DESCRIPTION
i.landsat.toar is used to transform the calibrated digital
number of Landsat imagery products to top-of-atmosphere radiance or
top-of-atmosphere reflectance and temperature (band 6 of the sensors
TM and ETM+). Optionally, it can be used to calculate the at-surface
radiance or reflectance with atmospheric correction (DOS method).
Usually, to do so the production date, the acquisition date, and the
solar elevation is needed. Moreover, for Landsat-7 ETM+ it is also
needed the gain (high or low) of the nine respective bands.
Optionally (recommended), the data can be read from metadata file (.met or MTL.txt) for all
Landsat MSS, TM and ETM+. However, if the solar elevation or the
product creation date are given the values of the metadata file are
overwriten (only with .met files). This is necessary when the data in the .met file is
incorrect or not accurate.
Attention: Any null value or smaller than QCALmin in the input
raster is set to null in the output raster and it is not included in
the equations.
Uncorrected at-sensor values (method=uncorrected, default)
The standard geometric and radiometric corrections result in a
calibrated digital number (QCAL = DN) images. To further standardize
the impact of illumination geometry, the QCAL images are first
converted first to at-sensor radiance and then to at-sensor
reflectance. The thermal band is first converted from QCAL to
at-sensor radiance, and then to effective at-sensor temperature in
Kelvin degrees.
Radiometric calibration converts QCAL to at-sensor radiance, a
radiometric quantity measured in W/(m² * sr * µm) using the
equations:
- gain = (Lmax - Lmin) / (QCALmax - QCALmin)
- bias = Lmin - gain * QCALmin
- radiance = gain * QCAL + bias
where, Lmax and Lmin are the calibration constants,
and QCALmax and QCALmin are the highest and the
lowest points of the range of rescaled radiance in QCAL.
Then, to calculate at-sensor reflectance the equations are:
- sun_radiance = [Esun * sin(e)] / (PI * d^2)
- reflectance = radiance / sun_radiance
where, d is the earth-sun distance in astronomical
units, e is the solar elevation angle, and Esun is
the mean solar exoatmospheric irradiance in W/(m² * µm).
Corrected at-sensor values (method=corrected)
At-sensor reflectance values range from zero to one, whereas at-sensor
radiance must be greater or equal to zero. However, since Lmin can be
a negative number then the at-sensor values can also be negative. To
avoid these possible negative values and set the minimum possible
values at-sensor to zero, this method corrects the radiance to output
a corrected at-sensor values using the equations (not for thermal
bands):
- radiance = (uncorrected_radiance - Lmin)
- reflectance = radiance / sun_radiance
Note: Other possibility to avoid negative values is set to zero
this values (radiance and/or reflectance), but this option is ease
with uncorrected method
and r.mapcalc.
Simplified at-surface values (method=dos[1-4])
Atmospheric correction and reflectance calibration remove the path
radiance, i.e. the stray light from the atmosphere, and the spectral
effect of solar illumination. To output these simple at-surface
radiance and at-surface reflectance, the equations are (not
for thermal bands):
- sun_radiance = TAUv * [Esun * sin(e) * TAUz + Esky] / (PI * d^2)
- radiance_path = radiance_dark - percent * sun_radiance
- radiance = (at-sensor_radiance - radiance_path)
- reflectance = radiance / sun_radiance
where, percent is a value between 0.0 and 1.0 (usually
0.01), Esky is the diffuse sky irradiance, TAUz is
the atmospheric transmittance along the path from the sun to the
ground surface, and TAUv is the atmospheric transmittance
along the path from the ground surface to the
sensor. radiance_dark is the at-sensor radiance calculated
from the darkest object, i.e. DN with a least 'dark_parameter'
(usually 1000) pixels for the entire image.
The values are,
- DOS1: TAUv = 1.0, TAUz = 1.0 and Esky = 0.0
- DOS2: TAUv = 1.0, Esky = 0.0, and TAUz = sin(e) for all bands
with maximum wave length less than 1. (i.e. bands 4-6 MSS, 1-4 TM,
and 1-4 ETM+) other bands TAUz = 1.0
- DOS3: TAUv = exp[-t/cos(sat_zenith)],
TAUz = exp[-t/sin(e)], Esky = rayleigh
- DOS4: TAUv = exp[-t/cos(sat_zenith)],
TAUz = exp[-t/sin(e)], Esky = PI * radiance_dark
Attention: Output radiance remain untouched (i.e. no set to
0. when it is negative) then they are possible negative
values. However, output reflectance is set to 0. when is obtained a
negative value.
NOTES
In verbose mode (flag --verbose), the program write basic
satellite data and the parameters used in the transformations.
Production date is not an exact value but it is necessary to apply
correct calibration constants, which were changed in the dates:
- Landsat-1 MSS: never
- Landsat-2 MSS: July 16, 1975
- Landsat-3 MSS: June 1, 1978
- Landsat-4 MSS: August 26, 1982 and April 1, 1983
- Landsat-4 TM: August 1, 1983 and January 15, 1984
- Landsat-5 MSS: April 6, 1984 and November 9, 1984
- Landsat-5 TM: May 4, 2003 and April, 2 2007
- Landsat-7 ETM+: July 1, 2000
EXAMPLES
Transform digital numbers of Landsat-7 ETM+ in band rasters 203_30.1,
203_30.2 [...] to uncorrected at-sensor reflectance in output files
203_30.1_toar, 203_30.2_toar [...] and at-sensor temperature in output
files 293_39.61_toar and 293_39.62_toar:
i.landsat.toar input_prefix=203_30. output_prefix=_toar \
metfile=p203r030_7x20010620.met
or
i.landsat.toar input_prefix=L5121060_06020060714. \
output_prefix=L5121060_06020060714_toar \
metfile=L5121060_06020060714_MTL.txt
or
i.landsat.toar input_prefix=203_30. output_prefix=_toar \
sensor=tm7 product_date=2004-06-07 date=2001-06-20 \
sun_elevation=64.3242970 gain="HHHLHLHHL"
REFERENCES
- Chander G., B.L. Markham and D.L. Helder, 2009: Remote Sensing of
Environment, vol. 113
- Chander G.H. and B. Markham, 2003.: IEEE Transactions On Geoscience And
Remote Sensing, vol. 41, no. 11.
- Chavez P.S., jr. 1996. Image-based atmospheric corrections -
Revisited and Improved. Photogrammetric Engineering and Remote
Sensing 62(9): 1025-1036.
- Huang et al: At-Satellite Reflectance, 2002: A First Order Normalization
Of Landsat 7 ETM+ Images.
- R. Irish: Landsat
7. Science Data Users Handbook. February 17, 2007; 15 May 2011.
- Markham B.L. and J.L. Barker, 1986: Landsat MSS and TM Post-Calibration
Dynamic Ranges, Exoatmospheric Reflectances and At-Satellite
Temperatures. EOSAT Landsat Technical Notes, No. 1.
- Moran M.S., R.D. Jackson, P.N. Slater and P.M. Teillet, 1992: Remote
Sensing of Environment, vol. 41.
- Song et al, 2001: Classification and Change Detection Using Landsat TM
Data, When and How to Correct Atmospheric Effects? Remote Sensing
of Environment, vol. 75.
SEE ALSO
i.atcorr,
r.mapcalc,
r.in.gdal
AUTHOR
E. Jorge Tizado (ej.tizado unileon es), Dept. Biodiversity and Environmental Management,
University of León, Spain
Last changed: $Date$