In just over a year it should be finished: the new land cover map that VITO is developing as part of ESA’s WorldCover project. By improving the resolution from 100 to 10 metres, many more details will become visible, ensuring that the map will meet the requirements of its many users even better.
Every year, VITO creates a new land cover map with global coverage. It does this on behalf of the EU’s Copernicus Earth Observation Programme, with the data being supplied by the European Space Agency, the ESA. The resolution of these maps is 100 metres. As a result of this, large fields, forests and urban districts can be distinguished, but smaller agricultural parcels and individual trees, roads and buildings cannot.
However, in 2021 the resolution of the map will be increased tenfold, with pixels that match squares of 10 by 10 metres on the ground. VITO was tasked with developing this map by the ESA within the scope of the WorldCover project, which runs for two years and finishes in September 2021. “This will allow us to see individual trees rather than forests, plus we will be able to distinguish the roofs of houses and buildings – all of this on a global scale,” says Ruben Van De Kerchove of VITO. “The amount of detail will therefore increase massively, and processing all of this is a very big challenge.”
Checking through human eyes
Even though the methodology of the data processing will largely remain the same, quite a few modifications are still required. The legend will have to be modified and the algorithms in complex environments (like cities) will have to recognise many more objects. “One example is a green roof: that should be marked on the map as a building, not as vegetation,” says Van De Kerchove. “So the algorithms need to be properly trained.” In collaboration with partners, VITO has marked out a total of 150,000 labelled points across the whole world for this. “Each point matches an area of 100 by 100 metres and is further subdivided into a hundred points of 10 by 10 metres. Each of these points is checked visually through human eyes.”
The increased resolution also leads to a massive increase in the amount of data that needs to be processed. Until recently, VITO did this mainly using its own computer clusters, but the new map involves one hundred times as much data. “We’re working with big data here; the amount of input data is around 2 to 3 petabytes. We process this in the cloud.” This does come with a price tag, so VITO is trying to minimise the processing time. “This is a technical exercise in which we want to have our algorithms run as quickly as possible without loss of quality. Every second in the cloud costs money.”
Update in real time
The data for the 10-metre map is supplied by the European Sentinel satellites. Sentinel-2 (which is actually a combination of two satellites) passes the same location above the earth’s surface every five days. This will allow the land cover map to be updated almost in real time. The satellites also have a large range: they can photograph a strip 290 kilometres wide in a single image. Van De Kerchove: “So a country like Belgium can be almost fully covered by a single image, and this can be done every five days.”
The Sentinel-1 satellites (which are also two) can help if areas are cloudy for prolonged periods of time. In contrast to Sentinel-2, which only takes pictures in the visible and infrared spectra, Sentinel-1 has an active sensor that essentially probes the earth’s surface with radar waves. “This allows us, for example, to distinguish water from vegetation, and urban areas from rural areas – we do this based on the extent to which the waves are scattered and reflected. However, these radar images have quite a lot of noise, so we always combine them with the optical images from Sentinel-2.”
In close consultation with users
Although the results of WorldCover will be publicly available, the map will be mainly used by international organisations. One example of these is UNCCD, the United Nations Convention to Combat Desertification. “Countries and organisations can use the WorldCover map to monitor themselves and others with regard to land cover – and they can check, for example, that soil degradation is not increasing.”
The map is also a welcome tool for climate research, as well as for monitoring biodiversity and food security. As every user has its own wishes and priorities, VITO is developing the 10-metre map in close consultation with the users. “A climate modeller, for example, will benefit more from stable maps on which the details remain the same as much as possible, while a biodiversity researcher would want to zoom in on those details. We want to satisfy all of these requirements,” explains Van De Kerchove.
