Remote sensing of total suspended solids
in penang coastal waters, malaysia
K. Abdullah, Z. B. Din, Y. Mahamod , R. Rainis, and M. Z. MatJafri
Universiti Sains Malaysia, 11800 Penang, Malaysia
Tel: 604-6577888 Fax: 604-6579150
E-mail: khirudd@usm.my
Keywords: algorithm, regression, total suspended solids, Thematic Mapper
Abstract
An attempt to estimate the concentration of total suspended solids (TSS) using Thematic Mapper (TM) data was carried out in the coastal waters of Penang. The algorithm used is based on the reflectance model which is a function of the inherent optical properties of water which can be related to its constituents concentrations. A multiple regression algorithm was derived using multi-band data for retrieval of the water constituent. The digital numbers coinciding with the sea truth locations were extracted and converted to radiance and exoatmospheric reflactence units. Solar angle and atmospheric corrections were performed on the data sets. These data were combined for multi-date regression analysis. The efficiency of the present algorithm versus other forms of algorithms was also investigated. Based on the observations of correlation coefficient and root-mean-square deviations with the sea-truth data, the results indicated the superiority of the proposed algorithm. The solar corrected data gave good results, and comparable accuracy was obtained with the atmospherically corrected data. The calibrated TSS algorithm was employed to generate water quality maps.
Introduction
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 use remote sensing data acquired simultaneously with the surface reference data. Thematic Mapper (TM) data were used because of the spatial resolution and availability of suitable bands for the present applications. Throughout 1999, only two TM scenes were successful acquired.
TM data 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). To utilize the multi-spectral radiance responses detected by the sensor as a means of water quality monitoring, a model or algorithm is required to relate the TM signals to the scattering and absorption phenomena occurring within the sea.
The objectives of the study are (i) to develop remote sensing retrieval algorithms for total suspended solids (TSS) in coastal waters based on water optical model through regression with coincident sea-truth data and (ii) to perform data correction for multi-date analysis in order to establish calibrated algorithm for local applications using TM data.
Study Area and Data Acquisition
The study area is the Penang Straits which is located within latitudes 100° 15' E to 100° 25'E and longitudes 5° 15'N to 5° 30'N. The corresponding satellite track for the TM scenes is 128/56. Based on the bathymetric map the water depth within the study may reach up to about 20 m. There are two major channels within the straits, namely, the Eastern and Western channels with mean depths of about 10m and 8m respectively.
The water samples were collected from February to November 1999 on dates that the satellite was supposed to overpass the study area. Surface water samples for the determination of TSS concentration were collected using 1 liter polyethylene bottles and later analyzed in the laboratory using standard methods as proposed by Strickland and Parsons (1972). The TM scenes acquired on 2 February 1999 and 22 March 1999 were available for applications.
Water Optical Model
A physical model relating radiance from the water column and the concentrations of the water quality constituents provide 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
R(l)=0.33bb(l)/a(l) (1)
where
l is the spectral wavelength, b
b 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 b
bw(i) is the backscattering coefficient of water, b
bc*and b
bp* are the specific backscattering coefficients of chlorophyll and sediment respectively, a
w(i) is the absorption coefficient of water, a
c*(i) and a
p*(i) are the specific absorption coefficients of chlorophyll and sediment respectively (Gallie and Murtha,1992).
Regression Algorithm
Solving the above simultaneous equations for TSS concentration yields the series consisting of the terms R
1 and R
2
P=a0+a1R1+a2R2+a3R1R2+a4R12+a5R22+a6R12R2+
a7R1R22+a8R12R22+... (3)
where a
j, 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
P=e0+e1R1+e2R2+e3R3+e4R1R2+e5R1R3+e6R2R3+
e7R12+e8R22+e9R32 (4)
and the coefficients e
j are then empirically determined.
