  In helioseismic studies, an observational parameter of primary concern is the
P-angle, the angle along which lies the solar axis of rotation for a given
image. For the six observing sites employed by The Global Oscillation Network
Group (GONG), this angle acts additionally as a marker of relative image
orientation, allowing concurrent images to be precisely aligned and merged to
provide the highest possible quality data. In this report, we present and
investigate two methods of determining the P-angle via the rotational signature
embedded in solar Dopplergram images by examining the large-scale structure of
the observed velocity field. As with other studies, we find that the
Dopplergram produces a time-varying 'P-angle' signature according to the
presentation of various physical phenomena across the solar surface, but with
the potential for sub-degree identification of the axis of rotation. However,
close agreement between separate P-angle-finding techniques also reveals
current limitations to P-angle determination that are imposed by the
calibration state of the GONG-site Dopplergrams, leaving these P-angle-finding
methods for GONG with errors on the scale of less than a degree between two
site.
