The WCS is validated when an object is read in or written to a file. However, this happens transparently to the end user and knowing the details of the validation machinery is not necessary to use or construct a WCS object.
GWCS uses the Advanced Scientific Data Format (ASDF)
to validate the transforms, coordinate frames and the overall WCS object structure.
ASDF makes use of abstract data type
schemas. The serialization and deserialization happens in classes,
referred to as
tags. Most of the transform schemas live in the
asdf-standard package while most of the transform tags live in
astropy. GWCS Schema Definitions are available for the WCS object, coordinate frames and some WCS specific transforms.
Packages using GWCS may create their own transforms and schemas and register them as an
Asdf Extension. If those are of general use, it is recommended they be included in astropy.