Remote Sensing of Turbidity Mapping from Digital Camera Imagery
2.0 STUDY ARAE AD DATA ACQUISITION
The location of the study area is in the vicinity of the Prai river estuary, Penang. It is situated
between latitudes 5º 22’ N to 5º 24’ N and longitudes 100º 21’ E to 100º 23’ E (Figure 1). Images
were taken during the flight between 9 a.m. to 11 a.m. on 1 September 2003. Turbidity readings
were measured by using a handheld turbidity meter. Digital camera imagery was captured
simultaneously during the acquisition of the water samples. Images were taken from an altitude of
4400 feet. Water samples locations were determined using a handheld GPS. In this study, airborne
image was used instead of satellite imagery because of the difficulty in obtaining a cloud free
satellite image in the equatorial region. River estuary is selected as the study area because of the
TSS concentration is clearly differentiated.
Figure 1 The study area
3.0 WATER OPTICAL MODEL
A physical model relating radiance from the water column and the concentrations of the water
quality constituents provides the most effective way for analysing remotely sensed data for water
quality studies. Reflectance is particularly dependent on inherent optical properties: the absorption
coefficient and the backscattering coefficient. The irradiance reflectance just below the water
surface, R(l), is given by
Where l is the spectral wavelength, bb is the backscattering coefficient and a is the absorption
coefficient (Kirk, 1984). The inherent optical properties are determined by the contents of the water.
The contributions of the individual components to the overall properties are strictly additive
(Gallegos and Correll, 1990).
For the case of two water quality components, i.e. chlorophyll, C, and suspended sediment,
P, the simultaneous equations for the two channels can be expressed as
where bbw(i) is the backscattering coefficient of water, bbc*and bbp* are the specific backscattering
coefficients of chlorophyll and sediment respectively, aw(i) is the absorption coefficient of water,
ac*(i) and ap*(i) are the specific absorption coefficients of chlorophyll and sediment respectively
(Gallie and Murtha, 1992).
4.0 REGRESSION ALGORITHM
Solving the above simultaneous equations (2a and 2b) for TSS concentration yields the series
consisting of the terms R1 and R2
Where aj, j = 0, 1, 2, … are the functions of the coefficients in equation (3) which are to be
determined empirically using multiple regression analysis. The algorithm can be extended to the
three-band method for turbidity
And the coefficients ej, j = 0, 1, 2, … is then empirically determined.
