  Diffuse interstellar bands (DIBs) hold a lot of information about the state
and the structure of the ISM. Structure can most directly be observed by
extensive spectroscopic surveys, including surveys of stars where DIBs are
especially important, as they are conveniently found in all observed bands.
Large surveys lack the quality of spectra to detect weak DIBs, so many spectra
from small regions on the sky have to be combined before a sufficient
signal-to-noise ratio (SNR) is achieved. However, the clumpiness of the DIB
clouds is unknown, which poses a problem, as the measured properties can end up
being averaged over a too large area. We use a technique called Gaussian
processes to accurately measure profiles of interstellar absorption lines in
145 high SNR and high resolution spectra of hot stars. Together with Bayesian
MCMC approach we also get reliable estimates of the uncertainties. We derive
scales at which column densities of 18 DIBs, CH, CH$^+$, Ca I, and Ca II show
some spatial correlation. This correlation scale is associated with the size of
the ISM clouds. Scales expressed as the angle on the sky vary significantly
from DIB to DIB between $\sim0.23^\circ$ for the DIB at 5512 {\AA} and
3.5$^\circ$ for the DIB at 6196 {\AA}, suggesting that different DIB carriers
have different clumpiness but occupy the same general space. Our study includes
lines-of-sight all over the northern Milky Way, as well as out of the Galactic
plane, covering regions with different physical conditions. The derived
correlation scales therefore represent a general image of the Galactic ISM on
the scales of $\sim5$ pc to $100$ pc.

Astrophysics of Galaxies