3D city models
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A 3D city model is digital model of urban areas that represent terrain surfaces, sites, buildings, vegetation, infrastructure and landscape elements in three-dimensional scale as well as related objects (e.g., city furniture) belonging to urban areas. Their components are described and represented by corresponding two- and three-dimensional spatial data and geo-referenced data. 3D city models support presentation, exploration, analysis, and management tasks in a large number of different application domains. In particular, 3D city models allow "for visually integrating heterogeneous geoinformation within a single framework and, therefore, create and manage complex urban information spaces."


Storage

To store 3D city models, both file-based and database approaches are used. There is no single, unique representation schema due to the heterogeneity and diversity of 3d city model contents.


Encoding of components

The Components of 3D city models are encoded by common file and exchange formats for 2D raster-based GIS data (e.g., GeoTIFF), 2D vector-based GIS data (e.g.,
AutoCAD DXF AutoCAD DXF (Drawing Interchange Format, or Drawing Exchange Format) is a CAD data file format developed by Autodesk for enabling data interoperability between AutoCAD and other programs. DXF was introduced in December 1982 as part of AutoCAD ...
), 3D models (e.g.,
.3DS 3DS is one of the file formats used by the Autodesk 3ds Max 3D modeling, animation and rendering software. It was the native file format of the old Autodesk 3D Studio DOS (releases 1 to 4), which was popular until its successor (3D Studio MAX ...
, .OBJ), and 3D scenes (e.g.,
Collada COLLADA (for ''COLLA''borative ''D''esign ''A''ctivity) is an interchange file format for interactive 3D applications. It is managed by the nonprofit technology consortium, the Khronos Group, and has been adopted by ISO as a publicly available ...
,
Keyhole Markup Language Keyhole Markup Language (KML) is an XML notation for expressing geographic annotation and visualization within two-dimensional maps and three-dimensional Earth browsers. KML was developed for use with Google Earth, which was originally named Ke ...
) such as supported by CAD, GIS, and
computer graphics Computer graphics deals with generating images with the aid of computers. Today, computer graphics is a core technology in digital photography, film, video games, cell phone and computer displays, and many specialized applications. A great de ...
tools and systems. All components of a 3D city model have to be transformed into a common geographic coordinate system.


Databases

A database for 3D city models stores its components in a hierarchically structured, multi-scale way, which allows for a stable and reliable data management and facilitates complex GIS modeling and analysis tasks. For example, the 3D City Database is a free 3D geo database to store, represent, and manage virtual 3D city models on top of a standard spatial relational database. A database is required if 3D city models have to be continuously managed. 3D city model databases form a key element in 3D spatial data infrastructures that require support for storing, managing, maintenance, and distribution of 3D city model contents. Their implementation requires support of a multitude of formats (e.g., based on FME multi formats). As common application, geodata download portals can be set up for 3D city model contents (e.g., virtualcityWarehouse).


CityGML

The
Open Geospatial Consortium The Open Geospatial Consortium (OGC), an international voluntary consensus standards organization for geospatial content and location-based services, sensor web and Internet of Things, GIS data processing and data sharing. It originated in 199 ...
(OGC) defines an explicit
XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable ...
-based exchange format for 3D city models,
CityGML CityGML is an open standardised data model and exchange format to store digital 3D models of cities and landscapes. It defines ways to describe most of the common 3D features and objects found in cities (such as buildings, roads, rivers, bridges, ...
, which supports not only geometric descriptions of 3D city model components but also the specification of semantics and topology information.


CityJSON

CityJSON is a JSON-based format for storing 3D city models. It mostly follows the CityGML data model, but aims to be developer- and user-friendly by avoiding most of the complexities of its usual GML encoding. Due to its simple encoding and the use of JSON, it is also suitable for web applications.


Construction


Level of detail

3D city models are typically constructed at various levels of detail (LOD) to provide notions of multiple resolutions and at different levels of abstraction. Other metrics such as the level of spatio-semantic coherence and resolution of the texture can be considered a part of the LOD. For example,
CityGML CityGML is an open standardised data model and exchange format to store digital 3D models of cities and landscapes. It defines ways to describe most of the common 3D features and objects found in cities (such as buildings, roads, rivers, bridges, ...
defines five LODs for building models: * LOD 0:
2.5D 2.5D (two-and-a-half dimensional) perspective refers to gameplay or movement in a video game or virtual reality environment that is restricted to a two-dimensional (2D) plane with little to no access to a third dimension in a space that other ...
footprints * LOD 1: Buildings represented by block models (usually extruded footprints) * LOD 2: Building models with standard roof structures * LOD 3: Detailed (architectural) building models * LOD 4: LOD 3 building models supplemented with interior features. There exist also approaches to generalize a given detailed 3D city model by means of automated generalization. For example, a hierarchical road network (e.g., OpenStreetMap) can be used to group 3D city model components into "cells"; each cell is abstracted by aggregating and merging contained components.


GIS data

GIS data provide the base information to build a 3D city model such as by digital terrain models, road networks, land use maps, and related geo-referenced data. GIS data also includes cadastral data that can be converted into simple 3D models as, for example, in the case of extruded building footprints. Core components of 3D city models form
digital terrain model A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer graphics representation of elevation data to represent terrain or overlaying objects, commonly of a planet, moon, or asteroid. A "global DEM" refers to a discrete g ...
s (DTM) represented, for example, by TINs or grids.


CAD data

Typical sources of data for 3D city model also include CAD models of buildings, sites, and infrastructure elements. They provide a high level of detail, possible not required by 3D city model applications, but can be incorporated either by exporting their geometry or as encapsulated objects.


BIM data

Building information models represent another category of geo-spatial data that can be integrated into a 3D city model providing the highest level of detail for building components.


Integration at visualization level

Complex 3D city models typically are based on different sources of geodata such as geodata from GIS, building and site models from CAD and BIM. It is one of their core properties to establish a common reference frame for heterogeneous geo-spatial and geo-referenced data, i.e., the data need not to be merged or fused based on one
common data model A common data model (CDM) can refer to any standardised data model which allows for data and information exchange between different applications and data sources. Common data models aim to standardise logical infrastructure so that related applicat ...
or schema. The integration is possible by sharing a common geo-coordinate system at the visualization level.


Building reconstruction

The simplest form of building model construction consist in extruding the footprint polygons of buildings, e.g., taken from the cadaster, by pre-compute average heights. In practice, 3D models of buildings of urban regions are generated based on capturing and analyzing 3D
point cloud Point or points may refer to: Places * Point, Lewis, a peninsula in the Outer Hebrides, Scotland * Point, Texas, a city in Rains County, Texas, United States * Point, the NE tip and a ferry terminal of Lismore, Inner Hebrides, Scotland * Poin ...
s (e.g., sampled by terrestrial or aerial laser scanning) or by photogrammetric approaches. To achieve a high percentage of geometrically and topologically correct 3D building models, digital terrain surfaces and 2D footprint polygons are required by automated building
reconstruction Reconstruction may refer to: Politics, history, and sociology *Reconstruction (law), the transfer of a company's (or several companies') business to a new company *'' Perestroika'' (Russian for "reconstruction"), a late 20th century Soviet Unio ...
tools such as BREC. One key challenge is to find building parts with their corresponding roof geometry. "Since fully automatic image understanding is very hard to solve, semi-automatic components are usually required to at least support the recognition of very complex buildings by a human operator." Statistical approaches are common for roof reconstruction based on airborne laser scanning point clouds. Fully automated processes exist to generate LOD1 and LOD2 building models for large regions. For example, the Bavarian Office for Surveying and Spatial Information is responsible for about 8 million building models at LOD1 and LOD2.


Visualization

The
visualization Visualization or visualisation may refer to: * Visualization (graphics), the physical or imagining creation of images, diagrams, or animations to communicate a message * Data visualization, the graphic representation of data * Information visuali ...
of 3D city models represents a core functionality required for interactive applications and systems based on 3D city models.


Real-time rendering

Providing high quality visualization of massive 3D city models in a scalable, fast, and cost efficient manner is still a challenging task due to the complexity in terms of 3D geometry and textures of 3D city models.
Real-time rendering Real-time computer graphics or real-time rendering is the sub-field of computer graphics focused on producing and analyzing images in real time. The term can refer to anything from rendering an application's graphical user interface ( GUI) to ...
provides a large number of specialized 3D rendering techniques for 3D city models. Examples of specialized real-time 3D rendering include: * Real-time 3D rendering of road networks on high resolution terrain models. * Real-time 3D rendering of water surfaces with cartography-oriented design. * Real-time 3D rendering of day and night sky phenomena. * Real-time 3D rendering of grid-based terrain models. * Real-time 3D rendering using different levels of abstraction, ranging between 2D map views and 3D views. * Real-time 3D rendering of multiperspective views on 3D city models. Real-time rendering algorithms and data structures are listed by the virtual terrain project.


Service-based rendering

Service-oriented architecture In software engineering, service-oriented architecture (SOA) is an architectural style that focuses on discrete services instead of a monolithic design. By consequence, it is also applied in the field of software design where services are provide ...
s (SOA) for visualizing 3D city models offer a separation of concerns into management and rendering and their interactive provision by client applications. For SOA-based approaches, 3D portrayal services are required, whose main functionality represents the portrayal in the sense of 3D rendering and visualization. SOA-based approaches can be distinguished into two main categories, currently discussed in the
Open Geospatial Consortium The Open Geospatial Consortium (OGC), an international voluntary consensus standards organization for geospatial content and location-based services, sensor web and Internet of Things, GIS data processing and data sharing. It originated in 199 ...
: * Web 3D service (W3DS): This type of service handles geodata access and mapping to computer graphics primitives such as scene graphs with textured 3D geometry models as well as their delivery to the requesting client applications. The client applications are responsible for the 3D rendering of delivered scene graphs, i.e., they are responsible for the interactive display using their own 3D graphics hardware. * Web view service (WVS): This type of service encapsulates the 3D rendering process for 3D city models at the server side. The server generates views of the 3D scene or intermediate, image-based representations (e.g., virtual panoramas or G-buffer cube maps), which are streamed and uploaded to requesting client applications. The client applications are responsible for re-construction the 3D scene based on the intermediate representations. Client applications do not have to process 3D graphics data, but to provide management for loading, caching, and displaying the image-based representations of 3D scenes and do not have to process the original (and possibly large) 3D city model.


Map-based visualization

A map-based technique, the "smart map" approach, aims at providing "massive, virtual 3D city models on different platforms namely web browsers, smartphones or tablets, by means of an interactive map assembled from artificial oblique image tiles."J. Klimke et al.:
Scalable Multi-Platform Distribution of Spatial 3D Contents
. ISPRS 8th 3D GeoInfo Conference & WG II/2 Workshop 27–29 November 2013, Istanbul, Turkey, (U. Isikdag, ed.), vol. II-2/W1, ISPRS Annals, ISPRS, pp. 193-200, 2013.
The map tiles are synthesized by an automatic 3D rendering process of the 3D city model; the map tiles, generated for different levels-of-detail, are stored on the server. This way, the 3D rendering is completely performed on the server's side, simplifying access and usage of 3D city models. The 3D rendering process can apply advanced rendering techniques (e.g., global illumination and shadow calculation, illustrative rendering), but does not require client devices to have advanced 3D graphics hardware. Most importantly, the map-based approach allows for distributing and using complex 3D city models with having to stream the underlying data to client devices - only the pre-generated map tiles are sent. This way, "(a) The complexity of the 3D city model data is decoupled from data transfer complexity (b) the implementation of client applications is simplified significantly as 3D rendering is encapsulated on server side (c) 3D city models can be easily deployed for and used by a large number of concurrent users, leading to a high degree of scalability of the overall approach."


Applications

3D city models can be used for a multitude of purposes in a growing number of different application domains. Examples: *
Navigation system A navigation system is a computing system that aids in navigation. Navigation systems may be entirely on board the vehicle or vessel that the system is controlling (for example, on the ship's bridge) or located elsewhere, making use of radio or othe ...
s: 3D navigation maps have become omnipresent both in automotive and pedestrian navigation systems, which include 3D city models, in particular, terrain models and 3D building models, to enhance the visual depiction and to simplify the recognition of locations. *
Urban planning Urban planning, also known as town planning, city planning, regional planning, or rural planning, is a technical and political process that is focused on the development and design of land use and the built environment, including air, water, ...
and
architecture Architecture is the art and technique of designing and building, as distinguished from the skills associated with construction. It is both the process and the product of sketching, conceiving, planning, designing, and constructing building ...
: To set up, analyze, and disseminate urban planning concepts and projects, 3D city models serve as communication and participation medium. 3D city models provide means for project communication, better acceptance of development projects through visualization, and therefore avoid monetary loss through project delays; they also help to prevent planning errors. * Spatial data infrastructures (SDIs): 3D city models extend spatial data infrastructures and support the management, storage, and usage of 3D models within SDIs; they require not only tools and processes for the initial construction and storage of 3D city models but also have to provide efficient data management and data distribution to support workflows and applications. * GIS: GIS support 3D geodata and provide computational algorithms to construct, transform, validate, and analyze 3D city model components. * Emergency management: For emergency, risk, and disaster management systems, 3D city models provide the computational framework. In particular, they serve to simulate fire, floodings, and explosions For example, the DETORBA project aims at simulating and analyzing effects of explosion in urban areas at high precision to support prediction of effects for the structural integrity and soundness of the urban infrastructure and safety preparations of rescue forces. *
Spatial analysis Spatial analysis or spatial statistics includes any of the formal techniques which studies entities using their topological, geometric, or geographic properties. Spatial analysis includes a variety of techniques, many still in their early deve ...
: 3D city models provide the computational framework for 3D spatial analysis and simulation. For example, they can be used to compute solar potential for 3D roof surfaces of cities, visibility analysis within the urban space, noise simulation,
thermographic inspection Thermographic inspection refers to the nondestructive testing (NDT) of parts, materials or systems through the imaging of the temperature fields, gradients and/or patterns ("thermograms") at the object's surface. It is distinguished from medical ...
s of buildings * Geodesign: In geodesign, virtual 3D models of the environment (e.g., landscape models or urban models) facilitate exploration and presentation as well as analysis and simulation. *
Gaming Gaming may refer to: Games and sports The act of playing games, as in: * Legalized gambling, playing games of chance for money, often referred to in law as "gaming" * Playing a role-playing game, in which players assume fictional roles * Playin ...
: 3D city models can be used to obtain base data for virtual 3D scenes used in online and video games. * Cultural heritage: 3D city model tools and systems are applied for modeling, design, exploration, and analysis tasks in the scope of cultural heritage. For example, archeological data can be embedded in 3D city models. * City information systems: 3D city models represent the framework for interactive 3D city information systems and 3D city maps. For example, municipalities apply 3D city models as centralized information platform for location marketing. *
Property management Property management is the operation, control, maintenance, and oversight of real estate and physical property. This can include residential, commercial, and land real estate. Management indicates the need for real estate to be cared for and monit ...
: 3D city model technology can extend systems and applications used in real-estate and property management. * Intelligent transportation systems: 3D city models can be applied to intelligent transportation systems. * Augmented reality: 3D city models can be used as reference frame for augmented reality applications.


See also

*
City map A city map is a large-scale thematic map of a city (or part of a city) created to enable the fastest possible orientation in an urban space. The graphic representation of objects on a city map is therefore usually greatly simplified, and reduce ...


References

{{Reflist


External links


3D City Model Systems and Tools
Management and infrastructure components for 3D city models.
Map-based Visualization of 3D City Models
Components for 3d city model applications.
OGC 3D Portrayal IE
3D portrayal interoperability experiment of the Open Geospatial Consortium.

Example of a massive 3D city models for an urban area.

Example of a 3D city model for cultural heritage applications. Applications of geographic information systems Earth sciences graphics software Geographic data and information 3D computer graphics Maps of cities