  We report on our ongoing, high-resolution study of H2O masers in the
innermost parsec of NGC 4258. Over thirty epochs of VLBA and VLA data, taken
over six years, are being used to monitor the velocities, accelerations,
positions and proper motions of water masers rotating in a warped, Keplerian
disk about a supermassive central object. Our extensive monitoring results in
an improved accuracy in the distance determination to this galaxy (i) via a
reduction in experimental random errors due to a longer time-baseline than in
previous work; and (ii) via a better modeling of sources of systematic error,
such as disk eccentricity. These data can therefore yield an extremely accurate
geometric distance to NGC 4258.
