i.landsat.toar(1grass) | GRASS GIS User's Manual | i.landsat.toar(1grass) |
i.landsat.toar - Calculates top-of-atmosphere radiance or reflectance and temperature for Landsat MSS/TM/ETM+/OLI
imagery, radiometric conversion, radiance, reflectance, brightness temperature, atmospheric correction, satellite, Landsat
i.landsat.toar
i.landsat.toar --help
i.landsat.toar [-rnp] input=basename
output=basename [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]
[lsatmet=string[,string,...]]
[scale=float] [--overwrite] [--help]
[--verbose] [--quiet] [--ui]
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.
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:
Then, to calculate at-sensor reflectance the equations are:
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):
The output raster cell values can be rescaled with the scale parameter (e.g., with 100 in case of using reflectance output in i.gensigset).
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
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
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).
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:
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=203_30. output=_toar \
metfile=p203r030_7x20010620.met
or
i.landsat.toar input=L5121060_06020060714. \
output=L5121060_06020060714_toar \
metfile=L5121060_06020060714_MTL.txt
or
i.landsat.toar input=LC80160352013134LGN03_B output=toar \
metfile=LC80160352013134LGN03_MTL.txt sensor=oli8 date=2013-05-14
DN to reflectance using DOS1:
# rename channels or make a copy to match i.landsat.toar’s input scheme: g.copy raster=lsat7_2002_10,lsat7_2002.1 g.copy raster=lsat7_2002_20,lsat7_2002.2 g.copy raster=lsat7_2002_30,lsat7_2002.3 g.copy raster=lsat7_2002_40,lsat7_2002.4 g.copy raster=lsat7_2002_50,lsat7_2002.5 g.copy raster=lsat7_2002_61,lsat7_2002.61 g.copy raster=lsat7_2002_62,lsat7_2002.62 g.copy raster=lsat7_2002_70,lsat7_2002.7 g.copy raster=lsat7_2002_80,lsat7_2002.8
Calculation of reflectance values from DN using DOS1 (metadata
obtained from p016r035_7x20020524.met.gz):
i.landsat.toar input=lsat7_2002. output=lsat7_2002_toar. sensor=tm7 \
method=dos1 date=2002-05-24 sun_elevation=64.7730999 \
product_date=2004-02-12 gain=HHHLHLHHL
The resulting Landsat channels are named lsat7_2002_toar.1 .. lsat7_2002_toar.8.
i.atcorr, i.colors.enhance, r.mapcalc, r.in.gdal
Landsat Data Dictionary by USGS
E. Jorge Tizado (ej.tizado unileon es), Dept. Biodiversity and Environmental Management, University of León, Spain
Available at: i.landsat.toar source code (history)
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