Seeing cyclone damage through the clouds

Cyclone Idai hit Beira on 14 March 2019. Clouds hid the city from optical satellites for days, but radar sees through them. Drag the slider to compare the changes detected by Sentinel-1 radar (left) with the damage that analysts later tagged by hand on optical images (right).

Radar changes across the city, by period

Most affected cells (count per 115 m cell)

Only the most affected 35% of cells are coloured, ranked separately for radar (using the landfall week) and for the manual tags, since the two count on different scales. Only changes and tags on or next to a house (OpenStreetMap building footprints) are counted.

How this map was made

We compared pairs of Sentinel-1 radar images taken 6–12 days apart and flagged pixels whose radar signal changed significantly (95% level). Then we counted the flagged pixels that fall on houses in each 115 m cell of Beira's residential area.

The manually tagged damage comes from UNOSAT and REACH, who marked damaged houses on optical images taken on 26 March. The two measures agree most strongly in the period of the cyclone. Across cells, the rank correlation is about 0.55, compared with about 0.2 in normal periods. Radar can't replace manual tagging, but it gives a first picture of the damage days earlier.

With David Malmgren-Hansen (DTU) and the World Bank. Read the paper.

Radar change

Tagged damage

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