8.7.3.4 IfcAlignmentVerticalSegment

8.7.3.4.1 Semantic definition

Individual segment along the IfcAlignmentVertical, being defined in the distance-along/z coordinate space.

The vertical alignment is defined by segments that connects end-to-start. The vertical alignment curve geometry is defined in a plane with x = distance along horizontal, the y = height (or elevation). The transition at the segment connection is not enforced to be tangential, The IfcSegment Transition enumeration specifies the type of transition explicitly.

The following vertical segment types are defined:

line segment - IfcAlignmentVerticalSegmentTypeEnum .LINE.
circular arc segment - IfcAlignmentVerticalSegmentTypeEnum .ARC.
parabolic arc segment - IfcAlignmentVerticalSegmentTypeEnum .PARABOLICARC. which can describe symmetric parabolas, unsymmetric parabolas can be created by defining two symmetric parabolas segments
transition segment with linear curvature variation - IfcAlignmentVerticalSegmentTypeEnum .CLOTHOID.

For each vertical segment, the following non-redundant information is provided:

the start point (in distance along/ height coordinates)
the start gradient (as a ratio measure with horizontal being 0, uphill positive, and downhill negative) usually between 1 < n < -1 (equal to a percentage of 100% < n < -100%, or to a degree of 45° < n < -45° but higher values are possible)
the length (as horizontal length along the distance along (not the curve segment length))
the curve parameter needed for circular and parabolic arc segments

The following information can be calculated (and is therefore not exchanged explicitly to avoid redundancy and inconsistencies)

the end distance along (from the distance along and segment length)
the end height (from start distance along, gradient, length and curve parameter)
the end direction (from start direction, segment length and curve parameter)
the point of vertical intersection (from start direction and end direction)

The following checks can be done to validate the correct exchange:

continuity – does the calculated end distance along of the previous segment matches with the provided start distance along of this segment
tangential continuity – does the calculated end gradient of the previous segment matches with the provided start gradient of this segment

Figure 8.7.3.4.A — Alignment vertical segment

8.7.3.4.2 Entity inheritance

- IfcAlignmentParameterSegment

8.7.3.4.3 Attributes

#	Attribute	Type	Description
IfcAlignmentParameterSegment (2)
1	StartTag	OPTIONAL IfcLabel	Tag to annotate the start point of the alignment segment.
2	EndTag	OPTIONAL IfcLabel	Tag to annotate the end point of the alignment segment.
Click to show 2 hidden inherited attributes Click to hide 2 inherited attributes
IfcAlignmentVerticalSegment (7)
3	StartDistAlong	IfcLengthMeasure	Distance along the horizontal alignment as measured along the corresponding IfcAlignmentHorizontal. NOTE 1 The distance along is measured from the start point of IfcAlignmentHorizontal, any optionally provided offset expressed by IfcAlignmentHorizontal.StartDistanceAlong is not taken into account. 2 The unit of measurement is the global length unit, as set by IfcContext.UnitInContext
4	HorizontalLength	IfcNonNegativeLengthMeasure	Length measured as distance along the horizontal alignment of the segment.
5	StartHeight	IfcLengthMeasure	Elevation in Z of the start point relative to the IfcAlignment coordinate system. NOTE It is strongly advised to not offset the IfcAlignment coordinate system from the project engineering coordinate system.
6	StartGradient	IfcRatioMeasure	Start gradient of the segment.
7	EndGradient	IfcRatioMeasure	End gradient of the segment. In the case of a PredefinedType='.CONSTANTGRADIENT.' the value is the same as StartGradient.
8	RadiusOfCurvature	OPTIONAL IfcLengthMeasure	Radius of parabola or arc. Positive values imply a CCW direction whereas negative CW. NOTE1 For PredefinedType is ARC. The radius of the basis circle for the arc. NOTE2 For PredefinedType is PARABOLICARC. Parabola constant (determining the “steepness” of the parabola). The parabola constant is provided by the “minimum parabola radius”, the true radius of a parabola at its vertical axis (the zero-gradient point of the parabola). The minimum radius is twice the focal length of the parabola (the distance between the focal point and the vertex). NOTE3 For PredefinedType that is not either ARC or PARABOLICARC the value should be empty.
9	PredefinedType	IfcAlignmentVerticalSegmentTypeEnum	Predefined type of the vertical alignmnent segment.

Table 8.7.3.4.C

8.7.3.4.4 Examples

Figure 8.7.3.4.B

UT_Alignment_12d_11

8.7.3.4.5 Formal representation

ENTITY IfcAlignmentVerticalSegment
 SUBTYPE OF (IfcAlignmentParameterSegment);
	StartDistAlong : IfcLengthMeasure;
	HorizontalLength : IfcNonNegativeLengthMeasure;
	StartHeight : IfcLengthMeasure;
	StartGradient : IfcRatioMeasure;
	EndGradient : IfcRatioMeasure;
	RadiusOfCurvature : OPTIONAL IfcLengthMeasure;
	PredefinedType : IfcAlignmentVerticalSegmentTypeEnum;
END_ENTITY;

8.7.3.4.6 References

IfcAlignmentParameterSegment