In by far the most extensive area of a turbine map, the Mach numbers at the exits of both the stator (absolute) and rotor (relative) are high. Similarity laws for incompressible flow are of little value for extrapolation purposes in this region. However, correlations derived from incompressible flow theory are extremely helpful to extend a turbine map to the low mass flow region, where turbines operate in a windmilling engine. In the incompressible flow region, a linear relationship exists between torque/flow and flow. The slope is independent of speed and can be found from the speed lines for which data are available. This knowledge enables turbine maps to be extended into regions where the pressure ratio is less than unity. Analysis of turbine velocity diagrams is used effectively to link turbine fundamentals to the complex behavior under consideration. Finally, the models are validated against physically meaningful parameters.