Represent Point Based Surface Model

Overview

Identifies the information requirements for a surface representation through the provision of points and, optionally, breaklines.
A point based surface model may be obtained from:

the results of a survey
a sample set of points derived from an existing map
other point based representation of a spatial region
Points may be simple geometric points identified in terms of their coordinates or they may be qualified annotation points that have a unique identifier for reference (in which case they have an underlying simple geometric point). In this functional part, only the underlying point representation is considered.
Breaklines are restricted to only connect points given in the set of survey points. Breaklines, if given, are used to constrain the triangulation

Results

A point based representation is provided from which a surface model may be derived. The representation may contain breaklines.

The following table describes representing a point based surface model in IFC 2x2

Description	Entity/Pset/Functional Part	MAN	REC	OPT
Determine the set of points and their location coordinates. Points are given as 3D cartesian points within the coordinate system of the spatial region concerned.	IfcCartesianPoint.Coordinates::IfcLengthMeasure
Derive dimensionality of points Note that the dimensionality of the points is determined by the number of coordinates given (2 or 3)	IfcCartesianPoint.Dim::IfcDimensionCount
Determine the set of break lines between Cartesian points. Breaklines should be specified by occurrences of IfcPolyline. The points defining the breakline polyline should already exist as occurrences of the set of points identified. Dimensionality of break lines should be set to 3.	IfcPolyline.Points::IfcCartesianPoint
Create a geometric set Collect together the occurrences of cartesian points and break lines into a geometric set	IfcGeometricSet.Elements::IfcCartesianPoint
Derive dimensionality of the geometric set The value of the dimensionality of the geometric set is derived from the value of the dimensionality of the first item in the set of elements within the geometric set. This is determined by the WHERE rule on the entity IfcGeometricSet. It should default to a value of 3 (for a 3 dimensional geometric set)	IfcGeometricSet.Dim::IfcDimensionCount
Identify a shape representation for the point model. Specific values of the identifier for the representation are given in exchange requirements.	IfcShapeRepresentation.RepresentationIdentifier::IfcLabel
Identify a shape type for the point model. Specific values of the type for the representation are given in exchange requirements.	IfcShapeRepresentation.RepresentationType::IfcLabel
Assign the geometric set of points and break lines to the shape representation.	IfcShapeRepresentation.Items::IfcGeometricSet

The following table describes modifications that enable an alternative (and recommended) way representing a point based surface model in IFC 2x3 and above

Description	Entity/Pset/Functional Part	MAN	REC	OPT
Establish the annotation points Each annotation point is uniquely identified and established.
Set the global unique identifier	IfcAnnotation.GlobalId::IfcGloballyUniqueId
Apply the owner history	IfcAnnotation.OwnerHistory::fp_apply_owner_history
Specify the name of the site	IfcAnnotation.Name::IfcLabel
Specify the description for the site	IfcAnnotation.Description::IfcText
Get the set of points
Determine the set of points and their location coordinates. For points given as IfcAnnotation, there will be a 1:1 correlation with an IfcCartesianPoint and this should be determined for the point based representation. Points are given as 3D cartesian points within the coordinate system of the spatial region concerned.	IfcAnnotation.Representation::fp_represent_annotation[point]
The collection of Cartesian points into a geometric set

IFC Entities Required

IfcAnnotation (** IFC 2x3 and above only)
IfcBoundedCurve
IfcCartesianPoint
IfcCurve
IfcGeometricRepresentationItem
IfcGeometricSet
IfcPoint
IfcPolyline
IfcRepresentationItem

IFC Datatypes Required

IfcDimensionCount
IfcGeometricSetSelect
IfcGloballyUniqueId (** IFC 2x3 and above only)
IfcLabel
IfcLengthMeasure
IfcText (** IFC 2x3 and above only)

IFC Functions Required

-

IDM Functional Parts Required

fp_apply_owner_history (** IFC 2x3 and above only)
fp_represent_annotation[point] (** IFC 2x3 and above only)

EXPRESS-G

The graphical notation corresponds to the IFC 2x2 release only at this point

EXPRESS Schema

The schema corresponds to the IFC 2x2 release only at this point

SCHEMA FP_REPRESENT_POINT_BASED_SURFACE_MODEL;

  TYPE IfcDimensionCount = INTEGER;
    WHERE
      WR1 : { 0 < SELF <= 3 };
  END_TYPE;

  TYPE IfcLengthMeasure = REAL;
  END_TYPE;

  TYPE IfcGeometricSetSelect = SELECT
    (IfcPoint,
     IfcCurve);
  END_TYPE;

  ENTITY IfcGeometricRepresentationItem
    ABSTRACT SUPERTYPE OF (ONEOF(IfcCurve, IfcPoint))
    SUBTYPE OF(IfcRepresentationItem);
  END_ENTITY;

  ENTITY IfcRepresentationItem
    ABSTRACT SUPERTYPE;
  END_ENTITY;

  ENTITY IfcCurve
    ABSTRACT SUPERTYPE
    SUBTYPE OF(IfcGeometricRepresentationItem);
    DERIVE
      Dim : IfcDimensionCount := IfcCurveDim(SELF);
  END_ENTITY;

  ENTITY IfcBoundedCurve
    ABSTRACT SUPERTYPE
    SUBTYPE OF(IfcCurve);
  END_ENTITY;

  ENTITY IfcPolyline
    SUBTYPE OF(IfcBoundedCurve);
      Points : LIST [2:?] OF IfcCartesianPoint;
    WHERE
      WR41 : SIZEOF(QUERY(Temp <* Points | Temp.Dim <> Points[1].Dim)) = 0;
  END_ENTITY;

  ENTITY IfcCartesianPoint
    SUBTYPE OF(IfcPoint);
      Coordinates : LIST [1:3] OF IfcLengthMeasure;
    DERIVE
      Dim         : IfcDimensionCount := HIINDEX(Coordinates);
    WHERE
      WR1 : HIINDEX(Coordinates) >= 2;
  END_ENTITY;

  ENTITY IfcPoint
    ABSTRACT SUPERTYPE
    SUBTYPE OF(IfcGeometricRepresentationItem);
  END_ENTITY;

  ENTITY IfcGeometricSet;
      Elements : SET [1:?] OF IfcGeometricSetSelect;
    DERIVE
      Dim      : IfcDimensionCount := Elements[1].Dim;
    WHERE
      WR21 : SIZEOF(QUERY(Temp <* Elements | Temp.Dim <> Elements[1].Dim)) = 0;
  END_ENTITY;

END_SCHEMA;

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