# Tunnel Geometry

The space curve of the tunnel axis consists of a sequence of track segments. In TuMan each track segment is approximated either by a three-dimensional line or a three-dimensional spline. A line is defined by two nodes (3D coordinates). At most four interpolation points (nodes) define a spline. This method allows a handy and flexible input of the tunnel axis by positioning the interpolation points.

A tunnel often has several kinds of cross-sections. For example, a rectangular cross-section near the portals and a mouth-shaped cross-section inside. With TuMan you can draw cross-section geometries of arbitrary shape. To generate a valid tunnel, you must assign a cross-section geometry to every track segment.

A valid tunnel geometry of every single track must match the following conditions:

The space curve of the tunnel axis must be valid. Every interpolation point has to be connected by a single non-closed curve without intersections.

A valid cross-section geometry has to be assigned to all track segments.