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Water quality mapping using multi-date images from digital camera
H. S. Lim, K. Abdullah, M. Z. MatJafri School of Physics, Universiti Sains Malaysia, 11800 Pulau Pinang, Malaysia Tel: 604-6577888 Fax: 604-6579150 Email : khirudd@usm.my, mjafri@usm.my
Introduction Remote sensing techniques have been widely used for water quality studies in coastal regions and in inland lakes (Ekstrand 1992, Ritchie et al.1990, Baban 1993, Dekker and Peters 1993, Forster et al. 1993, Allee and Johnson 1999, Koponen et al. 2002). To utilize the multi-spectral radiance responses detected by the satellite sensors as a means of water quality monitoring, a model and an algorithm were required to relate the remotely sensed signals to the scattering and absorption phenomena occurring within the sea. Sea-truth data that coincide with satellite over passing the study area is very important in remote sensing studies. However, quantitative measurements from satellite remote sensing through analysis with coincident sea-truth data are rarely conducted in the equatorial region. The main reason for this drawback is the difficulty in obtaining cloud free scenes. In this study we attempted to introduce a new method to overcome the above problem. In this study we used digital images captured by a conventional digital camera from light aircraft. In this work, the algorithms developed for use in remote sensing applications were tested with the airborne digital images. Multi-date data were used in the multi-band algorithm calibration and validation analysis. In this work we used the digital numbers as independent variables in the calibration of the algorithm. The newly developed algorithm was used to determine the distribution of TSS in the coastal water and to generate the water quality map. Study area The study area is located in the vicinity of the Prai river estuary, Penang Malaysia (within latitudes 5º 22' N to 5 º 24' N and longitude100o 21' E to 100o 23' E) as shown in Figure 1. The images were captured from a light aircraft flown at the altitudes of 3000 ft on the 28 October 2001 and 8000 ft on 9 March 2002. A digital camera (Kodak DC 290) was used as our airborne remote sensor. During the image acquisitions, the water samples were collected from a small boat within the study area. 
Optical model of water A physical model relating radiance from the water column and the concentrations of the water quality constituents provides the most effective way of analyzing 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 Kirk (1984) as
R(l) = 0.33b(l)/a(l) ----------------------------(1)
where l = the spectral wavelength b = the backscattering coefficient a = the absorption coefficient 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 Correl, 1990). For a case involving two water quality components, i.e. chlorophyll, C, and suspended sediment, P, the simultaneous equations for the two channels given by Gallie and Murtha (1992) can be expressed as 
where bbw(i) = backscattering coefficient bbc* = chlorophyll coefficient bbp = sediment coefficient aw(i) = absorption coefficient ac* = chlorophyll specific absorption coefficient ap* = sediment specific absorption coefficient C = chlorophyll P = suspended sediment
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