  Aperture synthesis observations with full polarisation have long been used to
study the magnetic fields of synchrotron emitting sources. Recently proposed
closure invariants give us a powerful method for extracting information from
measured visibilities which are corrupted by antenna and polarisation dependent
gains. In this paper, a formalism developed earlier for complete graphs (where
all visibilities are available) is extended to incomplete graphs. The formalism
provides a complete and independent set of closure invariants from the measured
visibilities in a general situation where not all visibilities are available.
We then show in a simulated, quasi-realistic case that the invariants developed
here contain usable information even in the presence of noise.
