3D city models on the up

By Matthias Merian for laser scanning

Detailed modeling of a church from terrestrial laser scan data. Brenner Claus Brenner / IKG Hannover
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Whether tourism or city marketing, architecture or traffic planning: virtual three-dimensional models are now used in many areas. Thus, mobile operators plan their networks, architects visualize planned construction projects and the city administration accelerates approval procedures. The advantages are obvious: virtual models make it possible to make location-specific decisions without actually having to be there.

Three-dimensional views of the city are not inventions of the modern age: More than 350 years ago, Mattias Merian the Elder created in his firm more than 2, 150 views of European cities as copperplate engraving. However, it was not necessarily the truthful and dimensionally accurate image in the foreground, but rather a favorable representation - at the time already for the purpose of city marketing.

Unlike at the time, however, today the greatest possible accuracy of the models is in the foreground. In the meantime, a large number of modern measuring platforms such as satellites or near-range sensors, which are also suitable for the measurement of millimeter-sized structures, are now available for recording reality. "Aerial photogrammetry is the most widely used measurement method for capturing three-dimensional city models, " explains Claus Brenner from the Institute of Cartography and Geoinformatics at the University of Hanover. "Images are taken from an airplane in such a way that every point on the ground is visible in several images. Similar to human stereo vision, three-dimensional information is derived from this, "adds Brenner.

In addition to this process, which is already 150 years old in its principles, airborne laser scanning has recently become established, which detects the surface directly in three dimensions by means of a laser beam. "Due to the very fast measurement of up to 100, 000 points per second, very extensive data sets are created, so-called point clouds. These are then converted into geometric models, "Brenner explains the procedure. "At close range, we use analog terrestrial scanners, which can capture facades and buildings in great detail with several million points, " says Brenner.

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Modeling a building from airborne laser scan data. © Claus Brenner / IKG Hannover

The biggest effort in the subsequent creation of virtual 3D city models is in the modeling. Depending on the application and the amount of data, different "Levels of Detail" (LOD) can be calculated. These range in increasing complexity from simple flat roof representations over detailed roofs with fine structures such as dormers and chimneys to virtually accessible architectural models, Brenner describes the possibilities. Due to the high cost, however, the survey of cities is usually limited to complex roof landscapes with a more differentiated modeling of selected buildings. display

Current research questions deal with the automation of data acquisition and the fully automatic transition from detailed to less detailed models, the so-called generalization. In recent decades, however, the reliable automation of the collection has proven to be very complex.

Future of virtual three-dimensional city models

The systematic acquisition of three-dimensional city models is currently steadily increasing. For example, in the coming years, Switzerland will completely convert the capture and storage of all topography data to 3D models. Also, a directive adopted by the EU in 2002 to reduce ambient noise demands precise information on noise pollution, which can not be derived without 3D models, "Brenner describes the practical use of the models. Lastly, vehicle navigation systems are already on the market in Japan that display three-dimensional city models. Similar developments are currently taking place in Europe, "says Brenner. Accordingly, the demand for uniform, up-to-date and comprehensive city models will probably continue to increase in the future.

(Dr. Claus Brenner / University of Hannover, 12.05.2006 - AHE)