Landsat 8 & Sentinel-2 Atmospheric Correction / Surface Reflectance Background
(Values below are based on the Relative Scatter Calculator [includes the most recent scatter values])
GIS Ag Maps surface reflectance is a dark object subtraction (DOS; Chavez 1988, 1996) method with the low (dark) value established by custom Landsat 8 & Sentinel-2 methods developed here (explained in tutorials). If you are not familiar with DOS, it a method where a single scatter (haze) reflectance amount is deducted (also known as the dark object) uniformly across an entire image from Top of Atmosphere (TOA) reflectance to retrieve surface reflectance – this corrects for atmospheric conditions at the time of imaging (you can learn more about DOS in the tutorials). USGS Landsat 8 Algorithm surface reflectance is not purely DOS. In other words, if you subtract USGS Landsat 8 Algorithm surface reflectance from TOA reflectance, there will be varying values across a Landsat scene; whereas, if you subtract DOS surface reflectance from TOA reflectance, there will be a single (scatter) value across the scene.
GIS Ag Maps DOS methods for Landsat 8 and Sentinel-2 (European-based satellite system) are based on similar DOS principles though the scatter value can be established by different methods depending on satellite and the software used - both include applying the Relative Scatter Calculator or Lookup Table. Conversion to surface reflectance can be accomplished whether using ArcGIS, free QGIS, or other capable GIS software. Sentinel-2 is offered in a TOA reflectance integer raster format upon download (divide by 10,000 to covert to decimal reflectance units), and is recommended here to be downloaded from Copernicus (Sentinel-2A & 2B; Copernicus can be accessed in Data Sources folder in Free section to the left).
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.