  We analyze a technique of obtaining turbulence power spectrum using spectral
line data along the velocity coordinate, which we refer to as Velocity
Coordinate Spectrum (VCS). We formalize geometrical aspects of observation
through a single factor, "geometric term". We find that all variety of
particular observational configurations can be described using correspondent
variants of this term, which we explicitly calculate. This allows us to obtain
asymptotics for both parallel lines of sight and crossing lines of sight. The
latter case is especially important for studies of turbulence within diffuse
ISM in Milky Way. For verification of our results, we use direct calculation of
VCS spectra, while the numerical simulations are presented in a companion
paper.
