Flags:¶

-i

Output raster map as integer

-r

Input raster map converted to reflectance (default is radiance)

-a

Input from ETM+ image taken after July 1, 2000

-b

Input from ETM+ image taken before July 1, 2000

--overwrite

Allow output files to overwrite existing files

--help

Print usage summary

--verbose

Verbose module output

--quiet

Quiet module output

--ui

Force launching GUI dialog

Parameters:¶

input=name [required]

Name of input raster map

range=min,max

Input range
Default: 0,255

elevation=name

Name of input elevation raster map (in m)

visibility=name

Name of input visibility raster map (in km)

parameters=name [required]

Name of input text file with 6S parameters

output=name [required]

Name for output raster map

rescale=min,max

Rescale output raster map
Default: 0,255

A. Geometrical conditions¶

B. Atmospheric model¶

C. Aerosols model¶

D. Aerosol concentration model (visibility)¶

If you have an estimate of aerosol optical depth, enter 0 for the visibility and in a following line enter the aerosol optical depth at 550nm (iaer means ’i’ for input and ’aer’ for aerosol), for example:

0                            - visibility
0.112                        - aerosol optical depth 550 nm

NOTE: if iaer is 0, enter -1 for visibility.

E. Target altitude (xps), sensor platform (xpp)¶

Sensor platform (xpp, in negative [km] or -1000):
xpp = -1000 means that the sensor is on board a satellite.
xpp = 0 means that the sensor is at the ground level.
-100 < xpp < 0 defines the altitude of the sensor expressed in [km]; this altitude is given relative to the target altitude as negative value.

For aircraft simulations only (xpp is neither equal to 0 nor equal to -1000): puw,po3 (water vapor content,ozone content between the aircraft and the surface)
taerp (the aerosol optical thickness at 550nm between the aircraft and the surface)

If these data are not available, enter negative values for all of them. puw,po3 will then be interpolated from the us62 standard profile according to the values at the ground level. taerp will be computed according to a 2km exponential profile for aerosol.

F. Sensor band¶

Define your own spectral conditions:

Pre-defined satellite bands:

Atmospheric correction of a LANDSAT-7 channel¶

g.region raster=lsat7_2002_40 -p

r.info lsat7_2002_40

If the sun position metadata are unavailable, we can also calculate them from the overpass time as follows (r.sunmask uses SOLPOS):

r.sunmask -s elev=elevation out=dummy year=2002 month=5 day=24 hour=10 min=42 sec=7 timezone=-5
# .. reports: sun azimuth: 121.342461, sun angle above horz.(refraction corrected): 65.396652

Conversion of digital number (DN) to radiance at top-of-atmosphere (TOA)¶

In case of different satellites, the conversion of DN (digital number = pixel values) to radiance at top-of-atmosphere (TOA) can also be done manually, using e.g. the formula

# formula depends on satellite sensor, see respective metadata
Lλ = ((LMAXλ - LMINλ)/(QCALMAX-QCALMIN)) * (QCAL-QCALMIN) + LMINλ

We extract the coefficients and apply them in order to obtain the radiance map:

CHAN=4
r.info lsat7_2002_${CHAN}0 -h | tr ’\n’ ’ ’ | sed ’s+ ++g’ | tr ’:’ ’\n’ | grep "LMIN_BAND${CHAN}\|LMAX_BAND${CHAN}"
LMAX_BAND4=241.100,p016r035_7x20020524.met
LMIN_BAND4=-5.100,p016r035_7x20020524.met
QCALMAX_BAND4=255.0,p016r035_7x20020524.met
QCALMIN_BAND4=1.0,p016r035_7x20020524.met

r.mapcalc "lsat7_2002_40_rad = ((241.1 - (-5.1)) / (255.0 - 1.0)) * (lsat7_2002_40 - 1.0) + (-5.1)"

Creation of parameter file for i.atcorr¶

# find mean elevation (target above sea level, used as initialization value in control file)
r.univar elevation

8                            - geometrical conditions=Landsat ETM+
5 24 15.70 -78.691 35.749    - month day hh.ddd longitude latitude ("hh.ddd" is in GMT decimal hours)
2                            - atmospheric mode=midlatitude summer
1                            - aerosols model=continental
50                           - visibility [km] (aerosol model concentration)
-0.110                       - mean target elevation above sea level [km]
-1000                        - sensor on board a satellite
64                           - 4th band of ETM+ Landsat 7

i.atcorr -r -a lsat7_2002_40_rad elev=elevation parameters=icnd_lsat4.txt output=lsat7_2002_40_atcorr

Note that the process is computationally intensive.
Note also, that i.atcorr reports solar elevation angle above horizon rather than solar zenith angle.