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Crop Agriculture

Landsat 8 DOS vs. COST Soybean Field NIR Reflectance / Visible and NIR Band Reflectance

* Important: Landsat 8 Surface Reflectance is now available from the USGS for Free; Landsat 7, 5, and 4 Surface Reflectance is also available from the USGS (LEDAPS) for Free and is also recommended.

Below, Landsat 8 NIR imagery is shown on left (lighter shades are higher reflectance, scaled from ± 2 standard deviations from the mean). NIR reflectance is calculated with ESUN values shown in the Landsat 8 ESUN, Radiance, and TOA Reflectance page. The Landsat 8 image date is 8/23/13; the picture date (on the right) is 8/26/13. The soybean field with reflectance values calculated has an arrow on it which approximates the view of the picture of the field. For scale, the NIR image is 2.5 miles wide (north is ↑). Though the COST method has been shown to be more accurate for previous Landsat data (Chavez, 1996), based on the NIR reflectance values below, the DOS method should be used for Landsat 8 atmospheric correction. (See below graphic for more details.)

For the soybean field below with the arrow, DOS (Chavez, 1988) NIR reflectance median and mode for recently reprocessed Landsat imagery (the most recent imagery released for area by the USGS) are both .60, realistic for this stage; while COST (Chavez, 1996) NIR reflectance median is .73 and mode is .74, too high (n = 497 pixels; for scale, the image is 2.5 miles wide; north is ↑). See graph below for soybean reflectance information.  

NIR surface reflectance comparison between DOS and COST atmospheric corrections methods for a soybean field

Spectral reflectance for above field for visible and NIR bands for reprocessed Landsat 8 imagery (most recently released imagery for area by USGS). Reflectance is calculated with the Landsat 8 DOS Method as detailed in the Landsat 8 Imagery & Reflectance Tutorial page (uses the Relative Scatter Lookup Table); values show well-documented relationships for a soybean field at this stage (median values are shown on graph; mean values are nearly identical). Imagery is for 8/23/13; reflectance for visible bands may have been slightly lower and reflectance for NIR might have been slightly higher earlier in August. On x-axis are Landsat 8 center bandwidths for, from left to right, blue, red, green and NIR; on y-axis are Preferred Method reflectance units.

Soybean field surface reflectance spectral reflectance graph for a soybean field

 

There is no NIR haze (path radiance) deduction necessary for this particular image (sometimes an NIR haze deduction is necessary and sometimes it is not as is explained in the Atmospheric Correction Guide and Landsat 8 Atmospheric Correction pages); as a result, DOS and TOA reflectance are the same whether calculated with the standard TOA equation (using the ESUN values in the Landsat 8 ESUN, Radiance, and TOA Reflectance page) or with the Landsat 8 USGS (2013) equation shown below.

The COST atmospheric correction method has been shown to be more accurate than DOS to derive reflectance for Landsat 5 (Chavez, 1996) and Landsat 7 has nearly identical bandwidths to Landsat 5; however, the COST method (which adds a factor in the denominator that increases reflectance) has not been validated with Landsat 8 data. Landsat 8 has different bandwidths, particularly the NIR band; the NIR band has been refined to decrease atmospheric absorption. The COST method results in NIR reflectance that is too high for soybeans while the DOS method results in realistic reflectance (see below). The COST and DOS methods are described in the Atmospheric Correction Guide and Landsat 8 Atmospheric Correction pages.

 

Pixels used for field for reflectance (n=497; north is ↑)

Stayed a minimum of 15 meters (half-pixel width or height) away from non-field areas (e.g. boundaries, channels, non-crop vegetation; tree stand in northwest seemed slightly more than 15 meters based on zooming in to background imagery) to help account for Landsat horizontal accuracy error.

Pixels used for field for surface reflectance comparison

 (OGRIP, 2013)

 

Plot from Gitelson (2004) that shows soybean NIR reflectance

Plot from Gitelson (2004) that shows soybean NIR reflectance per green leaf area index

Landsat 8 Conversion to TOA Reflectance Equation (USGS, 2013)

OLI band data can also be converted to TOA planetary reflectance using reflectance rescaling coefficients provided in the product metadata file (MTL file). The following equation is used to convert DN values to TOA reflectance for OLI data as follows:

ρλ' = MρQcal + Aρ 

where:              

ρλ'          = TOA planetary reflectance, without correction for solar angle.  Note that ρλ' does not contain a correction for the sun angle. 
Mρ         = Band-specific multiplicative rescaling factor from the metadata (REFLECTANCE_MULT_BAND_x, where x is the band number)
Aρ          = Band-specific additive rescaling factor from the metadata (REFLECTANCE_ADD_BAND_x, where x is the band number)
Qcal        = Quantized and calibrated standard product pixel values (DN)

 

TOA reflectance with a correction for the sun angle is then:

TOA reflectance with a correction for the sun angle

where:              

ρλ          = TOA planetary reflectance
θSE         = Local sun elevation angle. The scene center sun elevation angle in degrees is provided in the metadata (SUN_ELEVATION). 
θSZ         = Local solar zenith angle;  θSZ = 90° - θSE

For more accurate reflectance calculations, per pixel solar angles could be used instead of the scene center solar angle, but per pixel solar zenith angles are not currently provided with the Landsat 8 products.

 

 

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.

Gitelson, A.A. 2004. Wide dynamic Range vegetation index for remote quantification of biophysical characteristics of vegetation. Journal of Plant Physiology 161: pp. 165 –173.

OGRIP. 2013. Ohio Geographically Referenced Information Program. Website last updated in 2013. Imagery not from 2013.  Downloaded imagery from viewer at: http://gis4.oit.ohio.gov/osiptiledownloads/osip2.aspx

USGS. 2013b. Using the USGS Landsat 8 Product. Cited at: http://landsat7.usgs.gov/Landsat8_Using_Product.php