From OpenStreetMap Wiki
Jump to: navigation, search
Available languages
English norsk bokmål português do Brasil
Public-images-osm logo.svg boundary = maritime
Sea boundaries.png
Maritime boundaries of various levels as accordance with the UN Convention on the Law of the Sea
Group: Location
Used on these elements
should not be used on nodes
may be used on ways
should not be used on areas
use on relations unspecified
Status: unspecified

Almost every non-landlocked country has maritime borders, national borders at sea. United Nations Convention on the Law of the Sea establishes that the territorial waters of a country can extend up to 12 nautical miles (22.227 kilometres) from the low-tide mark of the sea (unless such extension conflicts with territorial waters of another country). This distance does not apply for all borders of this type: for instance, the English Channel is only 34 km at it's narrowest point. In this situation, the national border lies at the midway point of the Channel. However, a significant number of country borders lie 22 km from their coastline.

When drawing coastlines in OpenStreetMap, the rule is "water on the right, land on the left" thus a maritime border will always be to the right of the coastline.

Boundary relations

National borders

The territorial waters line (12 nm zone) should be included in boundary relations, the same way as land borders.

Internal borders

If a country has defined the maritime borders of sub-national entities, this way should be included in boundary relations for these entities.

In the United States of America, NOAA nautical charts specify in Note X:

"Within the 12-nautical mile territorial sea, established by Presidential Proclamation, some federal laws apply. The Three Nautical Mile Line, previously identified as the outer limit of the territorial sea, is retained as it continues to depict the jurisdictional limit of the other laws. The 9-nautical mile Natural Resource Boundary off the Gulf coast of Florida, Texas, and Puerto Rico, and the Three Nautical Mile Line elsewhere remain in most cases the inner limit of Federal fisheries jurisdiction and the outer limit of the jurisdiction of the states. The 24-nautical mile Contiguous Zone and the 200-nautical mile Exclusive Economic Zone were established by Presidential Proclamation. Unless fixed by treaty or the U.S. Supreme Court, these maritime limits are subject to modification."

Therefore, OpenStreetMap should extend state and county boundaries 3 nautical miles offshore (or 9 nautical miles offshore where applicable). These boundaries should not hug or intersect or roam around the coastline except where such boundaries separate one county (or county equivalent) from another.


The baseline from which the territorial sea is measured is mainly the low-water line along the coast as marked on large-scale charts officially recognized by the coastal state. Unfortunately, natural=coastline should be placed at mean high water spring (MHWS). The base line draws often lines across fjords or gulfs, all navigational waters inside the base line are defined as inshore or protected water.

On nodes: (optional)
ref=* reference of the waypoint.
source=* should be stated together with ref!

Territorial sea (12 nm zone)

A state's territorial sea extends up to 12 nautical miles (22 km) from its baseline. If this would overlap with another state's territorial sea, the border is taken as the median point between the states' baselines, unless the states in question agree otherwise. A state can also choose to claim a smaller territorial sea (usually 3 nm).

boundary=administrative since so many had a problem with grouping it together with maritime borders
maritime=yes to state that this is a maritime border, so it can be rendered correctly/different from land borders
admin_level=2 since it is a national boundary
border_type=territorial to distinguish it from borders on land

This line should also be member of the border relation surrounding the country.

This line differs from the other maritime boundaries as a result of the vote on Maritime Borders. The agreement was to keep it as boundary=administrative as the land border, but tag it with maritime=yes.

  • Potential data sources

Contiguous zone (24 nm zone)

The contiguous zone is a band of water extending from the outer edge of the territorial sea to up to 24 nautical miles (44 km) from the baseline, within which a state can exert limited control.

  • Potential data sources

Exclusive economic zone (EEZ) (200 nm zone)

An exclusive economic zone extends from the outer limit of the territorial sea to a maximum of 200 nautical miles (370.4 km) from the territorial sea baseline, thus it includes the contiguous zone. A coastal nation has control of all economic resources within its exclusive economic zone, including fishing, mining, oil exploration, and any pollution of those resources.

  • Potential data sources


This is a special border to mark the boundary between IALA Region A and IALA Region B, needed to distinguish the form of Lateral Sea Marks used. This boundary should not be edited, unless the board of IALA wishes to change this system. Region B is North America, South America, Caribbean, Japan, Republic of (South) Korea, the Philipines. Region A is the rest of the world.

border_type=IALA <- must be upper case

Other notes

The border of admin_level=2 is not where two entities of admin_level 2 meets, but a continuous line around one entity of admin_level 2, therefore use admin_level 2 on the territorial border.

See also

  • Key=boundary (German)
  • Original Proposal page
  • As a possible source, the United Nations database of treaties may be useful, but may not be free: the website terms of use imply non-commercial use, no redistribution and no derivative work. But there are many commercial maps that refer to the database as a source.