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R.H. Charlier and Chr. P. De Meyer
a geometric correction, which eliminates the deformation of the image due to
the irregularities in the flight path. The correction procedure is based on the
use of ground control points.
a radiometric correction can be applied in order to compensate for
atmospheric influences (haze, reflection .... ).
For the quantification of the scanner image, a best fit relationship between the
suspended matter concentrations and a combination of several of the scanner
values is established. Several relationships (linear, logarithmic, etc.) shotfld be
considered. The one with the highest regression coefficient is chosen. This
relationship is then used for all the other non-calibrated scanner data to obtain a
representation of the sediment concentrations over the entire field of view. In
accordance with Ritchie & Cooper (1988), a simple linear relationship can be
used. Here, in each point, the suspended sediment concentration C is calculated
by
C=a+ Y, biB i
(48)
i
where a, bi =
i=
Bi =
regression coefficients, determined in the picture points where
ground truth is available;
the number of spectral bands used in the regression analysis;
pixel value of spectral band i.
A logarithmic relationship can also been applied (Tassan & Sturm,
1986) :
log C = a + b log X
(49)
where a, b =
X =
parameters inferred from best fits to in situ
measurements of suspended sediment concentration;
a function of the pixel values Bi in the i spectral bands.
Using the best relationship, one comprehensive picture is compiled based on all
the bands of the digital multispectral scanner which enter into the equation. Fig.
18 is a black-and-white representation of the compiled color image showing the
results in the Zeebnlgge region of a flight executed on 4 September 1987, one
hour before low water, i.e. at the time of the greatest flow velocities. The
sediment concentrations have been represented into 8 classes ranging from less
than 0.05 g/1 to over 0.40 g/1 (0.00177 to > 0.0142).