There is a requirement for handling height information in geographic information systems, in a way that falls short of full 3D modeling. This document defines 2.5D in a way which is a simple extension of the existing 2D OpenGIS Simple Features specification. Handling these topics thoroughly is a more complicated job, and is outside the scope of this document.
Height is modeled by storing a Z value at each point.
Heights are undefined within surface interiors. Any 2.5D geometry can be converted into a 2D geometry by discarding all its Z values. In general, a 2D geometry is simple if it contains no self-intersection. With this definition, a sloping road centerline is simple, but a vertical flagpole is not. The fact that a flagpole is not simple is one of the limitations of 2.5D, whereas in a 3D system the flagpole could be considered simple.