Summary: In this article, authors Jantien Stoter, Joris Goos and Athina Trakas describe CityGML and take a look at how National Spatial Data Infrastructure stakeholders in The Netherlands worked together to establish a national 3D standard that aligns to both the existing national 2D standards and the international OGC standard for 3D geoinformation, CityGML.
Adding a third dimension to the National Spatial Data Infrastructure (NSDI) makes a lot of sense in a heavily populated country where two-thirds of the land is vulnerable to flooding. The OGC City Geography Markup Language (CityGML) Encoding Standard is already in use in spatial data infrastructure programs in Germany, France, Malaysia, Abu Dhabi and other countries, where it provides an important platform for the transition from 2D to 3D data. The Netherlands, however, is the first country to have made CityGML a national standard.
In this article we describe CityGML and look at how NSDI stakeholders in The Netherlands worked together to establish a national 3D standard that aligns to both the existing national 2D standards and the international OGC standard for 3D geoinformation, CityGML.
CityGML: an open standards platform for 3D geoinformation
As populations concentrate in cities around the world, governments have an increasing mandate to manage urban resources, waste streams, risks, traffic flows, zoning, energy usage and more. Many of these management issues, such as storm water and wastewater management, energy usage, and pedestrian traffic involve 3D and 4D (temporal) information, and thus 3D geoinformation becomes increasingly important.
Economic growth is another important driver for adding a 3D component to an NSDI. A detailed, free-to-the-public, always-available, easy-to-update 3D dataset for a downtown business district, for example, provides a foundation for countless applications to support activities such as tourism, public safety, urban planning, building renovations, utility and street repairs, and location-based marketing. New jobs are thus created in the information technology (IT) sector and efficiencies and new capabilities are realized in all of the sectors served by IT. Many emerging areas of information technology innovation involve spatial information, including augmented reality (AR), sensor webs, building information models (BIM), Smart Grid, ubiquitous computing, location-based marketing, crowdsourcing and others. These are all part of visions such as Smart Cities, Smarter World, and the Internet of Things. Each of these areas of innovation offers potential benefits for citizens, consumers, and sustainable and resilient economies and societies. Each of them, where they involve spatial data, shares a dependency on open access to a rich infrastructure of 3D data and services.
SDIs depend on open standards for ICT interfaces and encodings, and such standards for geospatial data and geospatial information systems must address a wide range of requirements. Their complexity derives from the diversity of geodesy systems (measurement of the Earth and Earth coordinate systems), 2D vector geographic information systems, 2D raster-based geographic information systems, 3D representations of natural and built objects, Earth imaging systems, navigation systems, mapping systems and systems for naming and describing geospatial features and phenomena. 3D data includes both geospatial and "building spatial" data; that is, data about the size, shape, appearance, function and content of buildings and physical infrastructure elements such as streets, bridges, pipes and wires.
Figure 1: Though most people think of 3D cityscapes when they think "3D," an urban 3D database contains important information about many out-of-sight systems under streets and in buildings that support life in the city. (Cartoon from presentation by Richard M. Vogel, Tufts University, CUAHSI Web Seminar, November 5, 2010: "Hydromorphology: The Shape of our Water Future." R. Crumb artist?)
CityGML is an information model and eXtensible Markup Language (XML)-based encoding for the representation, storage and exchange of virtual 3D city and landscape models. It provides a standard model and mechanism for describing 3D objects with respect to their geometry, topology, semantics and appearance. It defines five different levels of detail. CityGML is highly scalable and datasets can include a very wide variety of different urban entities. Thus it supports the general trend toward modeling individual buildings and urban landscape features, but also whole sites, districts, cities, regions and countries.
CityGML allows users to share virtual 3D city and landscape models for sophisticated analysis and display tasks in application domains such as environmental simulations, energy demand estimations, city lifecycle management, urban facility management, real estate appraisal, disaster management, pedestrian navigation, robotics, urban data mining and location-based marketing. Because CityGML is based on the OGC Geography Markup Language Encoding Standard (GML), it can be used with the whole family of OGC Web service interface and encoding standards for accessing, processing and cataloging geospatial data and sensor web data. CityGML also plays an important role in bridging urban information models with building information models to improve interoperability among information systems used in the design, construction, ownership and operation of buildings and capital projects.
3D Pilot NL
Much can be learned from the Dutch approach in using policy and other measures to accelerate the implementation and uptake of open standards for data, interfaces and encodings.
The leading organizations of the Dutch geospatial standard effort recognized the importance of enlisting the support and cooperation of a broad base of technology providers and users in the public, private, academic and research sectors. The four national organizations that helped establish this collaboration network are the Kadaster, Geonovum [the NSDI executive committee in the Netherlands], the Netherlands Geodetic Commission and the Dutch Ministry of Infrastructure and Environment. Their "3D Pilot NL" brought together over 65 private, public and scientific organizations to advance 3D developments in The Netherlands.
The Netherlands has well-established national standards on geo-domain models, but as in most countries, they are all 2D. The new 3D standard preserves valuable 2D concepts from the existing national standard for large-scale topography (information model geography (IMGeo)), and extends them with 3D concepts from CityGML. The 3D standard is therefore not just another standard on geoinformation; instead the realized CityGML implementation profile bridges the 2D and 3D standardization developments.
The pilot sought to create a test bed based on use cases related to a predefined test area in order to find consensus on a 3D standard NL. In January 2010, more than 45 organizations responded to the call for participation. Because the pilot received a lot of attention during its course, the number of participating organizations grew to about 65. Those organizations consisted of (large) municipalities, provinces, universities, GIS and DBMS vendors, 3D data suppliers, and engineering companies. Participation was not limited to The Netherlands.
Four work packages (WPs) were defined, and alignment of the participants’ interests in these work activities was important because no budget was available for individual contributions. Intermediate results were exchanged during plenary sessions every six to seven weeks. Social media were used to enhance the collaboration. Currently, the 3D Pilot NL LinkedIn group counts about 540 members and the 3D Pilot community has over 100 organizations.
Policy changes were important, because simply creating awareness may not be sufficient to ensure rapid uptake of the standard. Government data and system procurement requests for quotes were written to include specific requirements for standards. There is a legal requirement for government-funded data producers to "comply [with standards] or explain why." Approved open standards are on an officially endorsed list, and these standards are mandatory for public parties under "comply or explain" conditions. Since November 2011, IFCs (building information model standards) for 3D-BIM are included along with other SDI data layers.
A major result of the pilot was the proof of concept for a 3D SDI, covering acquisition, standardization, storage and use of 3D data in various use cases, including airstream simulation, 3D cadastre (as in high rise condominium developments), and planning and management of underground and aboveground municipal assets.
Also, it was demonstrated that 3D information automatically generated from laser point data -- tree heights and sizes, new buildings, roofs, etc. -- can easily become part of an OGC CityGML model.
The findings of the pilot were formally established in a national 3D standard realized as a CityGML Application Domain Extension (ADE). The ADE completely integrates the CityGML with a new version of the existing national IMGeo. IMGeo contains object definitions for large-scale representations of roads, water, land use/land cover, bridges, tunnels, etc. and prescribes 2D point, curve or surface geometry for all objects. As the new version of IMGeo is completely integrated with CityGML, IMGeo version 2.0 also facilitates extensions to 2.5D representations (i.e. as height surfaces; equivalent to the coarsest CityGML Level of Detail (LOD)) and 3D volumetric (CityGML LOD1, LOD2 and LOD3) representations of the objects according to geometric and semantic principles of CityGML.
3D Kadaster (Dutch Kadaster and Bentley)
Tree model generated by Alterra for use case 3D topography
Calculation of soil volumes (voxels) at the location of a planned tunnel, by Esri and TNO
Real-time interactive airstream simulation with voxels, Alterra
Spatial planning objects in CityGML, Crotec
3D change detection, NEO BV
Integrating design models in virtual environments, Gemeente Apeldoorn
3D Change detection based on differences between two point clouds, U Twente
Figure 2:Some of the use cases explored in 3D Pilot NL (Figure from Stoter, JE; Vosselman, G; Goos, J; Zlatanova, S; Verbree, E; Klooster, R (extern) & Reuvers, M (2011). Toward a national 3D spatial data infrastructure: case of the Netherlands. Geoinformation: Organ der Deutschen Gesellschaft für Photogrammetrie und Fernerkundung e.V., 2011(6), 405-420)
Topics were identified that require further attention before CityGML ADE IMGeo 2.0 can be widely implemented. These open issues are currently being studied in a follow-up project of the 3D Pilot.
In The Netherlands as elsewhere, there is a tremendous need for knowledge and expertise on standards at all levels, and the outreach done by the national partners of the 3D Pilot helped to raise awareness of this deficit. Many members of the OGC are calling for more training and education about open standards, as well as for improved lifecycle management of standards and research on a growing list of interoperability topics.
These are ongoing topics of discussion in the OGC. Users of the standards are invited to participate in these discussions so that they can plan accordingly and influence the roadmaps for standards development.
Editor’s Note: This article is abstracted from a longer article, "Advancing Open 3D Modelling Standards in National Spatial Information Policy," by P. Janssen, J.E. Stoter and A. Trakas that will soon appear in the ePractice Journal.
Authors: Jantien Stoter, TU Delft, Kadaster and Geonovum, the National Spatial Data Infrastructure (NSDI) executive committee in The Netherlands; Joris Goos, City of Rotterdam; Athina Trakas, Open Geospatial Consortium (OGC)