## 8.2 IfcGeometricConstraintResource

### 8.2.1 Schema Definition

The schema IfcGeometricConstraintResource defines the resources used to determine the placement of the shape representation of a product within the geometric representation context of a project. It also contains resource definitions to be assigned to product connectivity definitions to determine the connection geometry constraints between those products.

The primary application of this resource is to:

- determine the object placement used for the shape representation of the object
- determine the constraints applied to the connectivity between two shapes of objects

### Placement

The placement of an product's shape is given by the IfcObjectPlacement, referred to by the IfcProduct.ObjectPlacement. The object placement defines the object coordinate system in which all shape representations of that product are defined. It is given either as

- absolute placement
- relative placement
- placement relative to a grid

The absolute placement is specified by using IfcLocalPlacement and omitting the PlacementRelTo attribute. The relative placement is specified by using IfcLocalPlacement and pointing the PlacementRelTo attribute to an IfcObjectPlacement used in another IfcProduct instance.

A placement relative to a grid is given by IfcGridPlacement pointing to one (or two) virtual intersections of IfcGridAxis. If two virtual intersections are references, than the second virtual intersections specifies the orientation of the object placement.

### Connection geometry

The connection geometry constrains the connectivity between the shapes of two products. The constraint can be defined by geometric representation items:

- point
- curve
- surface

or by topological representation items with associated geometry:

- vertex point
- edge curve
- face surface

As a special type of point connection includes the provision to express an eccentricity, i.e. a physical distance between the two points involved in the connection.

EXPRESS-G diagram

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