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We investigate the effect of spiral structure on the Galactic disc as viewed by pencil beams centred on the Sun, relevant to upcoming surveys such as ARGOS, SEGUE and  GAIA . Using a steady‐state spiral structure model, 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 PBMs are a powerful tool for analysing spiral patterns but the uncertainty of the Milky Way structure prevents their simple interpretation. For the case of steady‐state spiral structure, 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 −1  , 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 inner Lindblad resonance (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 outer Lindblad resonance 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. The models presented here are preliminary as transient spiral structure models should also be considered. Forthcoming radial velocity surveys are likely to provide powerful constraints of the structure of the Milky Way disc.

Constraining spiral structure parameters through Galactic pencil‐beam and large‐scale radial velocity surveys