## 8.5 IfcGeometricConstraintResource

### 8.5.1 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

#### 8.7.1.1 Placement

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

- absolute placement,
  - the absolute placement is specified by using IfcLocalPlacement and omitting the PlacementRelTo attribute;
- relative placement,
  - the relative placement is specified by using IfcLocalPlacement and pointing the PlacementRelTo attribute to an IfcObjectPlacement used in another IfcProduct instance;
- placement relative to a grid,
  - the placement relative to a grid is specified by using 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. Alternatively the direction can also be provided explicitly by ifcDirection.

> NOTE When using relative placement the shape representation of each product is defined in the local object coordinate system provided by ObjectPlacement. That local object coordinate system is defined relative to the object coordinate system referred to by PlacementRelTo which may be a relative placement as well. Finally the ObjectPlacement not having an PlacementRelTo attribute defined the transformation into the global coordinate system. The transformation of the current coordinate system into the parent coordinate systems have to be applied in that order.

#### 8.7.1.2 Connection geometry

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

- point
- curve
- surface
- solid

or by topological representation items with associated geometry:

- vertex point
- edge curve
- face surface
- closed shell

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.