  Postnikov constructed a decomposition of a totally nonnegative Grassmannian
into positroid cells. We provide combinatorial formulas that allow one to
decide which cell a given point belongs to and to determine affine coordinates
of the point within this cell. This simplifies Postnikov's description of the
inverse boundary measurement map and generalizes formulas for the top cell
given by Speyer and Williams. In addition, we identify a particular subset of
Pluecker coordinates as a totally positive base for the set of non-vanishing
Pluecker coordinates for a given positroid cell.
