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S-121 Background

S-121 is a product specification for Maritime Limits and Boundaries (MLBs) for the administration of the marine domain. It allows for the representation of marine areas and their associated rights restrictions and responsibilities in alignment with the UN Convention of the Law of the Sea (UNCLOS) while also allowing freedom for States to represent their claims and views. The standard is based on IHO S-100 and also in the ISO-19152 standard on Land Administrative Domain Model (LADM).



Our planet’s oceans are subdivided by international and national laws into many areas and zones. Certainty over the locations of these zones and the rights, restrictions and responsibilities that apply to them facilitates the development of the world’s ocean resources while providing for the protection of the marine environment and safety of navigation.

At the highest level, these zones define where States exercise sovereignty and sovereign rights, and identify areas beyond national jurisdictions that fall under international jurisdiction. Together, these entities describe the common framework for the Law of the Sea.

Maritime Limits and Boundaries (MLBs) are the constructs used to delineate maritime zones and forms the legal foundation of the marine domain. These maritime zones are established in law by their geographic limits and where such limit is delimiting two neighboring States, this limit is described as a maritime boundary, hence the term Maritime Limits and Boundaries (MLBs).

To effectively distribute MLBs for the due publicity obligations under the Law of the Sea Convention and operational purposes, there needs to be a standard framework which ensures compatibility between users and encodes the legal character of the MLBs.

In January 2010, by adopting the S-100 Universal Hydrographic Data Model, the International Hydrographic Organisation (IHO) embarked on the development of a versatile standard framework aligned with the International Organization for Standardization (ISO) 19100 Geographic Information / Geomatics series of standards. The IHO S-100 standard aims to support a wide range of users by developing digital products and transfer standards for the marine community beyond the core hydrographic applications of the IHO. The standard opens the possibility of better marine administration by facilitating the integration of Hydrographic, Scientific and Legislative information.

The Maritime Limits and Boundaries standard - S-121 represents an essential extension of S-100 for the administration of the marine domain. It enables MLBs to be described in terms of what they are, what they embody and what they are used for. S-121 establishes a framework for communicating in a digital form the geographic extents of marine areas and the associated rights and restrictions that apply to them. The framework has been developed to rigorously apply the provisions of the United Nations Convention of the Law of the Sea (UNCLOS), while allowing sufficient flexibility to accommodate the diverse implementations of the Convention by States under domestic laws.

The primary purpose of S-121 is to allow States to communicate official digital representations of their maritime limits and boundaries to the public and international community. The standard is designed to be acceptable as a method for States to fulfill their deposit obligations under the Convention. S-121 is established to depict, describe and communicate national maritime limit or boundary positions without prejudice to maritime boundary disputes. S-121 does not seek to resolve disputes between States: Where disputes exist, the standard will facilitate visibility of those disputes by allowing each party to publish its own position. The S-121 secondary purpose is to provide a flexible and expandable framework able to support other maritime delimitation requirements such as defining areas of overlapping jurisdiction and Joint Development Areas, or any other management areas.

The current vision for this standard is to leverage the capabilities of the ISO-19152 Land Administration Domain Model. ISO-19152 supports the legal description of associated rights, restriction and responsibilities along with providing proper referencing through sourcing and versioning. This additional capability aligns the standard with legal practices of trace-ability. The use of the ISO-19152 standard leverages the significant community investment made in land administration, with which the management of maritime boundaries and limits has much in common. The use of ISO-19152 provides a foundation to extend S-121 into the management of other regulated boundaries, such as marine reserves and fisheries. Alignment with the land domain model will facilitate consistent administration of the littoral zone for those states that adopt S-121 for their marine spaces and ISO-19152 for their land jurisdiction.

The S-121 standard is designed to provide a flexible management and communication solution that can support any type of MLBs for the broadest range of users including the Owner State, other States, the international community, government organizations, courts of law, private industry, academic institutions, and the general public. By construction, the standard also remains compatible with S-101 (Electronic Nautical Chart Product Specification) to allow for the depiction of the MLBs information encoded by the standard to be displayed in electronic navigation charts.

The S-121 takes a practical step toward achieving the vision of S-100 as it was established: to expend the user base and better accommodate the requirements of our digital world. By building on ISO-19152, the S-121 framework provides the capacity to more efficiently and consistently administer across the land and maritime domains. It is essential that we evolve current practices to provide a foundation for sustainable administration of the world’s blue economy. Use of the S-121 standard will reduce costs of enforcement and compliance, and will support the extension of the digital economy into the offshore.


Coastal States, under articles 16, 47, 75, 76 and 84 of the United Nations Convention on the Law of the Sea, are required to deposit with the Secretary-General of the United Nations charts showing: straight baselines, including closing lines of mouths of rivers and bays, and archipelagic baselines; the outer limits, as well as lines of delimitation between States with adjacent or opposite coasts, of the territorial sea (including roadsteds, article 12); the contiguous zone; the exclusive economic zone and the continental shelf. Alternatively, the lists of geographical coordinates of points, specifying the geodetic datum, may be substituted.

In its resolutions 49/28 of 6 December 1994 and 52/26 of 26 November 1997, the General Assembly requested the Secretary-General to establish appropriate facilities, as required by the Convention, for the deposit by States of maps, charts and geographic coordinates concerning national maritime zones and establish a system for their recording and publicity and to develop and maintain [such] facilities for the deposit by States of charts and geographical coordinates concerning maritime zones, including lines of delimitation, and to give due publicity thereto, as required by article 16, paragraph 2, article 47, paragraph 9, article 75, paragraph 2, article 76, paragraph 9 and article 84, paragraph 2, of the Convention. The Division for Ocean Affairs and the Law of the Sea, Office of Legal Affairs of the United Nations is the unit of the Secretariat which performs these depositary functions on behalf of the Secretary-General, as part of an integrated program on the law of the sea and ocean affairs, distinct from the usual depositary functions of the Secretary-General in respect to multilateral treaties.

Subsequently, in its resolution 59/24 of 17 November 2004, the General Assembly requested the Secretary-General to improve the existing geographic information system for the deposit by States of charts and geographical coordinates concerning maritime zones, including lines of delimitation in particular by implementing, in cooperation with relevant international organizations technical standards for the collection, storage and dissemination of the information deposited, in order to ensure compatibility among the Geographic Information System, electronic nautical charts and other systems developed by these organizations. Recent General Assembly resolutions have noted ongoing efforts in this regard.

In addition, the General Assembly, in its annual resolutions on Oceans and the law of the sea, calls upon States Parties to the Convention to fulfill their deposit obligations. Most recently, General Assembly resolution 71/257 calls upon States Parties to the Convention that have not yet done so to deposit with the Secretary-General charts or lists of geographical coordinates, as provided for in the Convention, preferably using the generally accepted and most recent geodetic datums (para. 6).

To facilitate the implementation of the Secretary-General’s depositary functions, coastal States are encouraged to deposit the following information, as a minimum:

  • Geographic coordinates of points in decimal degrees on the straight baselines and archipelagic baselines in common global geodetic datum such as WGS 84, accompanied, as appropriate, by the relevant national legislation;
  • Geographic coordinates of points in decimal degrees on the outer limits as well as lines of delimitation between States with adjacent or opposite coasts for the following maritime zones: territorial sea (including roadsteds); contiguous zone; exclusive economic zone and continental shelf, in common global geodetic datum such as WGS 84, accompanied, as appropriate, by the relevant national legislation. Ideally the points defining the outer limits should be close enough to each other to ensure that the line formed by connecting the points with geodesic lines accurately reflects the outer limit of the maritime zone(s). States Parties are also encouraged to identify in the deposit the points that are part of an international boundary. States Parties are further encouraged to accompany such deposits with the relevant national legislation and/or with references to relevant international treaties.

Accordingly, the Division for Ocean Affairs and the Law of the Sea, of the Office of Legal Affairs of the United Nations, the unit entrusted with carrying out these responsibilities on behalf of the Secretary-General, approached the International Hydrographic Organization with a request to identifying appropriate technical standards. After consultations, the S-121 project team was formed by Member States of the International Hydrographic Organization.

Terms of Reference

The objective of the S121 project team is to develop IHO S-121 Maritime Limits and Boundaries Product Specification. The product specification should take into account the following:

  • Defining a proposed data model;
  • Create an S-100 conformant product specification for Maritime Limits and Boundaries to support coastal States’ depository obligations in accordance with the Convention;
  • Determine if S-100 needs to be extended to facilitate the implementation of the deposit obligation of coastal States’ under the Convention.


  • This Project Team (S121PT) is a subsidiary of the S-100 Working Group (S-100 WG). Its work will be guided by the 2016-18 Work Plan established by the S-100 WG and subject to its approval.
  • A new Terms and Reference Document was adopted by the S121PT after the its first meeting held in the United Nation Headquarters, NY, in December 2016.

Composition and Chair

  • The Project Team (S121PT) shall comprise representatives of IHO Member States (MS), Expert Contributors (EC) and observers from accredited NGIOs. The IHO Secretariat may also be represented. A membership list shall be maintained and posted on the IHO website.
  • EC membership is open to entities and organizations that can provide relevant and constructive contribution to the work of the S121PT. ECs shall seek approval of their membership from the Chair of the S121PT.
  • EC membership may be withdrawn, in the event that a majority of S121PT members agree that an EC’s continued participation is either irrelevant or not constructive to the work of the S121PT.
  • The Chair of the S121PT is designated by the parent body.
  • If a Secretary is required, he/she should normally be drawn from a member of the S121PT.


  • The S121PT should work primarily by written correspondence and teleconferences.
  • Decisions should be made by consensus. Dissenting opinions, if any, should be reflected in the S121PT report.
  • The S121PT should liaise with other IHO bodies, international organizations, end users and industry to ensure the relevance and currency of its work.
  • The S121PT should report in accordance with its Work Plan.

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