# 8.9.2.1 IfcArcIndex

## 8.9.2.1.1 Semantic definition

The IfcArcIndex describes a single circular arc segment within a poly curve by providing a list on indices. The first index is the start point of the circular arc, the second index is a point on arc, the third index is the end point of the circular arc. The three points shall not be co-linear.

Informal Propositions:

1. The second index, resolving to a point on arc, shall resolve into a Cartesian point that has approximately the same distance to the start point and the end point of the circular arc. This is due to avoid numeric instability, if the point on arc is too close to either the start or the end point.

## 8.9.2.1.2 Formal representation

TYPE IfcArcIndex = LIST [3:3] OF IfcPositiveInteger;
END_TYPE;