In the paper HI Intensity Mapping for Clustering-Based Redshift Estimation, published in Monthly Notices of the Royal Astronomical Society, Volume 482, Issue 3, January 2019, Pages 3341–3355, we showed how useful information can be gained on the redshift distributions of optical galaxy samples from spatial cross-correlations with intensity maps of unresolved neutral hydrogen (21cm) spectral line emission.
This means photometric redshifts from optical imaging surveys which are usually prone to systematics and imprecisions can be further constrained by comparing the angular distribution of structure from the resolved optical galaxies to that of HI intensity maps. Where the optical galaxies have a strong correlation with the HI in a particular redshift bin, then one can assume that the selected redshift bin is well represented. Following this process for all the redshift bins, a probability distribution P(z) for the optical galaxies can be constructed which is useful information to have when constraining errors.
I intend to investigate this method further and it is something that future optical surveys such as Euclid and LSST could massively benefit from.