Methodology for the creation of the Maritime Boundaries

Data collection and boundaries calculations

The Maritime Boundaries available for download on Marine Regions website are now on its 10th version. This version not only builds on what was made in earlier versions but it also adds shapefiles for areas and lines that, to the best of our knowledge, had never been published before at a global scale. In this new release, Marine Regions makes available all areas included in the provisions of the United Nations Convention on the Law of the Sea (UNCLOS). These areas are Exclusive Economic Zones (EEZ), Territorial Seas (TS), Contiguous Zones (CZ), Internal Waters (IW) and Archipelagic Waters (AW).


The maritime boundaries and areas are calculated from the baselines. In the Maritime Boundaries dataset, the baselines used were a combination of a coastline as a proxy for the low-water line (the normal baseline described in UNCLOS) and straight or archipelagic baselines. The source for the straight baselines was primarily the United Nations repository of all the claims from UNCLOS’s signatories: DOALOS. In addition to this main source, others were also used to complete the straight baselines database such as national legislation on maritime delimitations, agreements, treaties, USA Department of State, among others.

The ESRI Countries 2014 was the primary source for the Maritime Boundaries v10 baseline. The coastline was extracted from the ESRI Countries 2014 and then combined with reefs data extracted from Coral Reef Distribution UNEP for the countries where reefs were fundamental for the correct calculation of the maritime areas, as intended by the coastal state. Several other fixes were made to the coastline to ensure that the coastline could be used as an appropriate normal baseline for the drawing of the multiple Maritime Boundaries areas.

Following the UNCLOS distinction between Islands and Rocks – PART VIII REGIME OF ISLANDS - coastlines were created with the appropriate features for the drawing of EEZ, and another one for the drawing of TS, and CZ. Specific cases where coastal features were not contemplated for the drawing of EEZ include, among others, the Rockall rock off the west coast of Scotland and the Alijos rocks off Baja California.

Treaties and Joint Regimes

Most treaties and agreements between coastal states are submitted to the UN and gathered under the DOALOS. Points pertaining the relevant treaties were stored, converted to lines, and then added to the collection of baselines and median lines.

Median Lines

Unless borders at sea were defined by a treaty or agreement between coastal states, the boundary between nations was calculated by Marine Regions and defined as the Median Line – or the line connecting points which are located at equal distance from both coastal states. This was achieved by drawing Thiessen polygons rooted on points along both the straight and normal baselines.

The millions of Thiessen polygons generated are then merged and dissolved by attribute, in this case the Country name, exposing the median lines between countries.

In order to obtain regularly spaced points along the coast, both baselines were densified so that the maximum distance between points was reduced to 0.01 Degrees.

While the distance between points mentioned above was sufficient to grant equal weight to both coasts involved, this was not the case for international border estuaries and rivers, or in cases where the distance between coastal states was smaller than the maximum distance between points along the baselines. In these cases, the baselines were further densified.

Connection lines

There is an unavoidable mismatch between the median lines calculated by Marine Regions and those published by or retrieved from points from third parties. Of course, polygons cannot be drawn without an uninterrupted set of lines. Connection lines are lines added to bridge the gaps between, for instance, a median line to a treaty line. Connection lines were drawn by extending an existing line using the same angle or linking a line to the nearest relevant vertex.


Having calculated or stored the lines defining the coast and the maritime borders between adjacent or opposite coastal states, the remaining boundary left to be calculated is the seaward outermost boundary of 200, 12, and 24 nautical miles, representing the hypothetical outer limit of the EEZ, territorial seas, and contiguous zones, respectively. Unless, of course, said lines were not extracted from external official sources. These areas were drawn by calculating a buffer distance measured from the combination of straight and normal baselines mentioned above.

Map interface

It is possible to download the used treaties in PDF and the coordinates of the boundaries in GML (Geography MarkUp Language). The geodatabase is not only consultable through a web form, but also through a map interface where one can zoom, pan or query the GIS layers. Linking makes it simple to jump from the information pages to the map interface and back. The latter was implemented using the open-source GeoServer, which supports Open-GIS standards. This allows making the GIS layers available through WMS (Web Map Services) and WFS (Web Feature Services).

In order to optimize the dissemination of the global maritime boundaries database, a KML or or Keyhole Markup Language file has been created. KML is the filetype that can be viewed in Google Earth or on Google Maps. For performance reasons, the shapefile had to be generalised first to export it into a kml file. Within ArcGIS, the polygons were simplified on 10 arcsec (0,0027 deg).

Animation illustrating the drafting of the Maritime Borders from start to finish.

Check also the Frequently Asked Questions.