Atmospheric correction for a Landsat and Sentinel-2 product over water surfaces

In the last decade, satellite derived standard land products have increasingly been produced for medium resolution satellites such as Landsat and (more recently) Sentinel-2. These mostly involve estimating surface reflectance and surface temperature. The products generally remove or standardise atmospheric effects with some also normalizing for surface bidirectional reflectance distribution function (BRDF) and terrain illumination effects to provide consistent time series and mosaics. The products have been used in various land surface applications, e.g., land cover, fractional cover and water identification, including flooding, crop monitoring and other time series analysis. However, the products are generally not immediately sufficient for applications over persistent water areas, such as estimating water quality, benthic cover, sediment transport, erosion and shallow water bathymetry. These need additional corrections with different physics that are not included in standard land products. In this paper, a method is proposed that treats persistent water areas separately within the standard product and includes corrections not generally applied to the land. The processing has been designed to be fully consistent between water and land in atmospheric correction and definition of reflectance factors so that they can be combined in the same time series and form mosaics. The first step in this process was acquisition of an effective and up to date classification to separate the persistent water and land. The water areas are then atmospherically corrected in the same way as the land but not treated for BRDF or shading effects as are the land areas. For the water areas, adjacency effects are more significant near water-land interfaces and water surface effects have different physics from land surfaces. The extra corrections currently include correction for adjacency effects as well as regional sun glint and sky radiation effects. The water mask and these corrections have been added to the current existing atmospheric, BRDF and terrain corrected surface reflectance product (standard product) from Geoscience Australia (GA). However, at the scale of the Landsat and higher resolution satellite images, residual local surface and bidirectional effects still occur and are discussed in this paper. In this paper, results from the new processing strategy have been compared with GA standard products in test images of Canberra and the North Queensland coast near Ingham and used as a basis to discuss the likely residuals of surface and atmospheric effects and options for the inclusion of methods to overcome them in a standard product. The results show that:
•   Both inland and sea water signatures behave as expected from other data and models. 
•   Adjacency correction seems most useful where a water-Land interface is close to the water body.
•   Sky glint removal is sometimes too great in Canberra site when water is shielded by local terrain.
•   Sun and sky glint correction greatly improves the coast and deep sea water signatures.
This Abstract was presented at the 22nd International Congress on Modelling and Simulation (MODSIM2017) Hobart Tasmania (https://www.mssanz.org.au/modsim2017/)

Tags: EARTH SCIENCES, Published_External, adjacency correction, reflectance factor, sky and sun glint correction
Organization: Geoscience Australia Data