Geometric Aspects and Testing of the Galactic Center Distance
  Determination from Spiral Arm Segments
  We consider the problem of determining the geometric parameters of a Galactic
spiral arm from its segment by including the distance to the spiral pole, i.e.,
the distance to the Galactic center ($R_0$). The question about the number of
points belonging to one turn of a logarithmic spiral and defining this spiral
as a geometric figure has been investigated numerically and analytically by
assuming the direction to the spiral pole (to the Galactic center) to be known.
Based on the results obtained, in an effort to test the new approach, we have
constructed a simplified method of solving the problem that consists in finding
the median of the values for each parameter from all possible triplets of
objects in the spiral arm segment satisfying the condition for the angular
distance between objects. Applying the method to the data on the spatial
distribution of masers in the Perseus and Scutum arms (the catalogue by Reid et
al. (2014)) has led to an estimate of $R_0 = 8.8 \pm 0.5$ kpc. The parameters
of five spiral arm segments have been determined from masers of the same
catalogue. We have confirmed the difference between the spiral arms in pitch
angle. The pitch angles of the arms revealed by masers are shown to generally
correlate with $R_0$ in the sense that an increase in $R_0$ leads to a growth
in the absolute values of the pitch angles.
