This model implements the formulas published by the USGS for converting the quantized and calibrated scaled Digital Numbers (DN) representing multispectral image data acquired by Landsat 8 Operational Land Imager (OLI, bands 1 to 9) to Top of Atmosphere (ToA) Reflectance.
The conversion formulas require values stored in the metadata for each image (usually an ancillary file with a name such as <image>.met, or <image>_MTL.txt).
The published formula for TOA Reflectance is:
R l' = MlQcal + Ar
Rl' = TOA planetary reflectance without correction for solar angle.
Ml = Band-specific multiplicative rescaling factor from the metadata (REFLECTANCE_MULT_BAND_x, where x is the band number)
Qcal = Quantized and calibrated standard product pixel values (DN)
Ar = Band-specific additive rescaling factor from the metadata (REFLECTANCE_ADD_BAND_x, where x is the band number)
TOA reflectance can be corrected for the sun angle by:
Rl = Rl' / cos(solar zenith angle) OR Rl = rl' / sin(solar elevation angle)
Rl = TOA planetary reflectance
Rl' = product of TOA reflectance without correction
Solar Elevation angle = local sun elevation from the metadata (SUN_ELEVATION) NOTE: SUN_ELEVATION is unique to each Landsat 8 Scene
Solar Zenith angle = 90 - Solar Elevation angle
Landsat 8 MSI: Select the input Landsat 8 image to be converted to reflectance. This could be a multispectral image representing bands 1 to 7, a single band Panchromatic image (band 8), a single band Cirrus image (band 9) or a combination thereof (including individual band files). It should not be the Thermal bands (Bands 10 and 11) or the QA band.
Landsat 8 Reflectance: Name of the output image file containing the ToA Reflectance values. This will be a 32-bit Float data type.
REFLECTANCE_MULT_BAND: Enter a single Reflectance Multiplier correction value to be applied to every input band. The default is 0.00002, but the <image>_MTL.txt metadata file should be reviewed to determine the correct value.
REFLECTANCE_ADD_BAND: Enter a single Reflectance Additive correction value to be applied to every input band. The default is -0.1, but the <image>_MTL.txt metadata file should be reviewed to determine the correct value.
SUN_ELEVATION: Enter a single value for the Sun's elevation (in degrees) above the horizon at the time of image acquisition. This value is unique for every Landsat 8 image and can be determined by finding the SUN_ELEVATION keyword in the input image's <image>_MTL.txt metadata file.
Most Landsat 8 imagery provided by the USGS download site appears to have already been corrected to ToA Reflectance, but has subsequently had a consistent scale and offset applied to every band to fully utilize an unsigned 16-bit integer data range. Consequently the Reflectance_Mult_Band_n isusually 0.00002 and the Reflectance_Add_Band_n is usually -0.1 for every band in the image. So the current incarnation of the model both defaults to those values, and only has a single input to apply to every band.
If a review of the <image>_MTL.txt metadata file indicates that these values actually vary per band, you would need to either:
|How to modify the model for varying Multipliers and Additives:|