  We re-analyze data from the gravitational-wave event GW170817 and its host
galaxy NGC4993 to demonstrate the importance of accurate total and peculiar
velocities when measuring the Hubble constant using this nearby Standard Siren.
We show that a number of reasonable choices can be made to estimate the
velocities for this event, but that systematic differences remain between these
measurements depending on the data used. This leads to significant changes in
the Hubble constant inferred from GW170817. We present Bayesian Model Averaging
as one way to account for these differences, and obtain
$H_{0}=66.8^{+13.4}_{-9.2}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$. Adding additional
information on the viewing angle from high resolution imaging of the radio
counterpart refines this to
$H_{0}=64.8^{+7.3}_{-7.2}\,\mathrm{km\,s^{-1}\,Mpc^{-1}}$. During this analysis
we also present an alternative Bayesian model for the posterior on $H_{0}$ from
Standard Sirens that works more closely with observed quantities from redshift
and peculiar velocity surveys. Our results more accurately capture the true
uncertainty on the total and peculiar velocities of NGC4993 and show that
exploring how well different datasets characterize galaxy groups and the
velocity field in the local Universe could improve this measurement further.
These considerations impact any low-redshift distance measurement, and the
improvements we suggest here can also be applied to standard candles like type
Ia supernovae. GW170817 is particularly sensitive to peculiar velocity
uncertainties because it is so close. For future standard siren measurements
the importance of this error will decrease as (a) we will measure more distant
standard sirens, and (b) the random direction of peculiar velocities will
average out with more detections.
