Researchers watch the cities grow with 4D point clouds

Researchers watch the cities grow with 4D point clouds

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Researchers from the Technical University of Munich (TUM) have made a world record by making four-dimensional point clouds of Berlin, Las Vegas, Paris and Washington, D.C. from images stacks of the TerraSAR-X radar satellite.

Germany: Researchers from the Technical University of Munich (TUM) have made a world record by making four-dimensional point clouds of Berlin, Las Vegas, Paris and Washington, D.C. from images stacks of the TerraSAR-X radar satellite.

To make the record, Prof. Xiaoxiang Zhu and her team had to note three million measurement points in one square kilometer area. This is important because according to the UN, today approximately half of the world’s population is already living in cities. If the growth continues, then by 2050, the figure is expected to grow to two thirds of the world’s population.

“This growth places high demands on building and infrastructure safety, since damage events can endanger thousands of human lives at once,” says Xiaoxiang Zhu, Professor for Signal Processing in Earth Observation at TUM.

Together with her team she has developed a method for early detection of potential dangers: for example, subterranean subsidence can result in the collapse of buildings, bridges, tunnels or dams. The new method makes it possible to detect and visualize changes as small as one millimeter per year.

The data for the new urban images come from the German TerraSAR-X satellite, the most high resolution civilian radar satellite in the world. Orbiting the earth at an altitude of approximately 500 kilometers since 2007, the satellite sends microwave pulses to the earth and then collects their echoes.

“Initially these measurements are only in a two dimensional image with a resolution of one meter,” Zhu explains. The TUM professor collaborats with the German Aerospace Center (DLR), where she is also in charge of her own working group. The DLR is responsible for operating and use of the satellite for scientific purposes.

“The significance of the images is limited by the fact that reflections from different objects that are at an equal distance from the satellite will layover with each other. This effect reduces the three-dimensional world to a two-dimensional image.”

Not only has Zhu developed her own algorithm which makes it possible to reconstruct the third and even fourth (time) dimension, she set a world record at the same time: Four dimensional point clouds with a density of three million points per square kilometer are reconstructed. This rich retrieved information makes it possible to generate highly precise four-dimensional city models.

The fourth dimension

Since the images are taken at different times, it’s possible to estimate the change over time – the fourth dimension – at the order of a fraction of the radar wavelength.

The resulting 4-D model reveals the tiny changes with a precision on the order of approximately one millimeter per year, for example the thermal expansion of buildings in the summer or deformations resulting from subsidence below the earth’s surface.

Zhu explains, “The method is suitable for the detection of danger points. Satellite technology can thus make an important contribution to making our urban infrastructure safer.”

In the future the researchers even intend to watch major cities grow. In the recently launched ERC project “So2Sat”, all the world’s metropolitan areas are being mapped and observed on a long-term basis.

The investigations are centered on newly industrializing countries, where entire urban neighborhoods spring up almost overnight. Zhu and her team plan to use various Big Data sources for the first time: Measurements from satellites will be fused with map information from Open Street Map and the practically unlimited stream of images, text and activity patterns provided by social networks.