Writing CityGML from FME

The FME CityGML writer maps any feature type definitions that are not defined in CityGML into GenericCityObject(s). This topic explains how the FME CityGML writer can be used to output datasets with predefined thematic CityGML feature types, such as Building, CityFurniture, WaterBody, Road, Railway, etc…

CityGML Feature Types in FME

The simplest way to get CityGML writer feature type definitions for the supported CityGML features is to import them from an existing CityGML dataset.

FME ships with two sample CityGML datasets that can be used to populate FME Workbench with the required CityGML writer feature types. These file are located under the xml/CityGML/writer_feature_types folder of the FME installation. The CityGML_feature_types.xml and CityGML_NoiseADE_feature_types.xml sample datasets have the feature type definitions for CityGML (0.4 and 1.0) and CityGML-NoiseADE (0.4 and 1.0), respectively.

Note  These CityGML sample files are not complete datasets in the GML sense; they do not contain any meaningful data for reading, and are only meant to be used for importing feature type definitions into a writer.

CityGML Levels of Detail

CityGML specifies five different Levels of Detail (LODs), ranging from 0 (general topology) to 4 (detailed architectural features and furniture). Most features may contain geometry models for different LODs. LOD0 generally models terrain features. LOD1 models simple prismatic buildings and general landscape features. L0D2 incorporates some architectural features in building models, greater detail in transportation, vegetation, and outdoor furniture features. LOD3 models buildings and outdoor objects as they would actually appear. And LOD4 models detailed interior structures.

In order for various applications to correctly interpret the multiple geometries of a feature, GML geometries are enclosed in an element that indicates the geometry role. As an example, a Building feature with a solid geometry at LOD2 is shown below:

<cityObjectMember>

<bldg:Building gml:id="building890384">

<bldg:lod2Solid>

<gml:Solid gml:id="aebd7312">

FME geometries in CityGML features types must be tagged with their intended geometry role in order to be written out correctly. The geometry role is specified through the geometry trait citygml_lod_name. The geometry roles that are valid for each feature type are given in Valid LOD types for CityGML Feature Types, and the valid FME geometry types for each geometry role are shown in Valid Geometries for the citygml_lod_name Attribute Values.

Setting the Geometry Role in FME Workbench

The geometry role can be set using the AttributeCreator and the GeometryPropertySetter transformers in tandem.

The AttributeCreator sets citygml_lod_name as an attribute, and then transforms it into a trait using the GeometryPropertySetter.

Since we've passed in a valid FME geometry for the geometry role (an extrusion as a lod2Solid), we end up with a Building feature containing this geometry in our output:

<cityObjectMember>

<bldg:Building gml:id="boring_building_11">

<bldg:lod2Solid>

<gml:Solid srsDimension="3">

All of the other CityGML-specific attributes can also be easily set on an FME feature through the AttributeCreator, though many of these attributes have a restricted set of values that are defined in the CityGML external code lists.

See the tables below for more details.

Writing Multiple Geometry Roles to One Feature

To create additional geometry roles for a feature, simply label a copy of the same geometry, or any other geometry with the appropriate role, and aggregate all of the geometries for the feature into a single geometry:

Here is the CityGML Road feature:

<tran:Road gml:id="2nd_west_boring_street">
 <tran:lod0Network>
   <gml:LineString srsDimension="3">
    <gml:posList>12 0 0 12 12 0 0 12 0</gml:posList>
  </gml:LineString>
</tran:lod0Network>
  <tran:lod1MultiSurface>
    <gml:MultiSurface srsDimension="3">
     <gml:surfaceMember>
      <gml:Polygon>
       <gml:exterior>
        <gml:LinearRing>
         <gml:posList>13 0 0 11 0 0 11 11 0 0 11 0 0 13 0 13 13 0 13 0 0
                </gml:posList>
         </gml:LinearRing>
       </gml:exterior>
    </gml:Polygon>
   </gml:surfaceMember>
  </gml:MultiSurface>
 </tran:lod1MultiSurface>
 <tran:lod2MultiSurface>
   <!--The same gml:MultiSurface as for the lod1MultiSurface-->
 </tran:lod2MultiSurface>
</tran:Road>

Generally, the geometry at LOD2 should have more detail than the geometry at LOD1, but using the same geometry works for this example. In the case of this Road, it may have extra curves representing the separate lanes of the road.

Geometry Validation in FME Workbench

Note that in the above example, we tagged a polygon as a lodXMultiSurface, though it only represents a single area. The CityGML writer will check that the FME geometry can actually be written out as the specified geometry role. If it is unable to, and it is easy to convert the geometry to a type that is valid, the writer will make the conversion. For example, our polygon was converted to a surface and then wrapped in a multi-surface. If the FME geometry is not valid for the specified geometry role and cannot be converted to a valid geometry (for example, a point tagged with lod3MultiCurve), it will not be written out.

Setting CityGML Attributes and Properties in FME

The non-geometric attributes and properties of the CityGML features are specified through certain attributes of the FME feature types. Most of these attributes are prefixed with citygml_. For example, the value of the citygml_roof_type attribute of an FME Building feature will be written as the value of the roofType element of a CityGML Building feature.

The full list of valid attributes for a CityGML feature type will appear on feature types defined by importing feature types from a CityGML document that contains those features.

Most of the CityGML attributes and properties that take a string value have an enumerated list of valid values that are listed in the CityGML external code lists. For example, for the roofType element of a Building feature, “1070” indicates a pavilion roof. The corresponding attributes of CityGML features in FME should be set to one of these valid values, though FME currently does no validation of the values.

Writing CityGML Appearances from FME

FME supports reading and writing of CityGML appearances. In FME, Appearance objects are stored in a common library, and surfaces contain a reference to the Appearance that is applied to them.

The FME Appearance contains information about both constant (material) and non-constant (texture) surface properties. Any FME Appearances that are referenced by a surface passed into the CityGML Writer will be written out as a ParameterizedTexture or an X3DMaterial, depending on whether or not the Appearance contains textural information. The writer will try to re-use any existing elements in the case that ParameterizedTexture or X3DMaterial features are passed in (for example, during a CityGML-to-CityGML translation). In this case, the elements will be written out as members of the original CityGML Appearance and under their original theme. If new elements must be created, they will be members of a new Appearance under the CityModel.

The default theme name is FMETheme, but it can be changed through the writer parameter Theme name. ParameterizedTexture and X3DMaterial elements can belong to the same theme even under different Appearance features.

If any of the referenced FME Appearances contain texture information, the image files will be written out to a subfolder of the destination folder. By CityGML convention, this folder is named “appearance” by default. The folder name may be specified through the writer parameter Texture subfolder.

CityGML Feature Hierarchy

Many CityGML features are aggregations of other features. In FME, this relationship can be specified through the gml_id and gml_parent_id attributes.

The value of the gml_parent_id of a child element should be equal to the value of the gml_id of its parent element. The gml:id attribute must be unique for all features in a CityGML document.

Using the UUIDGenerator transformer will ensure that these values are unique, but they must be prefixed with a non-numeric character.

Note  The CityGML Writer will dynamically generate gml:id attribute values for all features it writes (essentially using the same method shown below), so it is not necessary to use these transformers unless the value of the gml_id attribute must be accessed.

Valid Geometry Role Lists

Note  Only one of the listed values for citygml_lod_name Value is needed.

Valid LOD types for CityGML Feature Types

CityGML Feature Type

citygml_lod_name Value

GenericCityObject

lod[0-4]Geometry

lod[0-4]TerrainIntersection

Address

multiPoint

Building

BuildingPart

lod[1-4]Solid

lod[1-4]MultiSurface

lod[2-4]MultiCurve

lod[1-4]TerrainIntersection

BuildingInstallation

lod[2-4]Geometry

RoofSurface

WallSurface

GroundSurface

ClosureSurface

FloorSurface

InteriorWallSurface

CeilingSurface

lod[2-4]MultiSurface

Door

Window

lod[3-4]MultiSurface

Room

lod4Solid

lod4MultiSurface

BuildingFurniture

IntBuildingInstallation

lod4Geometry

WaterBody

lod[0-1]MultiCurve

lod[0-1]MultiSurface

lod[1-4]Solid

WaterSurface

WaterGroundSurface

lod[2-4]Surface

TransportationComplex

Track

Road

Railway

Square

lod0Network

lod[1-4]MultiSurface

TrafficArea

AuxiliaryTrafficArea

lod[2-4]MultiSurface

SolitaryVegetationObject

lod[1-4]Geometry

PlantCover

lod[1-4]MultiSurface

lod[1-3]MultiSolid

CityFurniture

lod[1-4]Geometry

lod[1-4]TerrainIntersection

LandUse

lod[0-4]MultiSurface

GeoreferencedTexture

referencePoint

TINRelief

extent

MassPointRelief

reliefPoints

BreaklineRelief

ridgeOrValleyLines

breaklines

NoiseRoadSegment

NoiseRailwaySegment

NoiseCityFurnitureSegment

lod0BaseLine

Valid Geometries for the citygml_lod_name Attribute Values

LOD name

Valid FME Geometries

GML Geometry

lod0Geometry

lod1Geometry

lod2Geometry

lod3Geometry

lod4Geometry

relativeGMLGeometry

geometry

all except null, raster, and empty aggregates

gml:GeometryType

lod1MultiSolid

lod2MultiSolid

lod3MultiSolid

all solids

all surfaces

homogeneous aggregates of surfaces or areas

gml:MultiSolidType

lod0MultiSurface

lod1MultiSurface

lod2MultiSurface

lod3MultiSurface

lod4MultiSurface

all surfaces

all areas

homogeneous aggregates of surfaces or areas

gml:MultiSurfaceType

lod0MultiCurve

lod1MultiCurve

lod2MultiCurve

lod3MultiCurve

lod4MultiCurve

line

arc

all surfaces

all solids

homogeneous aggregates of lines, arcs, surfaces, or solids

gml:MultiCurveType

lod1Solid

lod2Solid

lod3Solid

lod4Solid

all solids

all surfaces

aggregates containing a single solid or surface

gml:SolidType

lod2Surface

lod3Surface

lod4Surface

all surfaces

all areas

aggregates containing a single surface or area

gml:SurfaceType

lod0TerrainIntersection

lod1TerrainIntersection

lod2TerrainIntersection

lod3TerrainIntersection

lod4TerrainIntersection

line

arc

all surfaces

all solids

homogeneous aggregates of lines, arcs, surfaces, or solids

gml:MultiCurveType

lod0Network

all points

line

arc

all areas

all surfaces

all solids

aggregates containing a single

point, line, arc, area, surface, or solid

gml:GeometricComplexType

noise:lod0BaseLine(move me)

line

arc

aggregates containing a single line or arc

gml:CurveType

referencePoint

point

aggregates containing a single point

gml:PointType

reliefPoints

point

homogenous aggregates of points

gml:MultiPointType

extent

face

rectangular face

all areas

aggregates containing a single face, rectangular face, or area

gml:PolygonType

ridgeOrValleyLines

line

arc

all surfaces

all solids

homogeneous aggregates of lines, arcs, surfaces, or solids

gml:MultiCurveType

breaklines

line

arc

all surfaces

all solids

homogeneous aggregates of lines, arcs, surfaces, or solids

gml:MultiCurveType

multiPoint

point

aggregate containing a single point

gml:MultiPointType

CityGML Feature Roles

CityGML Feature Type

citygml_feature_role Value

Appearance

appearance *

appearanceMember *

GeoreferencedTexture

ParameterizedTexture

X3DMaterial

surfaceDataMember

TINRelief

MassPointRelief

BreaklineRelief

reliefComponent

BuildingInstallation

outerBuildingInstallation

IntBuildingInstallation

interiorBuildingInstallation

roomInstallation

RoofSurface

WallSurface

GroundSurface

ClosureSurface

CeilingSurface

InteriorWallSurface

FloorSurface

Building/boundedBy **

Room

interiorRoom

BuildingPart

consistsOfBuildingPart

Address

address

Door

Window

opening

BuildingFurniture

interiorFurniture

WaterSurface

WaterGroundSurface

WaterBody/boundedBy **

TrafficArea

trafficArea

AuxiliaryTrafficArea

auxiliaryTrafficArea

noise:NoiseRailwaySegment

noise:noiseRailwaySegmentProperty

noise:NoiseRoadSegment

noise:noiseRoadSegmentProperty

noise:NoiseCityFurnitureSegment

noise:noiseCityFurnitureSegmentProperty

noise:Train

noise:usedBy

CityGML Feature Properties

CityGML Parent Feature Type

Valid properties

all feature types except

Address

X3DMaterial

ParameterizedTexture

GeoreferencedTexture

noise:Train

appearance *

appearanceMember *

Appearance

surfaceDataMember

ReliefFeature

reliefComponent

Building

BuildingPart

outerBuildingInstallation

interiorBuildingInstallation

Building/boundedBy **

interiorRoom

consistsOfBuildingPart

address

RoofSurface

WallSurface

GroundSurface

ClosureSurface

FloorSurface

InteriorWallSurface

CeilingSurface

opening

Door

address

Room

Building/boundedBy **

interiorFurniture

roomInstallation

WaterBody

WaterBody/boundedBy **

TransportationComplex

Track

Square

trafficArea

auxiliaryTrafficArea

Road

trafficArea

auxiliaryTrafficArea

noise:noiseRoadSegmentProperty

Railway

trafficArea

auxiliaryTrafficArea

noise:noiseRailwaySegmentProperty

CityFurniture

noise:noiseCityFurnitureSegmentProperty

noise:NoiseRailwaySegment

noise:usedBy

Footnotes

*

Appearance features that are properties of other features in CityGML v0.4 are enclosed in an appearanceMember element. In v1.0, Appearances that are properties of features are enclosed in an app:appearance element, but those that are properties of the CityModel are enclosed in an app:appearanceMember element.

**

boundedBy is a property of both the AbstractBuildingType and of the WaterBodyType, though the allowed values differ by module. The recognized citygml_feature_role value for both of these properties is boundedBy.