  Matroid varieties are the closures in the Grassmannian of sets of points
defined by specifying which Pl\"ucker coordinates vanish and which don't. In
general these varieties are very ill-behaved, but in many cases one can
estimate their codimension by keeping careful track of the conditions imposed
by the vanishing of each Pl\"ucker coordinates on the columns of the matrix
representing a point of the Grassmannian. This paper presents a way to make
this procedure precise, producing a number for each matroid variety called its
expected codimension that can be computed combinatorially solely from the list
of Pl\"ucker coordinates that are prescribed to vanish. We prove that for a
special, well-studied class of matroid varieties called positroid varieties,
the expected codimension coincides with the actual codimension.
