i.worldview.toar is a GRASS-GIS add-on module converting WorldView2 DN values
(Digital Numbers, that is relatively readiometrically corrected detector data)
to Spectral Radiance or Reflectance.
Conversion to top-of-atmosphere spectral radiance is a simple two step process that involves multiplying radiometrically corrected image pixels by the appropriate absolute radiometric calibration factor (also referred to as a K factor) to get band-integrated radiance [W-m-2-sr-1] and then dividing the result by the appropriate effective bandwidth to get spectral radiance [W-m-2-sr-1-μm-1] (Updike, 2010).
For the moment, the module requires as an input the acquisition's date and time
formatted as a UTC string OR the Day-of-Year (Julian Day), the (mean) Sun
Elevation Angle. These are required to calculate the Earth-Sun distance
parameter for the modules' internal computations and can be retrieved from the
imagery's metadata files. The UTC string can be overriden by using the optional
parameter doy, given the day of year (Julian Day) has been correctly estimated
for the acquisition that is to be processed.
Optionally, the module may operate on the current computational region, instead of a bands whole extent.
ToAdd: More details about retrieving the acquisition's metada.
Source: Updike and Comp, 2010
L(λ|Pixel, Band) = K(Band) * q(Pixel, Band) / Δ(λ|Band)
where:
- L(λ): Top-of-Atmosphere Spectral Radiance image pixels [W/sq.m./sr/μm]
- KBand: absolute radiometric calibration factor [W/sq.m./sr/count] for a given band
- q(Pixel,Band): radiometrically corrected image pixels [countsor or Digital Numbers]
- Δ(λ|Band): effective bandwidth [μm] for a given band.
ρ(p) = π x L(λ) x d^2 / ESUN(λ) x cos(θ(S))
where:
- ρ: Unitless Planetary Reflectance
- π: Mathematical constant
- L(λ): Spectral Radiance from equation (1)
- d: Earth-Sun distance in astronomical units [calculated using AcquisitionTime class]
- Radiometric Use of WorldView-2 Imagery, Technical Note (2010), by Todd Updike & Chris Comp
- http://landsat.usgs.gov/how_is_radiance_calculated.php