapping, change analysis, as well as image mosaicking and comparison.