  We use near-infrared (J-K)-colours of bright 2MASS galaxies, measured within
a 7"-radius aperture, to calibrate the Schlegel et al. (1998) DIRBE/IRAS
Galactic extinction map at low Galactic latitudes ($|b| < 10^{\rm o}$). Using
3460 galaxies covering a large range in extinction (up to $A_K$ = 1.15 or
E(B-V) ~ 3.19), we derive a correction factor $f = 0.83 \pm 0.01$ by fitting a
linear regression to the colour-extinction relation, confirming that the
Schlegel et al. maps overestimate the extinction. We argue that the use of only
a small range in extinction (e.g., $A_K$ < 0.4) increases the uncertainty in
the correction factor and may overestimate it. Our data confirms the
Fitzpatrick (1999) extinction law for the J- and K-band. We also tested four
all-sky extinction maps based on Planck satellite data. All maps require a
correction factor as well. In three cases the application of the respective
extinction correction to the galaxy colours results in a reduced scatter in the
colour-extinction relation, indicating a more reliable extinction correction.
Finally, the large galaxy sample allows an analysis of the calibration of the
extinction maps as a function of Galactic longitude and latitude. For all but
one extinction map we find a marked offset between the Galactic Centre and
Anticentre region, but not with the dipole of the Cosmic Microwave Background.
Based on our analysis, we recommend the use of the GNILC extinction map by
Planck Collaboration (2016b) with a correction factor $f = 0.86 \pm 0.01$.
