Extending the current "2.5D" terrain model is a necessary development in GIS. Existing 2D surface information must be extended to 3D, while trying to preserve the topological integrity. Computer-Aided Design (CAD) on the other hand is often fully 3D, which may be excessive for our "extended 2D" mapping needs. We have used the "boundary representation" (b-rep) design from CAD systems, based on manifold models, to address these problems. Triangulated Irregular Network (TIN) models of the terrain surface are well known in Geographic Information System (GIS), but are unable to represent cliffs, caves or holes, which are required to represent complex buildings. The simple TIN b-rep structures may be extended using formal Euler Operators to add simple CAD functionality while guaranteeing the connectivity required for terrain modelling. We believe this provides a simple and reliable extension that is sufficient for many applications.