When you are buying:
Only authorized distributors sell official ENCs as an ENC service, which includes the delivery of update information. The distributors are authorized either directly by the originating Hydrographic Office or by a cooperation of Hydrographic Offices. The list can be checked on the official site of the Hydrographic Office http://www.admiralty.co.uk/SitePages/Distributors.aspx
When used in an ECDIS:
ECDIS distinguishes an official ENC from unofficial data.When unofficial data is used, ECDIS informs mariners that they must navigate by means of an official up to date paper chart by a warning, which appears continuously on the screen.
If unofficial data is shown on the ECDIS display, its boundary is to be identified by a special line style.This boundary is visualized as a “one-sided” RED line with the diagonal stroke on the non-HO side of the line.
Where can we get official ENCs?
The International Hydrographic Organisation (IHO) provides an interactive web catalogue (www.iho.shom.fr) displaying the status of worldwide ENC production. This system has pointers for guiding users to ENC suppliers and distributors. A three-colour scheme is used to distinguish between degrees of accessibility.
This catalogue indicates that a considerable number of ENCs have been or are being produced by Hydrographic Offices. It also shows that only a part of the issued data has been released and made available to the market.