8.18.3.2 IfcCoordinateReferenceSystem
ABSTRACT This definition may not be instantiated
8.18.3.2.1 Semantic definition
The IfcCoordinateReferenceSystem is a definition of a coordinate reference system by means of qualified identifiers only. The interpretation of the identifier is expected to be well-known to the receiving software.
Definition from OpenGIS Abstract Specification, Topic 2: A coordinate reference system is a coordinate system which is related to the real world by a datum. The coordinate system is composed of a set of coordinate axes with specified units of measure. The datum specifies the relationship of a coordinate system to the earth. The resulting combination of coordinate system and datum is a coordinate reference system.The unambiguous identifier by which the coordinate reference system is know, is stored in the Name attribute. Well defined identifiers include the geodetic and the vertical CRS, each with its own datum. In these cases the GeodeticDatum can be omitted.
The code 'EPSG:5555' identifies the combination of ETRS89 / UTM zone 32N + DHHN92 height (i.e., a projected CRS + a vertical CRS). The code 'EPSG:6258' identifies the geodetic datum of the projected CRS ETRS89 / UTM zone 32N (itself identified as EPSG:25832). The code 'EPSG:5181' identifies the vertical datum of the vertical CRS DHHN92 height (itself identified as EPSG:5783). One widely-used, publicly-available authority is the European Petroleum Survey Group (EPSG), and use of this authority is currently specified in several OGC Implementation Specifications. Software used to transport IFC engineering models into GIS applications (and vice versa) is expected to have knowledge about the OGC Implementation Specifications. New entity in IFC4.8.18.3.2.2 Entity inheritance
8.18.3.2.3 Attributes
| # | Attribute | Type | Description |
|---|---|---|---|
| IfcCoordinateReferenceSystem (5) | |||
| 1 | Name | OPTIONAL IfcLabel |
Name by which the coordinate reference system is identified. The name shall be taken from the list recognized by the European Petroleum Survey Group EPSG. It should then be qualified by the EPSG namespace, for example as 'EPSG:5555'. The Name shall contain only one EPSG code. When there is not one EPSG that unambiguously identifies the CRS, IfcWellKnownText shall be used. Combining multiple EPSG codes in one string for Name (e.g., 'EPSG:2056,EPSG:5728') is not allowed. The name shall be 'WKT' if an EPSG code does not exist for the coordinate reference system (CRS). In this case, the CRS shall be further specified using the IfcWellKnownText entity. |
| 2 | Description | OPTIONAL IfcText |
Informal description of this coordinate reference system. |
| 3 | GeodeticDatum | OPTIONAL IfcIdentifier |
Name by which this datum is identified. The geodetic datum is associated with the coordinate reference system and indicates the shape and size of the rotation ellipsoid and this ellipsoid's connection and orientation to the actual globe/earth. It needs to be provided, if the Name identifier does not unambiguously define the geodetic datum as well. geodetic datums include: |
| HasCoordinateOperation | SET [0:1] OF IfcCoordinateOperation FOR SourceCRS |
Indicates conversion between coordinate reference systems. In particular it refers to an IfcCoordinateOperation between this coordinate reference system, and another coordinate reference system. |
|
| WellKnownText | SET [0:1] OF IfcWellKnownText FOR CoordinateReferenceSystem |
Well Known Text (WKT) definition for this coordinate reference system inversely associated as a IfcWellKnownText entity. |
|
8.18.3.2.4 Formal propositions
| Name | Description |
|---|---|
| NameOrWKT |
Ensures that the coordinate reference system is properly specified by either a reference to an EPSG code in Name or a well known text in WellKnownText. |
|
|
8.18.3.2.5 Formal representation
ENTITY IfcCoordinateReferenceSystem
ABSTRACT SUPERTYPE OF (ONEOF
(IfcGeographicCRS
,IfcProjectedCRS));
Name : OPTIONAL IfcLabel;
Description : OPTIONAL IfcText;
GeodeticDatum : OPTIONAL IfcIdentifier;
INVERSE
HasCoordinateOperation : SET [0:1] OF IfcCoordinateOperation FOR SourceCRS;
WellKnownText : SET [0:1] OF IfcWellKnownText FOR CoordinateReferenceSystem;
WHERE
NameOrWKT : (HIINDEX(WellKnownText) = 1) OR EXISTS(Name);
END_ENTITY;