  Recent advances in spectropolarimetry have allowed the reconstruction of
stellar coronal magnetic fields. This uses Zeeman-Doppler magnetograms of the
surface magnetic field as a lower boundary condition. The ZDI maps, however,
suffer from the absence of information about the magnetic field over regions of
the surface due to the presence of dark starspots and portions of the surface
out of view due to a tilt in the rotation axis. They also suffer from finite
resolution which leads to small scale field structures being neglected.
  This paper explores the effects of this loss of information on the
extrapolated coronal fields. For this we use simulated stellar surface magnetic
maps for two hypothetical stars. Using the potential field approximation, the
coronal fields and emission measures are calculated. This is repeated for the
cases of missing information due to, (i) starspots, (ii) a large area of the
stellar surface out of view, (iii) a finite resolution.
  The largest effect on the magnetic field structure arises when a significant
portion of the stellar surface remains out of view. This changes the nature of
the field lines that connect to this obscured hemisphere. Nonetheless, the
field structure in the visible hemisphere is reliably reproduced. Thus the
calculation of the locations and surface filling factors of accretion funnels
is reasonably well reproduced for the observed hemisphere. The decrease with
height of the magnetic pressure, which is important in calculating disc
truncation radii for accreting stars, is also largely unaffected in the
equatorial plane. The fraction of surface flux that is open and therefore able
to supply angular momentum loss in a wind, however, is often overestimated in
the presence of missing flux.
