International Hydrographic Organization

The International Hydrographic Organization (IHO) is the intergovernmental body that coordinates national hydrographic offices and sets the standards for nautical charts and hydrographic surveys. Based in Monaco and founded in 1921 as the International Hydrographic Bureau, it issues the S-series standards: S-44 for hydrographic-survey accuracy and coverage orders, S-57 and the newer S-100 framework with S-101 for electronic navigational chart (ENC) data, and S-23 for the limits of oceans and seas. IHO standards make charts and survey data interchangeable between member states and feed ECDIS. Compliance with S-44 is what lets a port declare a charted depth to a stated survey order and manage under-keel clearance against it.