Measure theory in the geometry of $GL(n,\mathbb Z) \ltimes \mathbb Z^{n}$

The $n$-dimensional affine group over the integers is the group $\mathcal G_n$ of all affinities on $\mathbb R^{n}$ which leave the lattice $\mathbb Z^{n}$ invariant. $\mathcal G_n$ yields a geometry in the classical sense of the Erlangen Program. In this paper we construct a $\mathcal G_n$-invariant measure on rational polyhedra in $\mathbb R^n$, i.e., finite unions of simplexes with rational vertices in $\mathbb R^n$, and prove its uniqueness. Our main tool is given by the Morelli-W{\l}odarczyk factorization of birational toric maps in blow-ups and blow-downs (solution of the weak Oda conjecture).