3
<em>i.landsat.toar</em> is used to transform the calibrated digital
4
number of Landsat imagery products to top-of-atmosphere radiance or
5
top-of-atmosphere reflectance and temperature (band 6 of the sensors
6
TM and ETM+). Optionally, it can be used to calculate the at-surface
7
radiance or reflectance with atmospheric correction (DOS method).
10
Usually, to do so the production date, the acquisition date, and the
11
solar elevation are needed. Moreover, for Landsat-7 ETM+ it is also
12
needed the gain (high or low) of the nine respective bands.
15
Optionally (recommended), the data can be read from metadata file
16
(.met or MTL.txt) for all Landsat MSS, TM, ETM+ and OLI/TIRS. However,
17
if the solar elevation is given the value of the metadata file is
18
overwritten. This is necessary when the data in the .met file is
19
incorrect or not accurate. Also, if acquisition or production dates are
20
not found in the metadata file then the command line values are used.
23
<b>Attention</b>: Any null value or smaller than QCALmin in the input
24
raster is set to null in the output raster and it is not included in
27
<h2>Uncorrected at-sensor values (method=uncorrected, default)</h2>
29
The standard geometric and radiometric corrections result in a
30
calibrated digital number (QCAL = DN) images. To further standardize
31
the impact of illumination geometry, the QCAL images are first
32
converted first to at-sensor radiance and then to at-sensor
33
reflectance. The thermal band is first converted from QCAL to
34
at-sensor radiance, and then to effective at-sensor temperature in
38
Radiometric calibration converts QCAL to <b>at-sensor radiance</b>, a
39
radiometric quantity measured in W/(m² * sr * µm) using the
42
<li> gain = (Lmax - Lmin) / (QCALmax - QCALmin)</li>
43
<li> bias = Lmin - gain * QCALmin </li>
44
<li> radiance = gain * QCAL + bias </li>
47
where, <em>Lmax</em> and <em>Lmin</em> are the calibration constants,
48
and <em>QCALmax</em> and <em>QCALmin</em> are the highest and the
49
lowest points of the range of rescaled radiance in QCAL.
52
Then, to calculate <b>at-sensor reflectance</b> the equations are:
55
<li> sun_radiance = [Esun * sin(e)] / (PI * d^2)</li>
56
<li> reflectance = radiance / sun_radiance </li>
59
where, <em>d</em> is the earth-sun distance in astronomical
60
units, <em>e</em> is the solar elevation angle, and <em>Esun</em> is
61
the mean solar exoatmospheric irradiance in W/(m² * µm).
63
<h2>Simplified at-surface values (method=dos[1-4])</h2>
65
Atmospheric correction and reflectance calibration remove the path
66
radiance, i.e. the stray light from the atmosphere, and the spectral
67
effect of solar illumination. To output these simple <b>at-surface
68
radiance</b> and <b>at-surface reflectance</b>, the equations are (not
72
<li> sun_radiance = TAUv * [Esun * sin(e) * TAUz + Esky] / (PI * d^2) </li>
73
<li> radiance_path = radiance_dark - percent * sun_radiance </li>
74
<li> radiance = (at-sensor_radiance - radiance_path) </li>
75
<li> reflectance = radiance / sun_radiance </li>
78
where, <em>percent</em> is a value between 0.0 and 1.0 (usually
79
0.01), <em>Esky</em> is the diffuse sky irradiance, <em>TAUz</em> is
80
the atmospheric transmittance along the path from the sun to the
81
ground surface, and <em>TAUv</em> is the atmospheric transmittance
82
along the path from the ground surface to the
83
sensor. <em>radiance_dark</em> is the at-sensor radiance calculated
84
from the darkest object, i.e. DN with a least 'dark_parameter'
85
(usually 1000) pixels for the entire image.
90
<li>DOS1: TAUv = 1.0, TAUz = 1.0 and Esky = 0.0</li>
91
<li>DOS2: TAUv = 1.0, Esky = 0.0, and TAUz = sin(e) for all bands
92
with maximum wave length less than 1. (i.e. bands 4-6 MSS, 1-4 TM,
93
and 1-4 ETM+) other bands TAUz = 1.0</li>
94
<li>DOS3: TAUv = exp[-t/cos(sat_zenith)],
95
TAUz = exp[-t/sin(e)], Esky = rayleigh</li>
96
<li>DOS4: TAUv = exp[-t/cos(sat_zenith)],
97
TAUz = exp[-t/sin(e)], Esky = PI * radiance_dark </li>
100
<b>Attention</b>: Output radiance remain untouched (i.e. no set to 0.0
101
when it is negative) then they are possible negative values. However,
102
output reflectance is set to 0.0 when is obtained a negative value.
106
The output raster cell values can be rescaled with the <b>scale</b>
107
parameter (e.g., with 100 in case of using reflectance output
108
in <em>i.gensigset</em>).
110
<h3>On Landsat-8 metadata file </h3>
112
NASA reports a structure of the L1G Metadata file
113
(<a href="http://landsat.usgs.gov/documents/LDCM-DFCB-004.pdf">LDCM-DFCB-004.pdf</a>)
114
for Landsat Data Continuity Mission (i.e. Landsat-8).
117
NASA retains in MIN_MAX_RADIANCE group the necessary information
118
to transform Digital Numbers (DN) in radiance values. Then,
119
<em>i.landsat.toar</em> replaces the possible standard values with the
120
metadata values. The results match with the values reported by the
121
metada file in RADIOMETRIC_RESCALING group.
124
Also, NASA reports the same values of reflectance for all bands
125
in max-min values and in gain-bias values. This is strange that all
126
bands have the same range of reflectance. Also, they wrote in the
127
web page as to calculate reflectance directly from DN, first with
128
RADIOMETRIC_RESCALING values and second divided by sin(sun_elevation).
131
This is a simple rescaling
134
<li> reflectance = radiance / sun_radiance = (DN * RADIANCE_MULT + RADIANCE_ADD) / sun_radiance</li>
135
<li> now reflectance = DN * REFLECTANCE_MULT + REFLECTANCE_ADD </li>
136
<li> then REFLECTANCE_MULT = RADIANCE_MULT / sun_radiance </li>
137
<li> and REFLECTANCE_ADD = RADIANCE_ADD / sun_radiance </li>
141
The problem arises when we need ESUN values (not provided) to
142
compute sun_radiance and DOS. We assume that REFLECTANCE_MAXIMUM
143
corresponds to the RADIANCE_MAXIMUM, then
146
<li> REFLECTANCE_MAXIMUM / sin(e) = RADIANCE_MAXIMUM / sun_radiance</li>
147
<li> Esun = (PI * d^2) * RADIANCE_MAXIMUM / REFLECTANCE_MAXIMUM </li>
150
where <em>d</em> is the earth-sun distance provided by metadata file
151
or computed inside the program.
154
The <em>i.landsat.toar</em> reverts back the NASA rescaling to continue
155
using Lmax, Lmin, and Esun values to compute the constant to convert
156
DN to radiance and radiance to reflectance with the "traditional"
157
equations and simple atmospheric corrections.
159
<b>Attention</b>: When MAXIMUM values are not provided,
160
<em>i.landsat.toar</em> tries to calculate Lmax, Lmin, and Esun from
161
RADIOMETRIC_RESCALING (in tests the results were the same).
163
<h3>Calibration constants</h3>
164
In verbose mode (flag <b>--verbose</b>), the program write basic
165
satellite data and the parameters used in the transformations.
168
Production date is not an exact value but it is necessary to apply
169
correct calibration constants, which were changed in the dates:
171
<li>Landsat-1 MSS: never </li>
172
<li>Landsat-2 MSS: July 16, 1975</li>
173
<li>Landsat-3 MSS: June 1, 1978</li>
174
<li>Landsat-4 MSS: August 26, 1982 and April 1, 1983</li>
175
<li>Landsat-4 TM: August 1, 1983 and January 15, 1984</li>
176
<li>Landsat-5 MSS: April 6, 1984 and November 9, 1984</li>
177
<li>Landsat-5 TM: May 4, 2003 and April, 2 2007</li>
178
<li>Landsat-7 ETM+: July 1, 2000</li>
179
<li>Landsat-8 OLI/TIRS: launched in 2013</li>
184
Transform digital numbers of Landsat-7 ETM+ in band rasters 203_30.1,
185
203_30.2 [...] to uncorrected at-sensor reflectance in output files
186
203_30.1_toar, 203_30.2_toar [...] and at-sensor temperature in output
187
files 293_39.61_toar and 293_39.62_toar:
189
<div class="code"><pre>
190
i.landsat.toar input=203_30. output=_toar \
191
metfile=p203r030_7x20010620.met
196
<div class="code"><pre>
197
i.landsat.toar input=L5121060_06020060714. \
198
output=L5121060_06020060714_toar \
199
metfile=L5121060_06020060714_MTL.txt
204
<div class="code"><pre>
205
i.landsat.toar input=203_30. output=_toar \
206
sensor=tm7 product_date=2004-06-07 date=2001-06-20 \
207
sun_elevation=64.3242970 gain="HHHLHLHHL"
212
<div class="code"><pre>
213
i.landsat.toar input=LC80160352013134LGN03_B output=toar \
214
metfile=LC80160352013134LGN03_MTL.txt sensor=oli8 date=2013-05-14
220
<li>Chander G., B.L. Markham and D.L. Helder, 2009: Remote Sensing of
221
Environment, vol. 113</li>
223
<li>Chander G.H. and B. Markham, 2003.: IEEE Transactions On Geoscience And
224
Remote Sensing, vol. 41, no. 11.</li>
226
<li>Chavez P.S., jr. 1996. Image-based atmospheric corrections -
227
Revisited and Improved. Photogrammetric Engineering and Remote
228
Sensing 62(9): 1025-1036.</li>
230
<li>Huang et al: At-Satellite Reflectance, 2002: A First Order Normalization
231
Of Landsat 7 ETM+ Images.</li>
233
<li>R. Irish: <a href="http://landsathandbook.gsfc.nasa.gov/orbit_coverage/">Landsat
234
7. Science Data Users Handbook</a>. February 17, 2007; 15 May 2011.</li>
236
<li>Markham B.L. and J.L. Barker, 1986: Landsat MSS and TM Post-Calibration
237
Dynamic Ranges, Exoatmospheric Reflectances and At-Satellite
238
Temperatures. EOSAT Landsat Technical Notes, No. 1.</li>
240
<li>Moran M.S., R.D. Jackson, P.N. Slater and P.M. Teillet, 1992: Remote
241
Sensing of Environment, vol. 41.</li>
243
<li>Song et al, 2001: Classification and Change Detection Using Landsat TM
244
Data, When and How to Correct Atmospheric Effects? Remote Sensing
245
of Environment, vol. 75.</li>
251
<a href="i.atcorr.html">i.atcorr</a>,
252
<a href="r.mapcalc.html">r.mapcalc</a>,
253
<a href="r.in.gdal.html">r.in.gdal</a>
256
<a href="https://lta.cr.usgs.gov/landsat_dictionary.html">Landsat Data Dictionary</a> by USGS
260
E. Jorge Tizado (ej.tizado unileon es), Dept. Biodiversity and Environmental Management,
261
University of León, Spain
264
<i>Last changed: $Date: 2014-11-28 17:25:40 +0100 (Fri, 28 Nov 2014) $</i>