  Imaging surveys of galaxies will have a high number density and angular
resolution yet a poor redshift precision. Intensity maps of neutral hydrogen
(HI) will have accurate redshift resolution yet will not resolve individual
sources. Using this complementarity, we show how the clustering redshifts
approach, proposed for spectroscopic surveys can also be used in combination
with intensity mapping observations to calibrate the redshift distribution of
galaxies in an imaging survey and, as a result, reduce uncertainties in
photometric redshift measurements. We show how the intensity mapping surveys to
be carried out with the MeerKAT, HIRAX and SKA instruments can improve
photometric redshift uncertainties to well below the requirements of DES and
LSST. The effectiveness of this method as a function of instrumental
parameters, foreground subtraction and other potential systematic errors is
discussed in detail.
