Mapping coastal turbidity around a coal-mining island
A Landsat model of total suspended solids, calibrated with field samples around Bunyu Island, North Kalimantan, and used to compare the coast across a decade.
- Year
- 2025
- Role
- Independent research, linked to my master’s thesis
- Tools
- Landsat, Google Earth Engine, Python, QGIS
The question
Bunyu Island lies off the coast of North Kalimantan, with coal mining on land and shallow coastal water all around it. Total suspended solids (TSS) is one of the clearest signs of how activity on land reaches the sea. Field sampling, though, only shows a handful of points on a handful of days. The aim was to turn satellite imagery into a TSS map I could trust, so the whole coast, and its past, could be read.
Field data
- 45 water samples across three zones, collected in April 2025.
- Each sample matched to a Landsat scene, with the allowed time gap between sampling and overpass justified from the data rather than chosen by habit.
Building the model
- Compared 44 combinations of bands and regression forms.
- Selected a power model with Duan smearing correction, which removes the bias that appears when a log-scale model is converted back to real units. R² ≈ 0.67.
- Checked the model with leave-one-out cross-validation.
Making different sensors agree
Comparing years meant combining images from Landsat’s ETM+ and OLI sensors. The widely used harmonisation coefficients (Roy et al., 2016) turned out to make agreement worse over water at low reflectance, so I derived local coefficients by quantile matching instead.
Standard corrections are built mostly on land. Over dark coastal water, it paid to test them before trusting them.
Results
- TSS maps of the coast with a three-tier zonation.
- A seasonal and multi-year comparison across 2016, 2019 and 2025.
- A manuscript submitted to Environmental Monitoring and Assessment (first author, under review).
What I delivered
Earth Engine scripts, Python analysis scripts, QGIS style files for consistent map symbology, and a step-by-step Excel workbook so the method can be repeated by others.