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Crop Agriculture

Landsat 8 & Sentinel-2 Low Solar Elevation Imagery

SEE BELOW DOWNLOADS FOR NAMING CONVENTION, IMAGERY EXTENT, AND BACKGROUND INFORMATION

Landsat Scene Center Solar Elevation is 26.2⁰ (Sentinel-2 scene center solar elevation is nearly identical). Chavez (1996) stated further research for the COST model is needed for solar elevations less than 35⁰. Imagery downloads include Landsat 8 & Sentinel-2 TOA & surface reflectance (for blue, red, green, NIR, SWIR1, and SWIR2 bands), as well as corresponding Landsat 8 USGS Algorithm Surface Reflectance (which is not a purely DOS method; surfaces will have different values if SR is subtracted from TOA). Sentinel-2 imagery is for the same extent four days earlier. Very similar atmospheric conditions exist for both image dates - Landsat 8 and Sentinel-2 green band scatter were virtually identical for the two dates (Landsat 8 and Sentinel-2 green bands have virtually identical band center wavelengths and should have virtually identical scatter for the same atmospheric conditions). Surface reflectance methods are noted in Naming Convention below (see drop-down menu on top and links below for methods). 

IMPORTANT: By comparing surface reflectance (SR) values in the downloads below, it can be seen that values are less accurate at this low sun angle than at higher solar elevations - Sentinel-2 seems more accurate than Landsat 8 (click here to access Sentinel-2 download with a solar elevation about 19⁰). SR values are particularly erroneous for the blue band (but also to a lesser, but meaningful, extent for the green and red band ); whereby, green vegetation (fields and forests) incorrectly has higher blue surface reflectance than green or red (incorrect spectral signature). To access higher solar elevation downloads with correct visible band relationship, access the following: 61.4⁰ (Tutorial downloads); 59.5⁰ (Rare L8-S2 Comparison downloads). Accuracy at low elevation increases as wavelengths increase, so blue is the most erroneous - though, NIR and longer wavelengths are accurate at these and lower solar elevations (there is no reason you cannot correct NIR and SWIR with the steps shown on this website and use the imagery for times throughout the year). For Landsat 8, use imagery with solar elevations greater than 50⁰ if you plan on applying visible band surface reflectance converted with the methods described on this website. Check back for more information about Sentinel-2.