Brief Introduction to Remote Sensing (3/3): Landsat image conversion

This is the third part of some basic definitions of remote sensing that are already in the user manual of the Semi-Automatic Classification Plugin.

This post provides information about the Landsat conversion to reflectance implemented in SCP Landsat.

Landsat images downloaded from http://earthexplorer.usgs.gov or through the SCP tool Download Landsat are composed of several bands and a metadata file (MTL) which contains useful information about image data.

Radiance at the Sensor’s Aperture

Radiance is the “flux of energy (primarily irradiant or incident energy) per solid angle leaving a unit surface area in a given direction”, “Radiance is what is measured at the sensor and is somewhat dependent on reflectance” (NASA, 2011, p. 47).
The Spectral Radiance at the sensor’s aperture (Lλ) is measured in [watts/(meter squared * ster * μm)] and for Landsat images it is given by (https://landsat.usgs.gov/Landsat8_Using_Product.php):

Lλ=ML∗Qcal+AL

where:

• ML = Band-specific multiplicative rescaling factor from Landsat metadata (RADIANCE_MULT_BAND_x, where x is the band number)
• AL = Band-specific additive rescaling factor from Landsat metadata (RADIANCE_ADD_BAND_x, where x is the band number)
• Qcal = Quantized and calibrated standard product pixel values (DN)

Top Of Atmosphere (TOA) Reflectance

“For relatively clear Landsat scenes, a reduction in between-scene variability can be achieved through a normalization for solar irradiance by converting spectral radiance, as calculated above, to planetary reflectance or albedo. This combined surface and atmospheric reflectance of the Earth is computed with the following formula” (NASA, 2011, p. 119):

Read more »