  We derive the shock strength area rule for a Noble-Abel stiffened gas (NASG)
equation of a state required in Whitham's geometrical shock dynamics approach
to determine shock wave dynamics in dense gases, liquids and solids. An exact
formulation requiring the solution of an ordinary differential equation is
provided. Closed form solutions of various levels of approximations are also
obtained as an expansion in shock wave strength. The leading order
approximation recovers the geometrical acoustic limit, while higher order
approximations account for the medium's compressibility. The exact shock
strength area relation and the various order approximations are illustrated for
shocks in liquid water. The simple closed form of the first order solution
predicts the shock strength area rule up to shock pressures of approximately
twice the stiffening pressure in water, i.e., approximately 1-2 GPa.
