Eli. Sediment Transport
107
Chapter III : Detection of Suspended Sediment
1
Principle
In order to detect and measure concentrations of suspended matter (mostly
sediment and chlorophyll), the data from aerial or satellite scanners must be
multispectral in nature (Doerffer, 1978; Morel & Prieur, 1977; Liedtke, 1987).
Especially the red, green and infrared play an essential role in accurately
determining these concentrations. The red portion of the spectrum provides the
most consistent information over the broadest range of sediment and atmospheric
conditions. The green spectral region is also useful for broad suspended sediment
concentration ranges where its behavior can be compared to that of the red band
by ratio techniques. A channel in the infrared portion of the spectrum is
necessary to extend the analytical range of the sensor system and to ensure that
image over-exposure (i.e, image saturation) does not occur at high suspended
sediment concentrations. In addition, the infrared band is used to help delineate
tile shallow water and the land-water interface, for these wave lengths are
attenuated within the first few centimeters of the water.
Multispectral scanner data can either be acquired by satellite borne scanners or by
a scanner operated in an aircraft. Satellite scanners have the advantage that
geometrical corrections to the data can more easily be performed; and in general
satellite data are available at relatively low cost. For the study of sedimentary
processes in the nearshore zone however, most benefits are gained from an
airborne multispectral scanner. First, an airborne scammr can be deployed at the
time that best suits the aim of the study (e.g. with respect to the tide). It is also
essential, for calibration of the multispectral recordings, that ground truth (i.e.
near-surface suspended sediment concentrations) be available of the precise time
of the recordings. Such data can more easily be obtained when the time of
recording can be planned, as with the airborne scanner. When satellite data are
used, one has to rely on empirical formulas in order to get any quantified ground
truth.
The corrections that have to be carried out to airborne digital multispectral
scanner recordings include :
- a panoramic correction, which recalculates the image plane, that is the result
of a central projection, to a straight plane.
a thermal calibration. This procedure recalculates tile recorded values of the
thermal band so as to simulate an instantaneous recording (in reality, a
certain time elapses between tile beginning and the end of the recording
during which the earth surface temperature gradually changes).
107
Chapter III : Detection of Suspended Sediment
1
Principle
In order to detect and measure concentrations of suspended matter (mostly
sediment and chlorophyll), the data from aerial or satellite scanners must be
multispectral in nature (Doerffer, 1978; Morel & Prieur, 1977; Liedtke, 1987).
Especially the red, green and infrared play an essential role in accurately
determining these concentrations. The red portion of the spectrum provides the
most consistent information over the broadest range of sediment and atmospheric
conditions. The green spectral region is also useful for broad suspended sediment
concentration ranges where its behavior can be compared to that of the red band
by ratio techniques. A channel in the infrared portion of the spectrum is
necessary to extend the analytical range of the sensor system and to ensure that
image over-exposure (i.e, image saturation) does not occur at high suspended
sediment concentrations. In addition, the infrared band is used to help delineate
tile shallow water and the land-water interface, for these wave lengths are
attenuated within the first few centimeters of the water.
Multispectral scanner data can either be acquired by satellite borne scanners or by
a scanner operated in an aircraft. Satellite scanners have the advantage that
geometrical corrections to the data can more easily be performed; and in general
satellite data are available at relatively low cost. For the study of sedimentary
processes in the nearshore zone however, most benefits are gained from an
airborne multispectral scanner. First, an airborne scammr can be deployed at the
time that best suits the aim of the study (e.g. with respect to the tide). It is also
essential, for calibration of the multispectral recordings, that ground truth (i.e.
near-surface suspended sediment concentrations) be available of the precise time
of the recordings. Such data can more easily be obtained when the time of
recording can be planned, as with the airborne scanner. When satellite data are
used, one has to rely on empirical formulas in order to get any quantified ground
truth.
The corrections that have to be carried out to airborne digital multispectral
scanner recordings include :
- a panoramic correction, which recalculates the image plane, that is the result
of a central projection, to a straight plane.
a thermal calibration. This procedure recalculates tile recorded values of the
thermal band so as to simulate an instantaneous recording (in reality, a
certain time elapses between tile beginning and the end of the recording
during which the earth surface temperature gradually changes).