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Maritime information systems are innovative geographic information systems for study, monitoring and actiontaking in maritime areas. They respond to needs in the development of intelligent systems for applications such as scientific research and safety (monitoring the global ecosystem, the atmosphere, the oceans, the biosphere, ice fields, fish populations etc.) or the support of the maritime industry and its related organizations (tracking the position of vessels in motion, providing them with safe routing etc.). For these systems to efficiently handle the complex demands made on such specialized applications, up-to-date real-world data purchased or downloaded from official, trustworthy online data sources is needed. This article examines geospatial free-ofcharge data sources and discusses the various classes of available data. Several hundred resources and their available datasets were empirically tested and their quality and usefulness verified, producing a selective thesaurus. An accompanying website summarizing useful available information about the data sources and datasets also includes information which could not be mentioned in the article. The survey, covering a wide spectrum of online information regarding up-to-date sources for genuine valuable real-world high-precision maritime data worldwide, is, to the best of the authors’ knowledge, the only one of its kind at the time of writing