Résumé
We address the problem of testing when orderings on coordinates of n points in an (n-1)-dimensional affine space, one ordering for each coordinate, suffice to determine if these points are the vertices of a simplex (i.e. are affinely independent), and the orientation of this simplex, independently of the real values of the coordinates. In other words, we want to know when the sign (or the non-nullity) of the determinant of a matrix whose columns correspond to affine points is determined by orderings given on the values on each row. We completely solve the problem in dimensions 2 and 3, providing a direct combinatorial characterization, together with a formal calculus method, that can be seen also as a decision algorithm, which relies on testing the existence of a suitable inductive cofactor expansion of the determinant. We conjecture that the method we use generalizes in higher dimensions. The motivation for this work is to be part of a study on how oriented matroids encode shapes of 3-dimensional objects, with applications in particular to the analysis of anatomical data for physical anthropology and clinical research.