If I take a singular noun like 'person' and turn it into its plural 'people', I think I'm doing pluralisation (or, if you insist, pluralization).
What am I doing if I take a cardinal number like '13' and turn it into an ordinal, like 'thirteenth'? I liked ordination, but that already means some kind of religious thing.
Should I refer to it as ordinalisation? I couldn't find ordinali(s|z)ation in a dictionary, and all the Google hits are from software packages.