The Use of SeaWiFS Satellite Data For Ocean Colour Determination In Fisheries Application
Mohd Ibrahim Seeni Mohd and Tadris Ahmad
Department of Remote Sensing,
Faculty of Geoinformation Science and Engineering
Universiti Teknologi Malaysia,
81310 Skudai, Johor, Malaysia
Tel : (607) 5502969 Fax : (607) 5566163
E-mail : mism@fksg.utm.my ,
amir_ahmad@lycos.com
Keywords : SeaWiFS satellite data, normalised water leaving radiance(nLw) at 443nm, fish distribution, concentration of chlorophyll
Abstract
Several studies have explored the possibility of using remote sensing data for marine and oceanography applications in Malaysia. These include satellite bathymetry, suspended sediment and sea bottom features mapping. Remote sensing of ocean colour in the coastal zone is currently used to map a chlorophyll concentration. Through analysis of satellite data over long time of periods, a suitable algorithm can be test and evaluated in responsed to different seasons and environment. This research intends to study on the algorithm development using SeaWiFS data with the hope to improve the results. In - situ data were collected at the time of satellite pass in Terengganu coastal area during 20 to 22 June 2000. This data will be used with SeaWiFS data acquired during this period to calibrate and validate relevant algorithm. The preminilary result shows the correlation between normalised water leaving radiance (nLw) at 443 nm and chlorophyll concentration. At this wavelengths, as chlorophyll concentration increases, absorption will also increases. The R--squared value for the correlation of above parameters is 0.96. Further investigation will be carried out to evaluate the RMSE between the in-situ measurement and the chlorophyll concentration calculated using the SeaWiFS data. This research would be beneficial to the Fisheries Department to determine potential fishing grounds.
Introduction
The research includes the identification of regression coefficient through a modification of existing algorithm which is suitable to Malaysia coastal waters. The suitable algorithms will be used to estimate the concentration of chlorophyll using the satellite data. Time series satellite images are required for the analyses with in - situ data obtained simultaneously . This quantitative technique can be used to monitor the variability of chlorophyll spatially and temporally.
The study will be conducted in the coastal waters of Terengganu located between 5° - 6°30` N and 102° - 104° E ( Figure 1 ). The east coast of Peninsular Malaysia has a complex oceanic characteristics due to the monsoon winds which have a significant effect on the chlorophyll distribution.
Figure 1: Location of map study area.
Materials and Methods
This study was carried out using SeaWiFS satellite data which was taken simultaneously with in - situ measurement. SeaWiFS data was supplied by HRPT (High Resolution Picture Transmission) station at National University of Singapore. The raw level 1A data was processed using SEADAS image processing to extract normalised water leaving radiance (nLw). The influence of molecular scaterring from atmosphere is corrected by employing an atmospheric correction procedured, referred as Gordon and Wang algorithm( Wang 1994 ). It is necessary to normalised the radiance by the incoming solar irradiance to eliminate sun zenith angle effects and atmospheric effects.
Chlorophyll concentration was quantitatively estimated using an algorithm proposed by Robert Frouin from Scripps Institute for Oceanography. However the algorithm has been modified prior to regional variability.
C = phytoplankton concentration
A and B = regression coefficient ( site - spesific)
The algorithm above then has been iterated to retrieve the regression coefficient.
Log Chl = A ( NDPI ) - B (2)
Results
SeaWiFS data taken on 20 June 2000 over Terengganu coast of Malaysia was available on compact disc. Sea - truth of chlorophyll had been taken at 4 stations by Unipertama 7 and Unipertama 3 ships .
The values of normalised water leaving radiance at several bands corresponding to chlorophyll concentration are given in table 1.
Table 1. Normalised water leaving radiance ( nLw ) and chlorophyll concentration for several points along the on - shore to off shore ship track.
| Station |
nLw 443 |
nLw 490 |
nLw 555 |
Chlorophyll |
| 1 |
0.285 |
0.685 |
0.617 |
0.146 |
| 2 |
0.526 |
1.025 |
0.678 |
0.138 |
| 3 |
0.741 |
1.254 |
0.895 |
0.117 |
| 4 |
0.749 |
0.815 |
0.405 |
0.121 |
| 5 |
0.195 |
0.496 |
0.323 |
0.075 |
| 6 |
0.155 |
0.413 |
0.237 |
0.089 |
| 7 |
0.109 |
0.326 |
0.177 |
0.113 |
| 8 |
0.251 |
0.454 |
0.230 |
0.063 |
