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NAME
i.landsat.toar - Calculates top-of-atmosphere radiance or reflectance and temperature for Landsat MSS/TM/ETM+/OLI
KEYWORDS
imagery, Landsat, radiance, reflectance, brightness temperature, atmospheric correction
SYNOPSIS
i.landsat.toar
i.landsat.toar help
i.landsat.toar [-rnp] input_prefix=string output_prefix=string [metfile=name] [sensor=string] [method=string] [date=yyyy-mm-dd] [sun_elevation=float] [product_date=yyyy-mm-dd] [gain=string] [percent=float] [pixel=integer] [rayleigh=float] [--verbose] [--quiet]
Flags:
- -r
- Output at-sensor radiance instead of reflectance for all bands
- -n
- Input raster maps use as extension the number of the band instead the code
- -p
- Print output metadata info
- --verbose
- Verbose module output
- --quiet
- Quiet module output
Parameters:
- input_prefix=string
- Base name of input raster bands
- Example: 'B.' for B.1, B.2, ...
- output_prefix=string
- Prefix for output raster maps
- Example: 'B.toar.' generates B.toar.1, B.toar.2, ...
- metfile=name
- Name of Landsat metadata file (.met or MTL.txt)
- sensor=string
- Spacecraft sensor
- Required only if 'metfile' not given (recommended for sanity)
- Options: mss1,mss2,mss3,mss4,mss5,tm4,tm5,tm7,oli8
- mss1: Landsat_1 MSS
- mss2: Landsat_2 MSS
- mss3: Landsat_3 MSS
- mss4: Landsat_4 MSS
- mss5: Landsat_5 MSS
- tm4: Landsat_4 TM
- tm5: Landsat_5 TM
- tm7: Landsat_7 ETM+
- oli8: Landsat_8 OLI/TIRS
- method=string
- Atmospheric correction method
- Atmospheric correction method
- Options: uncorrected,dos1,dos2,dos2b,dos3,dos4
- Default: uncorrected
- date=yyyy-mm-dd
- Image acquisition date (yyyy-mm-dd)
- Required only if 'metfile' not given
- sun_elevation=float
- Sun elevation in degrees
- Required only if 'metfile' not given
- product_date=yyyy-mm-dd
- Image creation date (yyyy-mm-dd)
- Required only if 'metfile' not given
- gain=string
- Gain (H/L) of all Landsat ETM+ bands (1-5,61,62,7,8)
- Required only if 'metfile' not given
- percent=float
- Percent of solar radiance in path radiance
- Required only if 'method' is any DOS
- Default: 0.01
- pixel=integer
- Minimum pixels to consider digital number as dark object
- Required only if 'method' is any DOS
- Default: 1000
- rayleigh=float
- Rayleigh atmosphere (diffuse sky irradiance)
- Required only if 'method' is DOS3
- Default: 0.0
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 are 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, ETM+ and OLI/TIRS. However,
if the solar elevation is given the value of the metadata file is
overwritten. This is necessary when the data in the .met file is
incorrect or not accurate. Also, if acquisition or production dates are
not found in the metadata file then the command line values are used.
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).
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 0.0
when it is negative) then they are possible negative values. However,
output reflectance is set to 0.0 when is obtained a negative value.
NOTES
On Landsat-8 metadata file
NASA reports a structure of the L1G Metadata file
(LDCM-DFCB-004.pdf)
for Landsat Data Continuity Mission (i.e. Landsat-8).
NASA retains in MIN_MAX_RADIANCE group the necessary information
to transform Digital Numbers (DN) in radiance values. Then,
i.landsat.toar replaces the possible standard values with the
metadata values. The results match with the values reported by the
metada file in RADIOMETRIC_RESCALING group.
Also, NASA reports the same values of reflectance for all bands
in max-min values and in gain-bias values. This is strange that all
bands have the same range of reflectance. Also, they wrote in the
web page as to calculate reflectance directly from DN, first with
RADIOMETRIC_RESCALING values and second divided by sin(sun_elevation).
This is a simple rescaling
- reflectance = radiance / sun_radiance = (DN * RADIANCE_MULT + RADIANCE_ADD) / sun_radiance
- now reflectance = DN * REFLECTANCE_MULT + REFLECTANCE_ADD
- then REFLECTANCE_MULT = RADIANCE_MULT / sun_radiance
- and REFLECTANCE_ADD = RADIANCE_ADD / sun_radiance
The problem arises when we need ESUN values (not provided) to
compute sun_radiance and DOS. We assume that REFLECTANCE_MAXIMUM
corresponds to the RADIANCE_MAXIMUM, then
- REFLECTANCE_MAXIMUM / sin(e) = RADIANCE_MAXIMUM / sun_radiance
- Esun = (PI * d^2) * RADIANCE_MAXIMUM / REFLECTANCE_MAXIMUM
where d is the earth-sun distance provided by metadata file
or computed inside the program.
The i.landsat.toar reverts back the NASA rescaling to continue
using Lmax, Lmin, and Esun values to compute the constant to convert
DN to radiance and radiance to reflectance with the "traditional"
equations and simple atmospheric corrections.
Attention: When MAXIMUM values are not provided,
i.landsat.toar tries to calculate Lmax, Lmin, and Esun from
RADIOMETRIC_RESCALING (in tests the results were the same).
Calibration constants
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
- Landsat-8 OLI/TIRS: launched in 2013
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"
or
i.landsat.toar input_prefix=LC80160352013134LGN03_B output_prefix=toar \
metfile=LC80160352013134LGN03_MTL.txt sensor=oli8 date=2013-05-14
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
Landsat Data Dictionary by USGS
AUTHOR
E. Jorge Tizado (ej.tizado unileon es), Dept. Biodiversity and Environmental Management,
University of León, Spain
Last changed: $Date: 2014-11-25 08:54:02 -0800 (Tue, 25 Nov 2014) $
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