  In this article we prove that Schubert union codes are Tanner codes
constructed with the point--line incidence geometry that Schubert varieties
inherit from the Grassmannian. We do this by first finding an lengthening
algorithm for Tanner codes. This algorithm finds the entries of a codeword of a
Tanner code from the entries in a given subset of its positions. We find
sufficient conditions on the initial set and the initial positions such that a
codeword is determined from the component codes only. We find an iterative and
systematic encoding algorithm for Schubert union codes with linear complexity.
With this encoder we also determine the minimum distance of Schubert union
codes.
