Photometric distances to young stars in the inner Galactic disk. I. The
  L = 314$^{o}$ direction
  The spiral structure of the Milky Way is nowadays receiving renewed attention
thanks to the combined efforts of observational campaigns in different
wavelength regimes, from the optical to the radio. We start in the paper the
exploration of a number of key sectors in the inner Milky Way, where the spiral
structure is still poorly known. We search for density enhancements of young
stars that might plausibly be associated with spiral structure. To this aim we
collect sufficiently wide-field UBVI photometry. The intensive usage of U-band
photometry ensures robust determination of reddening and hence distance for
stars of spectral type earlier than A0, which are well-known spiral arm
tracers. The fields we use are large enough to include in most cases
well-studied Galactic clusters, which we use as bench-marks to assess the
quality and standardisation of the data, and to validate our method. We focus
in this paper on the line of sight to the Galactic longitude l= 314$^o$. First
of all, we made exhaustive tests to show that our data-set is in the standard
system, and calibrated our method using the two open clusters NGC 5617 and
Pismis 19 which happen to be in the field, and for which we found estimates of
the basic parameters in full agreement with the literature. We then applied the
method to the general field stars and detected signatures of three different
groups of stars, evenly distributed across the field of view, at
1.5$^{+0.5}_{-0.2}$, 2.5$^{+0.3}_{-0.5}$, and 5.1$^{+1.5}_{-1.1}$ kpc,
respectively. These distances are compatible with the location of the nowadays
commonly accepted description of the Carina-Sagittarius and Scutum-Crux arms,
at heliocentric distance of $\sim$ 2 an 5 kpc,
