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Crop Agriculture

Landsat 8 & Sentinel-2 Low Solar Elevation Imagery

Related Page: Sentinel-2 Lower Solar Elevation (~19⁰) Imagery Downloads

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 Surface Refletance (SR) model is needed for solar elevations less than 35⁰. (IMAGERY DOWNLOADS & MORE INFORMATION ARE INCLUDED BELOW.)

12/18/2016: Central California, USA. Image extent (in kilometers) is 12.7 E-W x 14.6 N-S. Orange is download extent for Landsat 8 and Sentinel-2. Yellow segments are counties; extent is mostly in Madera County; the northwest portion is Merced County; the smallest portion in the north is Mariposa County. Background image is corresponding Sentinel-2 True Color imagery for 12/18/16. Surface is very similar between the Sentinel-2 12/18/16 date and Landsat 8 12/22/16 date, which can be seen when viewing the downloads.


Imagery downloads below include Landsat 8 & Sentinel-2 Top of Atmosphere (TOA) & SR converted with the Frequency 50 atmospheric correction method using relative scatter based on the red band (summed up on the Simplified Landsat 8 & Sentinel-2 SR Steps page) for blue, red, green, NIR, SWIR1, and SWIR2 bands, as well as corresponding Landsat 8 USGS Algorithm Surface Reflectance for the same bands (USGS SR is not a purely DOS method; as a result, surfaces will have different values if SR is subtracted from TOA). There is also a Sentinel-2 true color image than can be download for a larger area. 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.

IMPORTANT : By comparing SR values in the downloads below, it can be seen that SR values are somewhat higher (and somewhat higher than they should be) at this low sun angle angle than at higher solar elevations (for this particular imagery, it seems that Sentinel-2 is modestly more accurate than Landsat 8 [though it is just on example]). To download higher solar elevation imagery with more accurate SR, access the following: 61.4⁰ (Tutorial downloads); 59.5⁰ (Rare L8-S2 Comparison downloads).

When comparing Landsat 8 F50 SR (converted with the Frequency 50-based method described here) to USGS Landsat 8 Algorithm SR (not a pure DOS method), F50 SR values are particularly high for the blue band (about .05 to .06 higher for vegetation pixels), but also to a lesser, but meaningful, extent for the green (about .02 to .03 higher for vegetation pixels) and red band (about .01 to .03) higher for vegetation pixels). For green vegetation, green SR can be slightly higher than blue but can also be lower than blue SR (particularly for healthy cropfields with low visible band SR) which is a clear incorrect relationship. A point on the histogram (for all visible bands) representing too many values would need to be selected as scatter in order to lower SR enough to match USGS Landsat 8 Algorithm SR (further providing evidence of problems visible bands have at low solar elevations).

When comparing Sentinel-2 8 F50 SR (converted with the Frequency 50-based method described here) to USGS Landsat 8 Algorithm SR (not a pure DOS method), F50 SR values are particularly high for the blue band (about .03 to .04 higher for vegetation pixels), but also to a lesser, but meaningful, extent for the green (about .01 to .02 higher for vegetation pixels) and red band (about .01 to .02 higher for vegetation pixels). For green vegetation, green SR can be fairly higher than blue but can also be even or very slightly lower than blue SR (particularly for healthy cropfields with low visible band SR) which is a clear incorrect relationship (though not a distinct as the Landsat image above). A point on the histogram (for all visible bands) representing too many values would need to be selected as scatter in order to lower SR enough to match USGS Landsat 8 Algorithm SR (further providing evidence of problems visible bands have at low solar elevations).

Accuracy at low elevation increases as wavelengths increase, so blue is the most erroneous - NIR AND SWIR WAVELENGTHS ARE ACCURATE AT THESE, AND EVEN LOWER, SOLAR ELEVATIONS.  

DOWNLOADS FOLLOW (SEE BELOW FOR NAMING CONVENTION):