In the past different spheroids and reference systems (horizontal datum) based on these spheroids have been calculated. Reference systems have only been calculated for limited regions, e.g. ED 50 for the European main land.
In general, conversions between different reference systems are possible.
Transformation parameters are available in ECDIS systems to convert between different horizontal datums.
The IHO has published the User´s Handbook on Datum Transformations involving WGS 84, 3 rd edition (July 2003) which contains an extensive documentation for datum conversion.
The development of reference systems is not finished. Geodesists have developed the European Terrestrial Reference System (ETRS 89) which takes the dynamics between the Eurasian plate and a global reference system into account. The differences between ETRS 89 and WGS 84 are so small that they can be ignored for navigation.