Unsymmetric Parabolas

The entity definition for IfcAlignmentVerticalSegment ( says

parabolic arc segment - IfcAlignmentVerticalSegmentTypeEnum.PARABOLICARC. which can describe symmetric and unsymmetric parabolas.

How is an unsymmetric parabola defined? It seems the HorizontalLength attribute is insufficient since the horizontal length on either side of the PVI (L1 and L2 in the image below) is needed to define the asymmetry.


The semantic definition of vertical in IFC is by segments and not by PVI.

In 4x3 RC3 the parabola is defined by giving the start gradient, end gradient and the Radius.
So I believe in this case it would be made up of two segments:

  1. the first one with start gradient g1 and end gradient the gradient above the PVI (is that always 0 ?)
  2. the second segment would start with the gradient above the PVI and end with gradient g2.

Thanks for the feedback. Breaking an unsymmetrical vertical curve into two symmetrical curves is the common solution. For accuracy of the documentation, it seems that “unsymmetric parabolas” needs to be defined clearly.

I’ve checked a couple of route surveying references and the common definition of unsymmetrical vertical curve is different length before and after the PVI.

I suspect the intended definition in IFC is the entry and exit grades does not have equal and opposite values.