About Landsat & Sentinel-2 Surface Reflectance (SR) or Conversion to Surface Reflectance
IMPORTANT: YOU CAN NOW DOWNLOAD SENTINEL-2 IMAGERY FROM COPERNICUS THAT HAVE ALREADY BEEN CONVERTED TO SURFACE REFLECTANCE (THESE ARE LABLED "L2A"), THOUGH WE HAVE SEEN VALUES THAT SEEM VERY INACCURATE AND RECOMMEND USING THE TUTORIALS ON THIS WEBSITE ALONG WITH SENTINEL-2 1C IMAGERY (PARTICULARLY FOR BANDS SMALLER THAN NIR).
New: Simplified Landsat 8 & Sentinel-2 Conversion to Surface Reflectance Steps
IMPORTANT : Links in the above L8 & S2 Surface Reflectance Tutorials drop-down menu take you directly to surface reflectance (SR) tutorials (also referred to as courses and guides) in order to independently convert to SR by applying image-based atmospheric correction to remove atmospheric scatter (see scatter correction methods here). Some guides are designed to be used with ArcGIS (but can be applied to any GIS software that can produce a raster attribute table); others are designed specifically for FREE QGIS software. Landsat can be ordered and downloaded in USGS SR format as is explained more in next paragraph, but you need to wait until it has been processed as opposed to downloading imagery right away and starting the conversion process independently. Sentinel-2 can only be downloaded in top of atmosphere (TOA) format, but a free toolkit is available online that can be used to convert Sentinel-2 to SR (explained more below). SIMPLY READ THIS PAGE AND FOLLOW A GUIDE AND YOU WILL BE ABLE TO ACQUIRE AND USE SATELLITE IMAGERY FOR FREE.
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IF YOU PREFER, YOU CAN ORDER LANDSAT FROM THE USGS ALREADY CONVERTED TO SR; keep in mind that USGS Landsat 8 Algorithm SR is not purely a DOS method (in other words, subtracting USGS SR from USGS TOA will result in variable values). You need an account to order USGS SR imagery (free and easy process). To order SR imagery, open Data Sources folder on the left and access EarthExplorer (click Login and Create New Account, if necessary); in Data Sets, open Landsat then access Landsat Collection 1 Level-2 (On-Demand) (that is SR). USGS SR is in an integer raster format; divide by 10,000 to convert to surface reflectance units. The USGS will email you a link to download the data when it is ready.
Tutorials on this website guide you through the process of converting Landsat 8 (as well as Landsat 5 & 7) imagery to SR (can be used with Free QGIS). Conversion to SR here is based on dark object subtraction (DOS; Chavez [1988 and 1996]), which is a well-documented and established method, and applies continuous relative scatter (suggested by Chavez [1988]) developed here. Learn about DOS here.
Landsat 8 DOS surface reflectance based on the GIS Ag Maps Lowest Valid Value Attribute Table Method has been cited in Remote Sensing of Environment (includes results from NDVI calculation). (The current scatter table has slightly different values than the Original Table used in the article, but the Original Table remains accessible and can be accessed through the previous link; however, we do recommend using the current table as is explained in tutorials). Methods, in addition to the Lowest Valid Value Attribute Table Method, have been developed for Landsat 8 to establish scatter, and are explained in the tutorials.
Landsat 5 and 7 SR are based on the one-percent dark object subtraction (DOS; Chavez [1988 and 1996]) COST Method (Chavez, 1996), which is a well-documented and established method for Landsat 5 & 7 (access the Chavez [1996] article here for details and results; PDF on this website). Continuous relative scatter (suggested by Chavez [1988]) is applied to Landsat 5 and 7 here, where the power correlation between band center wavelength and scatter has the proper value of R² = 1.0 (Chavez, 1988). Power exponents also have the proper range of values based on Chavez (1988) and research completed here. This is explained in more detail in the tutorials. Relative scatter for Landsat 5, 7, 8, and Sentinel-2 are based on values that all fit together with a correlation between band center wavelength and scatter reflectance of R² = 1.0. It is recommended that relative scatter uses the red band for the starting scatter (as is explained in the tutorials).
Sentinel-2 GIS Ag Maps DOS Surface Reflectance Method varies depending on the specific software used (as is explained in the tutorials). Sentinel-2 is downloaded in top of atmosphere (TOA) integer units, which eliminates a step in the SR conversion process of first converting to TOA. Methods to establish scatter use a raster attribute table (for ArcGIS or other GIS software that can generate a raster attribute table) or QGIS histogram (QGIS cannot generate a raster attribute table), and are explained in tutorials. Sentinel-2 cannot be downloaded in SR format which is considered Level-2A, but there is a free online toolbox that can be installed that can be used to convert to SR (Level-2A, also known as Bottom of Atmosphere reflectance). The following page can be used to access the toolbox: ESA Sentinel-2 Level 2A Toolbox (offsite ESA page; opens in new tab).
References
Chavez, P.S., Jr. 1996. Image-based atmospheric corrections–revisited and improved. Photogrammetric Engineering and Remote Sensing 62(9): pp.1025-1036.
Chavez, P.S., Jr. 1988. An improved dark-object subtraction technique for atmospheric scattering correction of multispectral data. Remote Sensing of Environment 24: pp.459-479.