49
ferred option. However, estimating the AOD from the image data requires some
assumptions to be made to derive some estimates of the surface reflectance of the
visible image bands and considerable computing resource to invert (at least partially) an atmospheric model.
Ground measurements, using a sun photometer, provide very accurate measures
of AOD but these are point measurements and therefore not necessarily representative of the whole scene. Additionally, because of the sparse nature of the ground
measurements, it is unlikely that data will be available for the image being processed.
Some weather stations provide visibility data (e.g., in the UK) but again these are
point measurements and not available everywhere.
Table 3 Parameters for the 6S model
Parameter
Description
Known
Solar zenith
The zenith angle of the sun with respect to the earth surface for
the area of acquisition.
✔
Solar azimuth
The azimuth angle of the sun with respect to the earth surface
for the area of acquisition.
✔
Sensor zenith
The zenith angle of the sensor with respect to the earth surface
for the area of acquisition.
✔
Sensor azimuth
The azimuth angle of the sensor with respect to the earth
surface for the area of acquisition.
✔
Acquisition date
and time
The exact date and time of the acquisition.
✔
Centre point of
scene (lat, long)
The point on the Earth’s surface for where the model is being
run.
✔
Altitude of sensor The height of the sensor above the Earth’s surface.
✔
Altitude of ground
surface
The height above sea level of the ground surface being
measured.
✔
Atmospheric
profile
The vertical distribution of the atmospheric layers at a given
altitude with pressure, temperature and water vapour and
ozone at that layer. However, this is commonly generalised to
standard profiles for tropical, mid-latitude summer, midlatitude winter, sub-arctic summer, sub-arctic winter.
Standardised profiles can be automatically selected based on
time and location.
✗ (✔)
Water vapour
The total amount of water in a vertical path through the
atmosphere (in g/cm
2 ).
✗
Ozone
The total amount of ozone in a vertical path through the
atmosphere (in cm-atm).
✗
Aerosol profile
The proportion of water-like, dust-like, oceanic-like and
soot-like aerosol partials in the atmosphere. However, this is
commonly generalised to standard profiles for continental,
maritime, urban, desert and biomass burning. Standardised
profiles can be automatically selected.
✗ (✔)
Aerosol optical
depth (AOD)
The total amount of AOD in the vertical path through the
atmosphere at 550 nm.
✗
Pre-processing of Remotely Sensed Imagery
ferred option. However, estimating the AOD from the image data requires some
assumptions to be made to derive some estimates of the surface reflectance of the
visible image bands and considerable computing resource to invert (at least partially) an atmospheric model.
Ground measurements, using a sun photometer, provide very accurate measures
of AOD but these are point measurements and therefore not necessarily representative of the whole scene. Additionally, because of the sparse nature of the ground
measurements, it is unlikely that data will be available for the image being processed.
Some weather stations provide visibility data (e.g., in the UK) but again these are
point measurements and not available everywhere.
Table 3 Parameters for the 6S model
Parameter
Description
Known
Solar zenith
The zenith angle of the sun with respect to the earth surface for
the area of acquisition.
✔
Solar azimuth
The azimuth angle of the sun with respect to the earth surface
for the area of acquisition.
✔
Sensor zenith
The zenith angle of the sensor with respect to the earth surface
for the area of acquisition.
✔
Sensor azimuth
The azimuth angle of the sensor with respect to the earth
surface for the area of acquisition.
✔
Acquisition date
and time
The exact date and time of the acquisition.
✔
Centre point of
scene (lat, long)
The point on the Earth’s surface for where the model is being
run.
✔
Altitude of sensor The height of the sensor above the Earth’s surface.
✔
Altitude of ground
surface
The height above sea level of the ground surface being
measured.
✔
Atmospheric
profile
The vertical distribution of the atmospheric layers at a given
altitude with pressure, temperature and water vapour and
ozone at that layer. However, this is commonly generalised to
standard profiles for tropical, mid-latitude summer, midlatitude winter, sub-arctic summer, sub-arctic winter.
Standardised profiles can be automatically selected based on
time and location.
✗ (✔)
Water vapour
The total amount of water in a vertical path through the
atmosphere (in g/cm
2 ).
✗
Ozone
The total amount of ozone in a vertical path through the
atmosphere (in cm-atm).
✗
Aerosol profile
The proportion of water-like, dust-like, oceanic-like and
soot-like aerosol partials in the atmosphere. However, this is
commonly generalised to standard profiles for continental,
maritime, urban, desert and biomass burning. Standardised
profiles can be automatically selected.
✗ (✔)
Aerosol optical
depth (AOD)
The total amount of AOD in the vertical path through the
atmosphere at 550 nm.
✗
Pre-processing of Remotely Sensed Imagery