Answer:
ATCOR employs the unit
[mW cm-2 sr-1 micron-1].
For each channel the at-sensor radiance
L(i) and digital number DN(i) are related by
L(i) = c0(i) + c1(i)*DN(i)
where c0(i) and c1(i) are the offset (bias) and slope (gain) of the linear calibration
equation.
The metadata files of different sensors use different units, so care has to be taken to
convert these numbers into the ATCOR radiance unit.
The c0, c1 coefficients (with unit mW cm-2 sr-1 micron-1) are specified in a "sensor.cal" file,
e.g. "Ikonos.cal".
Exception: for the SPOT cameras the calibration coefficient A(i) is specified in the metadata file
as
Answer:
Multiply the coefficients in the metadata file with 0.1 to convert into
mW cm-2 sr-1 micron-1.
Answer:
The radiance-DN relationship for sensors with gain setting g is
Answer:
The SPOT tilt angles are specified with respect to the direction of motion of the
sub-satellite point, so "right" means geographic west for the descending orbit, where
daytime data acquisition takes place, "left" means geographic east.
Answer:
There are several possibilities for this behavior:
Answer:
The composition of the aerosol particles is primarily controlled by
natural and man-made sources at the earth's surface. The aerosol content
of the atmosphere at a given location will therefore depend on the
trajectory of the local air mass during the preceding several days.
However, some general easy-to-use guidelines for the selection are :
In forested, agricultural areas and scrub land the rural aerosol
is usually the adequate choice. This also holds for polar, arctic, and snow covered land.
Answer:
A DEM spatial resolution comparable to the image resolution is sufficient for most applications.
Even if the DEM pixel size is larger than the image pixel a clear reduction of topographic
illumination effects is usually achieved. However, areas of steep slopes that are smoothed in
the coarse-resolution DEM may lead to artifacts.
Answer: Use a filter size of 3-10 pixels for the slope/ aspect calculation to smooth the gradients.
Answer:
There are 3 possibilities:
Answer:
The overcorrection usually occurs in areas with steep slopes oriented away from the
sun. These areas with a high local solar incidence angle (or low local solar
elevation angle) appear dark in the illumination image cos(solar_incidence) that is
automatically generated. In these areas the reflectance shows a behavior
that depends on the illumination and viewing conditions (the bidirectional reflectance
distribution function, BRDF). There is no general solution to this problem. However, the next Question/Answer discusses an empirical correction method implemented in ATCOR.
Answer:
Two output files are generated during the atmospheric / topographic correction: the
processed scene and the illumination image cos(beta) which is coded as byte data,
i.e. illumination illu = 100*cos(beta).
Answer:
Since the current LAI and FPAR equations are simple empirical relationships based on the selected
vegetation index (either NDVI or SAVI) they can only approximate typical trends.
This application is mainly intended to get the correct trends in multi-temporal studies,
therefore it is recommended to use the same parameter set for all multitemporal scenes,
whether it is the default or user-specified parameters.
DN(i) = A(i) * L(i)
and the A(i) coefficients (with unit [m2 sr micron W-1]) enter the ATCOR calibration file and c0(i)=0.
ATCOR converts the A(i) internally as
c1(i) = 0.1 / A(i)
to obtain the required unit [mW cm-2 sr-1 micron-1].
Please use the
L = c0 + {c1/g} * DN
However, in the Landsat-7 metadata file the slope (gain) value c1 is already adapted to g=1.
Therefore, multiply the value with the factor 0.1 to convert the Landsat radiance unit (see Question 2) into the ATCOR
unit [mW cm-2 sr-1 micron-1],
see Question 1, and specify a gain setting g=1 in the ATCOR panel.
(a) Edit a copy of the "sensor.cal" file, e.g. "sensor_edit2002.cal" and raise the offset c0 in the
appropriate channels until the reference spectrum (e.g., water or vegetation) is matched.
(b) use the "Inflight Calibration" module and select some ground target(s) whose reflectance spectra
serve as reference and will be matched. A new calibration file will be generated.
If in doubt, select the "rural" aerosol type. It represents conditions
one finds in continental areas not directly influenced by
urban/industrial sources.
This aerosol consists of dust-like and organic particles.
In urban areas, the rural aerosol background is often modified by the
addition of particles from combustion products and industrial sources
(carbonaceous soot-like particles).
However, depending on wind direction and shortly after raining, the
rural aerosol might also be applicable in urban areas.
In areas close to the sea or close to large lakes, the aerosol largely
consists of sea-salt particles, mixed with continental particles. In
these areas, the aerosol choice would depend on wind direction: for an
off-shore wind, the rural aerosol (or urban) type is still the best
choice, otherwise the maritime. Again, since the wind conditions are
often not known, select the rural type if in doubt.
As the name implies, this type is intended for desert-like conditions,
with dust-like particles of larger size.
Notice :
(i) The SPECTRA module can be used to assess the influence of the selected
aerosol type. The target spectrum for a certain target (e.g. vegetation,
water) can be displayed for the different aerosol types and compared to
library spectra. The aerosol with the closest match would be the logical
choice.
(ii) If the sensor has a blue spectral band and a 1.6 or 2.2 um band (e.g.
Landsat TM), ATCOR starts with the user-selected aerosol/atmosphere, but
is able to adapt the path radiance, i.e. aerosol properties, in cases
where reference areas (coniferous forest, dark soils) are found in the
scene. Thus, ATCOR will cope with non-standard aerosol conditions, e.g.
a mixture of rural and urban aerosols.
In practice we often have to live with DEM resolutions of 50 - 100m, because higher resolution
DEM's are not available or too expensive.
- reduce the atmospheric water vapor column
- do a re-calibration ("Inflight Calibration" with reference targets)
- try the "Spectral Polishing" module (only for small-amplitude spikes)
rho(corrected) = rho(isotropic) * cos(beta) / cos(beta_t)
where beta is the local solar incidence angle on a surface element, and beta_t is a
threshold angle to be specified by the user. How do I have to specify the beta_t to
reduce the reflectance of overcorrected bright areas in the processed scene ?
Link both files (e.g. with the ENVI software) and look for the dark areas in the
illumination image. These correspond to steep slopes oriented away from the sun
and these areas usually show strong BRDF effects, i.e. they correspond to the bright
overcorrected areas in the processed scene.
Example: the scaled overcorrected reflectance is 320 (scale factor 4), i.e.,
rho(isotropic) = 80%. The illumination file for this pixel says illu=34, i.e.,
beta = arccos(illu/100)=70 degree. Let us assume the 80% reflectance should be reduced
to 40%, which is the value for pixels in the flat-terrain neighborhood. Then the
threshold angle beta_t has to be specified such that cos(beta) / cos(beta_t) = 0.5,
in this case beta_t=47 degree. So if the desired geometry-dependent reflectance reduction factor is
G the required threshold angle can be calculated as
beta_t = arccos { cos(beta) / G } = arccos { illu / (100*G) }
Unfortunately, there is no general solution to this problem, the threshold parameter
is scene dependent and has to be defined interactively. A rule of thumb is
beta_t = solar_zenith_angle + 20 [degrees]
A quantitative agreement with field measurements of different crop types in
different seasons cannot be expected.