  Interstellar absorption in the galactic plane is highly variable from one
direction to another. In this paper colour excesses and distances from a new
open cluster sample are used to investigate the spatial distribution of the
interstellar extinction. An inverse method (Tarantola & Valette, 1982) is used
to construct the extinction map in the galactic plane below $|b| < 10^{o}$. The
$A_{v} (r,l) $ diagrams are compared with those derived from individual stars
(Arenou et al. 1992, Neckel & Klare 1980). An analytic expression for the
interstellar extinction as a function of galactic longitude and distance in the
solar neighborhood is given. The comparison of the model predictions with
Hipparcos observations in the 4-dimensional space of ($V$, $B-V$, $H_v$, $r$)
shows that our extinction model provides a better fit to the data. However, a
new and more detailed extinction model is still lacking.
