Materials
- Sea truth
The data from the sea truth campaigned of 24th August 2000 until 29th August 2000 were applied to obtain the correlation between chlorophyll-a concentration (mg/m3) and the radiance values in chosen channel of SeaWiFS image. The amount of concentration of chlorophyll-a was calculated based on blue, blue/green and green (442nm-555nm) reflectance ratios, this was done by selecting representative radiance values corresponding to in-situ data measurements.
- Image satellite data
For SeaWiFS data, are uses in
this study are Level 1A (LAC) data dated on 24th August 2000 and
29th August 2000 with FTP format can be downloaded from the
Internet. Both images are cloudy and only apart of sampling point
can be furthered analysis.
Methodology
This study can segregated into two parts, firstly, chlorophyll-a sampling and analysis and secondly to map the chlorophyll-a concentration from SeaWiFS data.
Laboratory analysis
Seawater samples were taken using Van Dorn Sampler for maximum chlorophyll-a layer according the information from acoustic equipment. Chlorophyll-a concentrations were measured in the laboratory.
The chlorophyll-a concentrations were estimated by using the spectrophotometer 6300 Jensey at the Aquatic laboratory. The chlorophyll-a concentration was measured by using the technique and calculation described by Parsons et.al (1984). Five litres of seawater were filtered through a filter paper (Millipore, size 0.5 mm). As the seawater is being filtered, a few drops of suspension of magnesium carbonate were added to prevent acidity on the filter. Pigments were extracted from the filters in 90% acetone. The wavelengths involved are .
All the extinction was corrected for a small turbidity blank by subtracting the from the , and absorptions. Then, the amounts of pigments in the original seawater sample were calculated using the equation given below. For Chlorophyll-a (Ca):
Ca = 11.85E
664 - 1.54E
647 - 0.08E
630 …… (3.1)
Where:
E stands for the absorbance at different wavelength obtained above (Corrected by the 750 nm reading).
Ca is the amount of chlorophyll in
mg / ml
Chlorophyll-a is Chlorophyll-a concentration, and its obtained from the following equation:
Chlorophyll-a(mg/m
3) = Cx
n/V x 10 …… (3.2)
Where:
C are substituted for Chlorophyll-a from equation (3.1)
n is the volume of acetone in ml (10 ml)
V is the volume of seawater in l (5l)
Image Processing
Image processing for SeaWiFS data in this study, was processed using an image processing software PCI Easi/Pace Version 8.0 and ENVI Version 3.6. The main operation in this study can be categorized onto four types of processing. There are geometric correction, radiometric correction, chlorophyll-a extraction and finally mapping chlorophyll-a concentration.
Chlorophyll-a Extraction
Based from curve fitting, a linear regression analysis was carried out between water parameters and spectral radiances were performed and their significant level examined. In this study, spectral attention was given to channel 2 (445nm), channel 3 (490nm) and channel 5 (555nm). Linear regression technique was applied between ratio of reflectance values and chlorophyll-a concentration samples from in-situ sampling. The best correlation coefficient (r2), would be used to extract the distribution of chlorophyll-a concentration.