Comparison of Turbidity Measurement by MODIS and AVHRR Images

The turbidity is commonly measured by turbidity meter. However, it is only accurate for single point measurement but ineffective for wide spatial and temporal coverage. The objective of this study was to calibrate the turbidity algorithm for AVHRR and MODIS images. We also want to compare the effectiveness of MODIS and AVHRR in turbidity retrieval. Some studies had been performed to correlate the relationship between turbidity and reflectance. The reflectance at the Top of Atmosphere (TOA) was different with the reflectance from the target (water surface). Therefore an algorithm for atmospheric corrected reflectance had to be performed before a regression analysis was done to correlate the turbidity measurement with reflectance. Some pre-processing steps had to be performed to retrieve the atmospheric corrected reflectance and perform the geometric correction. The classification of land, cloud and water pixels was done based on the ratio of reflectance for near-infrared channel and red visible channel. The cloud masking techniques were also applied to filter the cloud contaminated pixels. The good pixels were only chosen for regression analysis. The multiple regressions were done on turbidity data and atmospheric corrected reflectance. The turbidity data was turbidity in-situ data obtained from Department of Environment (DOE). The atmospheric corrected reflectance was calculated with method of Hu. The square of correlation coefficient, R2 and root mean square error, RMSE were calculated. The same procedure was done on both MODIS and AVHRR images. The scatter plot was generated to observe the correlation between turbidity estimated from the algorithm for MODIS and AVHRR images. The turbidity estimated from algorithm by MODIS image gave a higher correlation compared with AVHRR image. Besides of that, the turbidity estimated from algorithm by both images show only the moderate relationship.