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KNOWLEDGE BASE

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.

Limits. All satellite maps contain errors, especially at forest edges, in narrow creeks and for small patches.

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