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 Table 1, and the valid FME geometry types for each geometry role are shown in Table 2.

Setting the geometry role in FME Workbench

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

The reason we are using an AttributeCreator here is to set citygml_lod_name as an attribute, and then transforming 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 Tables 1-4 for more details.

Writing multiple geometry roles to one feature

Creating additional geometry roles for a feature is not much more difficult. Simply label a copy of the same, 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 (Eg. 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 2010 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 (Eg. 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 advanced FME Workbench 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 advanced FME Workbench 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 that 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.

This workspace produces the following output:

<bldg:Building gml:id="exciting_building_123">

   <bldg:boundedBy>

    <bldg:RoofSurface gml:id="UUID_47be0fd6-ccec-45ab-96f7-0868ff9208d0">

     <bldg:lod3MultiSurface>

      ...

     </bldg:lod3MultiSurface> 

    </bldg:RoofSurface>

   </bldg:boundedBy>

  <bldg:boundedBy>

    <bldg:WallSurface gml:id="UUID_dafb5306-41aa-4e07-8f58-80874167a2a7">

      <bldg:lod3MultiSurface>

       ...

       </bldg:lod3MultiSurface>

      </bldg:WallSurface>

     </bldg:boundedBy>

    <!--Three more WallSurfaces-->

  </bldg:Building>

Valid Geometry Role Lists

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

Table 1: 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

Table 2: 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

 

Table 3: 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

Table 4: 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

*	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”.