Greenery around the home

What we use

The NDVI vegetation index computed on Copernicus Sentinel-2 imagery, the European Union satellites that cover Italy every five days. The index compares the red and infrared light reflected by the ground: it rises the more the vegetation is alive and dense. Each measurement covers a square 10 metres across. Copernicus data is free and open.

How we use it

For each month of the last year we work out the average greenness in the circle of 500 metres around the address, which is about 7,850 points of ten metres each. The cycle of the seasons shows what kind of area it is:

• where greenery is abundant and steady there are trees, parks or woodland;
• where it drops sharply in summer it is mostly grass or farmland;
• where it stays low throughout it is nearly all built up, paved or bare.

The report says how green it is, whether the seasonal change is marked, which month was the greenest and which the barest, and shows the twelve-month series.

On large plots we measure the plot itself too. When the parcel is over 2,000 square metres, the report adds a reading of the land itself: there the question changes, namely whether the land is worked (it grows and is harvested, with a clear season) or idle (the same all year). Below that size the plot is not measured, because the satellite would have too few points and would end up measuring the road and the hedges around it.

Clouds do not count: every image carries a mask saying which points are cloud, shadow or snow, and those points are left out. A month with less than 60% visible is dropped.

When the report does not answer, and why

• There are fewer than eight months with clear skies: the seasonal cycle cannot be seen, and without it saying "it is green here" would be guesswork. It happens in the Alps more than elsewhere.
• The satellite service does not respond: the block says so and is redone on the next report.

Limits

• The block describes the colour seen from above. Whether the land can be farmed is answered by the soil and the water, and what may be built on it by the municipal planning scheme.
• A freshly mown meadow and abandoned land in August look much alike: it is the yearly cycle that tells them apart, not a single month.
• The satellite sees the roof, not what is under it: the greenness measured is that of gardens, courtyards, street trees and open areas.
• The 500 metre circle does not respect boundaries: if the address sits at the edge of a park, the park's greenery counts even though the park is not yours. That is deliberate, because it is what you see from the window.

Official page: Copernicus Data Space Ecosystem.