Spatial environmental processes often exhibit dependence in their large values. In order to model such processes their dependence properties must be characterized and quantified. In this paper we introduce a measure that evaluates the dependence among extreme observations located in two disjoint sets of locations of ${ℝ}^2$. We compute the range of this new dependence measure, which extends the existing $\lambda$-madogram concept, and compare it with extremal coefficients, finding generalizations of the known...