  We investigate the effect of spiral structure on the Galactic disk as viewed
by pencil beams centered on the Sun, relevant to upcoming surveys such as
ARGOS, SEGUE, and GAIA. We create synthetic Galactic maps which we call Pencil
Beam Maps (PBMs) of the following observables: line-of-sight velocities, the
corresponding velocity dispersion, and the stellar number density that are
functions of distance from the observer. We show that such maps can be used to
infer spiral structure parameters, such as pattern speed, solar phase angle,
and number of arms. The mean line-of-sight velocity and velocity dispersion are
affected by up to ~35 km/s which is well within the detectable limit for
forthcoming radial velocity surveys. One can measure the pattern speed by
searching for imprints of resonances. In the case of a two-armed spiral
structure it can be inferred from the radius of a high velocity dispersion ring
situated at the 2:1 ILR. This information, however, must be combined with
information related to the velocities and stellar number density in order to
distinguish from a four-armed structure. If the pattern speed is such that the
2:1 ILR is hidden inside the Galactic bulge the 2:1 OLR will be present in the
outer Galaxy and thus can equivalently be used to estimate the pattern speed.
Once the pattern speed is known the solar angle can be estimated from the
line-of-sight velocities and the number density PBMs. Forthcoming radial
velocity surveys are likely to provide powerful constraints of the structure of
the Milky Way disk.
