8.18.3.1 IfcCoordinateOperation
ABSTRACT This definition may not be instantiated
8.18.3.1.1 Semantic definition
IfcCoordinateOperation is an abstract supertype to handle any operation (transformation or conversion) between two coordinate reference systems (CRSs).
By convention, a coordinate operation is given between a source CRS, being the more local (or child) CRS; and target CRS, being the more remote (or parent) CRS.
A common coordinate operation is the one between the local engineering coordinate system of a construction project and any map or other coordinate reference system.An IfcCoordinateOperation allows to connect: - an IfcGeometricRepresentationContext – a reference system of a virtual model - to an IfcCoordinateReferenceSystem – the coordinate system which is related to the real world by datums.
This can is done by setting: - the IfcCoordinateOperation.SourceCRS to IfcGeometricRepresentationContext - the IfcCoordinateOperation.TargetCRS either to: - IfcProjectedCRS, for defined easting, northing and orthogonal height - IfcGeographicCRS, for defined latitude, longitude and ellipsoidal height IfcProjectedCRS can be used to represent a projected CRS or a compound CRS, comprising of a projected CRS with a vertical CRS. See IfcProjectedCRS for further details. Definition from OpenGIS Abstract Specification, Topic 2: If the relationship between any two coordinate reference systems is known, coordinates can be transformed or converted to another coordinate reference system. Coordinate operations are divided into two subtypes:
Coordinate conversion – mathematical operation on coordinates that does not include any change of datum. The best-known example of a coordinate conversion is a map projection. The parameters describing coordinate conversions are defined rather than empirically derived. Note that some conversions have no parameters.
Coordinate transformation – mathematical operation on coordinates that usually includes a change of datum. The parameters of a coordinate transformation are empirically derived from data containing the coordinates of a series of points in both coordinate reference systems. This computational process is usually ‘over-determined’, allowing derivation of error (or accuracy) estimates for the transformation. Also, the stochastic nature of the parameters may result in multiple (different) versions of the same coordinate transformation. Because of this several transformations may exist for a given pair of coordinate reference systems, differing in their transformation method, parameter values and accuracy characteristics.
There are three types of IfcCoordinateOperation. The one to be used depends on the specific use case:
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IfcMapConversion and IfcMapConversionScaled involve a transformation. These coordinate operations convey that the local engineering coordinate system of the virtual model (which is a topocentric CRS) is put in place in the referenced IfcProjectedCRS.
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IfcRigidOperation involves a translation in the 3D space. This coordinate operation convey that the whole virtual model is translated in the same way. For example, using lengths to translate along x,y,z; or using angles for a 2D translation (e.g., lambda, phi), plus a change in height.
8.18.3.1.2 Entity inheritance
8.18.3.1.3 Attributes
| # | Attribute | Type | Description |
|---|---|---|---|
| IfcCoordinateOperation (2) | |||
| 1 | SourceCRS | IfcCoordinateReferenceSystemSelect |
Source coordinate reference system for the operation. |
| 2 | TargetCRS | IfcCoordinateReferenceSystem |
Target coordinate reference system for the operation. |
8.18.3.1.4 Formal representation
ENTITY IfcCoordinateOperation
ABSTRACT SUPERTYPE OF (ONEOF
(IfcMapConversion
,IfcRigidOperation));
SourceCRS : IfcCoordinateReferenceSystemSelect;
TargetCRS : IfcCoordinateReferenceSystem;
END_ENTITY;