Section IV CityGML in Detail – Part 2...Section IV Section IV CityGML in Detail – Part 2 Prof....
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Section IV
Section IVCityGML in Detail – Part 2
Prof. Dr. Thomas H. Kolbe
Institute for Geodesy and Geoinformation ScienceBerlin University of [email protected]
May 2008
EduServ6 Course on CityGML
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Section IV2 T. H. Kolbe – Lectures on CityGML
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Section IV3 T. H. Kolbe – Lectures on CityGML
Overview
Section I
Introduction: Urban Information Modelling
CityGML Overview and Status
OGC Geography Markup Language (GML)
Section II
Further GML Concepts and Application Modelling
Section III – CityGML Details, Part 1
Section IV – CityGML Details, Part 2
Section V
Extending CityGML
Application Examples
Section VI
Relations to Other Standards
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Section IV4 T. H. Kolbe – Lectures on CityGML
Details
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Section IV5 T. H. Kolbe – Lectures on CityGML
Water Bodies
CityModel ExternalReference
gml:_AbstractFeature+name[0..*]: String
* *
WaterBody+ function[0..*]
CityObject
gml:Solid
gml:MultiCurveLoD0CurveGeometry
LoDXSolidGeometry,x in {1..4}
gml:_Surface LoDXGeometry, x in {1..4}
WaterSurface+ waterLevel[0..1]
GroundSurface ClosureSurface
LoDXGeometry, x in {1..4}
* **
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Section IV6 T. H. Kolbe – Lectures on CityGML
Illustration of a Water Body
Water
WaterSurface
GroundSurface
ClosureSurface
WaterBody
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Section IV7 T. H. Kolbe – Lectures on CityGML
Closure Surfaces
„Seal open 3D objects“in order to be able to compute their volumes
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Section IV8 T. H. Kolbe – Lectures on CityGML
Grouping of CityObjects
Feature type CityObjectGrouphas arbitrary CityObjects as members
CityObjectGroup is a CityObjectcan become again member of another groupevery member can denote its role in a group
usable for user-defined aggregationse.g. results of classifications or selection
usable also to group CityObjects wrt. somefunction or area, e.g.
city districts, building storeys, or evacuation areas
<<Feature>>_CityObject
*
CityObjectGroup
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Section IV9 T. H. Kolbe – Lectures on CityGML
CityGML Extensions to Standard GML3
Implicit geometries (Prototypic shapes)Shape of a 3D object in local coordinatesInstancing at anchor points (+ further transformations)
Surface MaterialsColors, Textures (adopted from X3D & COLLADA)Appearance information can be assigned to any surface
Both are concepts used in scene graphsdirectly transformable to VRML, X3D, U3D etc.however only simple & limited extensionstailored to the demand of 3D city modelseasy to support by exporting / importing applications
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Section IV10 T. H. Kolbe – Lectures on CityGML
Prototypic 3D Objects
3D city models often contain large numbersof geoobjects of identical shape but at different locations
Examples: trees, traffic lights, street lamps, benches, etc.
in GML3, all geometries have absolute coordinates
every copy / instance would have to be explicitlyrepresented
CityGML: Implicit GeometriesSeparation of shape definition and georeferencing (anchor point + transform.)Comparable to scene graph concepts
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Section IV11 T. H. Kolbe – Lectures on CityGML
City Furniture
CityFurniture
+class[0..1] +function[0..*]
relativeG
eometry
*0..1
0..1
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Section IV12 T. H. Kolbe – Lectures on CityGML
Appearance Model
explicit texture coodinatesgeoreferenced texturesparameterized texturesmaterial
multiple appearances per object (~ themes)
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Section IV13 T. H. Kolbe – Lectures on CityGML
Georeferenced Textures
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Section IV14 T. H. Kolbe – Lectures on CityGML
Parameterized Textures
Georeferenced Photography: Projected onto 3D surfaces:
using worldToTexture parameterization
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Section IV15 T. H. Kolbe – Lectures on CityGML
Further CityGML Concepts
Support for generalization of 3D dataGeneralized objects are linked to the original objects on thelarger scale
Object historyObjects may have a lifespan (creation & termination date)
Explicit linkingEvery CityGML object can have an arbitrary number of links to external resources (files, objects, database entries)
Support for spatial homogenization / integratione.g. Terrain Intersection Curves (for integration of 3D objects with the terrain)
Representation of topology
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Section IV16 T. H. Kolbe – Lectures on CityGML
External References
Every object (part) may have references to corresponding objects from external resourcesConnection with external information, e.g.:
building: link to cadastre, owner's contact information door, antenna: link to facility management systems
Cadastral DB
FacilityManagementSystem
Telecomm.DB
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Section IV17 T. H. Kolbe – Lectures on CityGML
Terrain Intersection Curve (TIC)
„Interface between 3D objects and the terrain“ensure matching of object textures with the DTMDTM may be locally warped to fit the TIC
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Section IV18 T. H. Kolbe – Lectures on CityGML
Geometric-topological Composition
recursive aggregationarbitrary depth
Solid 1 Solid 2
CompositeSolid
?
SolidGeometry
Solid
SolidAggregate
• Wall face should bepartitioned into 2 faces
S.1
S.2
explicit topol. connection- but: goes beyond B-Rep
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Section IV19 T. H. Kolbe – Lectures on CityGML
“Backdoor Topology“
How to allow for flexible usage of topology? until now, most 3D city models do not consider topologyneed to represent city models with geometry only
Topology model of GML3 sophisticated, but complexwould make it necessary to implement 2 options for therepresentation of spatial properties
Approach in CityGML:topological connections are represented by XlinksGML3 geometries are objects; composites/aggregatescan include subgeometries by value or by reference; references express topological relations
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Section IV20 T. H. Kolbe – Lectures on CityGML
Semantic Relations by Topology
Multiple referencing of geometry (components) by distinctgeospatial features (from different feature classes)
realizes topological, but also semantic relationsredundancy free description of space and surfaces possible, thus no overlaps occur
This surface is part ofthe geometries of thebridge object and the
road object
© Rheinmetall Defence Electronics
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Section IV21 T. H. Kolbe – Lectures on CityGML
(Some)CityGML Implementation Issues
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Section IV22 T. H. Kolbe – Lectures on CityGML
CityGML Implementation Issues (I)
(City)GML files become very large (several GB for biggercities)
file sizes can be effectively reduced by gzip compression (≈10%)but: XML validation and processing can be problematic (classicalDOM parsing not feasible due to main memory limitations)
WFS access might have to be realized in an asynchronous way in order to avoid timeouts
Complex data modelextensive use of OO modeling -> puts considerable demands on the modelling power of processing and storage components
Aggregation hierarchies: nested objects
Specialization hierarchies: inheritance of object properties
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Section IV23 T. H. Kolbe – Lectures on CityGML
CityGML Implementation Issues (II)
XLinksComplex objects can be represented inline, in a self-contained way
But: sub-objects may be also distributed over different files (evenWeb Services) and only referenced by their parent objects
GML object referencing employs the XLink standard of the W3C
Topologytopological relations are realized by reusing (partial) geometries;
reusage: referencing the same geometry from different objects
referencing uses XLinks, referenced geometries need to have IDs
Geometry ModelSee next slide
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Section IV24 T. H. Kolbe – Lectures on CityGML
Geometry Model of ISO 19107 / GML3
3D GML geometries are represented as B-Rep withabsolute (world) coordinates (but always with CRS!)
no scene graph concepts like transformation nodesthe CRS is (one) key to the integration of different spatial datasets
No generative modeling principles like CSG, Sweep Repr.Very few implicit (parametric) shape definitions (e.g. Box, TIN)
Reusability of geometry within a dataset is limitedHowever useful to express topological connectivity of different features or semantic relations between them
Advantages of the GML3 geometry modeleasy to spatially index and manage within spatial databases and GIS; native support by Oracle, PostGIS, MySQL etc.visualization (transformation to X3D) is immediate