How satellites see mangroves
Radar sees through cloud and optical sensors see colour; maps combine both.
Two families of satellite sensors are used.
Optical (Landsat, Sentinel-2). They measure sunlight reflected in visible and infrared bands, like a colour photograph. Healthy mangrove is green and reflects strongly in near-infrared. Limits: clouds are frequent in the tropics, and tide level changes what is visible.
Radar (SAR, for example JAXA's L-band ALOS PALSAR). The satellite sends microwave pulses and records the echo. Radar works through cloud and at night and the long L-band wavelength is sensitive to woody structure, which helps separate forest from low vegetation.
Resolution
- Landsat: 30 m pixels, archive from 1984.
- Sentinel-2: 10 m, since 2015.
- ALOS PALSAR mosaics: 25 m.
How the mapping is done. Algorithms classify pixels as mangrove using training samples and then clean up the map; change is tracked by comparing years.
Why this matters to you. At 30 m, a patch smaller than a few pixels (a few hundred square metres) is unreliable, which is why Atlas treats very small polygons with caution.
Related reading
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- What mangroves do for people (ecosystem services)
- What Atlas is, and why it is more than a mangrove map
- LAPSSET: why a corridor project matters to mangroves
- Mangroves in Africa: the big picture
- Sea-level rise and the "coastal squeeze"
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