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Global Urban Footprint Revealed in Unprecedented Resolution

Mapping urban areas is fraught with difficulty. But a database of radar images taken from space throws new light on the global urban footprint.

Urbanization is changing our planet. Today, about half the world’s seven billion people live in cities. By 2050, that proportion is expected to rise to two-thirds. And because the global population is increasing, by then, more than six billion people will live in cities.

Urban living places significant burdens on the environment. Cities need to be fed with water, power, and food, and so require the complex infrastructures that make that possible. All this needs to be done in a way that is sustainable.

And that raises some important questions. How many cities are there on Earth, and how much of the surface area is covered with built-up areas?

In this respect, developed countries are well mapped, but the same is not true of the developing world, where a significant proportion of people live. What’s more, attempts to spot urban spaces using images taken from space are bedeviled with problems: low resolution images, cloud cover, and ambiguities in the way the data is interpreted.

What’s needed is a way of mapping urban areas from space at high resolution and in a way that is unaffected by cloud cover and without the ambiguities that traditional imaging introduces.

Today, Thomas Esch from the German Aerospace Center in Wessling, Germany, and a few pals unveil a global map of urbanization that meets all these requirements. The result is a data set of the entire planet at a resolution of 12 meters that maps the global urban footprint with unprecedented accuracy and resolution.

“This paper introduces a new inventory of human presence on Earth ... that reflects the human settlements pattern in a so far unique spatial resolution,” says the team.

Until now, estimates of the global urban footprint have been largely based on optical images of the ground. These images are widely available but have significant drawbacks.

For a start, the images have a spatial resolution measured in hundreds of meters. That immediately raises questions over their ability to spot small rural villages and to resolve undeveloped areas in cities.

Cloud cover also obscures the ground so up-to-date coverage can be spotty in places. And then there is the question of how to identify built-up areas at all. There is no unambiguous way of spotting built-up areas in images of this kind.

As a result, geographers’ estimates of the global urban footprint differ in size by an order of magnitude. Clearly a better approach is needed.

Enter Esch and co, who have created a global database of built-up areas based on synthetic aperture radar images from an Earth-orbiting mission called TanDEM-X. This consists of a pair of spacecraft that have been orbiting Earth in close formation—just a few hundred meters apart—since 2007.

These spacecraft take radar images of the ground from slightly different angles, allowing researchers to create a 3-D map of the planet. In total, Esch and co have processed 470,000 pairs of images to create their map of the entire planet.

This map has a spatial resolution of 12 meters, but because it is in 3-D, the map reveals changes in vertical elevation, too. Indeed, it is a straightforward matter to spot vertical structures such as walls.

That immediately makes the task of automatically identifying built-up areas much easier. And because radar passes straight through clouds, the data is unaffected by cloud cover.

The result is a map of the global urban footprint of unprecedented accuracy and resolution. “We introduce a novel global settlements mask in a so far unique spatial resolution, the Global Urban Footprint,” say Esch and co.

That’s an impressive piece of work and one that has significant implications for the way geographers, sociologists, and policymakers will understand the way urbanization is changing our planet.

In 1950, the global rural population was twice the size of the urban one. In 2008, the urban population over took rural population for the first time and the rate of urbanization continues to gain pace.

The first step in understanding the impact of this process is to produce an accurate map. Now the difficult process of counting the number of cities and measuring the proportion of urban land use can begin in earnest.

The map is available at: https://geoservice.dlr.de/web/maps/eoc:guf:4326

Ref: arxiv.org/abs/1706.04862: Breaking New Ground In Mapping Human Settlements From Space—The Global Urban Footprint

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