Methodology for the creation of the Maritime Boundaries (MarBOUND)
Data collection and boundaries calculations
Maritime boundaries are calculated from the baseline on offshore. Different kinds of baselines are in use. Normal baselines are defined as ‘the low-water line along the coast as marked on large-scale charts officially recognized by the coastal state’. This rule has been extended for reefs, bays, river mouths, bays…. On the landward side of the baseline, one speaks about internal waters. On the other side of the baseline the territorial waters, the contiguous zone and the Exclusive Economic zone succeed each other, with widths of respectively 12, 24 and 200 nautical miles.
The Global Self-consistent, Hierarchical, High-resolution Shoreline Database (GSHHS), made available on the NOAA website (version 1.3) has been used to increase the resolution on the land-side of the MarBound polygon layer from version 3 on. This computation has been performed, because points located close to the coastline were falling on land, when we are certain, the sample has been taken in water or the species has been seen on sea; or vice versa.
The first step of the creation of the geodatabase was the integration of information that was already available. For three regions, data was available. (i) In Europe, EUROSION proposed data for all maritime boundaries: baseline, territorial sea, EEZ, delimitation between states, continental shelf, contiguous zones and fishery zones. (ii) Geoscience Australia developed the Australian Maritime Boundaries Information System (AMBIS [version 1.1, 2001). This information system includes all the maritime boundaries (baseline, territorial sea, contiguous zone, exclusive economic zone) of Australia and its overseas territories. The EEZ-limits were extracted from AMBIS (downloaded at 20/01/2005) and included in the VLIZ geodatabase (iii) The website of NOAA [NOAA] provides shapefiles of the Exclusive Economic Zones for different regions of the United States and its overseas territories. This data has been downloaded in 2005 as well as in 2008, for updates to use in the VLIZ Geodatabase of Maritime Boundaries.
Data from treaties
In a second phase the database of negotiated treaties from the United Nations Law of the Sea was consulted and imported into a GIS. The geographic coordinates from the documents were converted to decimal degrees and imported into a database. After importing them in ArcGIS 9, the points were connected by a line (Figure 1).
Figure 1: Negotiated boundaries between Greenland and Iceland: from coordinated in a treaty, over transformed coordinated (decimal degrees) in a database, to GIS, where the points were imported and a line has been created.
The remaining boundaries were calculated in a GIS in accordance with the United Nations Convention on the Law of the Sea: a 200 Nautical Mile buffer was drawn from the baseline or a median line between 2 countries was calculated. If there are no treaties available, based on the International Law of the Sea, there are still two possibilities: (i) the 200 nautical mile buffer around a country can be calculated. To do this, the built-in buffer tool of ArcGis was used. When possible the boundary was calculated based on the baseline of a country. When there was no information about the baseline, the shoreline was used instead. The shoreline that has been used is available for download at the National Geophysical Data Centre (Figure 2).
Figure 2: On the right, a 200 nautical mile buffer, from the baseline. On the left the 200 nautical miles were calculated from...
(ii) If the distance between two countries is less than 400 nautical miles, the maritime boundary is calculated as the median line between both countries. Each point from this line is on an equal distance from either baselines or shorelines, if the baselines are not available. To calculate the median line, the extension ‘Create Thiessen Polygons – v2. in ArcView 3.2 was used. A Thiessen Polygon is a polygon in which every place is closest to the specific centre point. The places on the boundary of a Thiessen Polygon are equidistant to two points. When those two points are part of the baseline or coastline of a different country, a segment of the boundary is drawn.
First, points had to be extracted from the baseline or shoreline, around which Thiessen Polygons can be drawn. To reduce the calculation time, only the points closest to the other country were taken into account after have been selected manually (Figure 3).
Through the compilation of the boundary lines and the coastline, an EEZ-polygon per country was composed. This layer was compared with the commercial Veridian database by Asif Iqbal at Kansas University and this lead to a 99% parallel.
Polygon and polyline creation from the boundaries
Two global GIS-covers in ESRI shape format were produced: one contains polylines representing the maritime boundaries, the second holds polygons representing the EEZs. When all the boundary lines were constructed, the lines could be aggregated and an EEZ-polygon per country was derived. For the land-side border, the boundaries of the world countries were used, on the sea-side the boundaries were aggregated for each country. Separate polygons were created for islands (ex.: Azores) and dependencies (ex.: Réunion) but those polygons are linked with their corresponding sovereign nation (in the attribute table).
Figure 4: Map with the three shapefiles that are available for download: (i) World Maritime Boundaries, (ii) World Maritime Exclusive Economic Zones, (iii) Extended continental Schelf of Australia.
Boundaries are dynamic, hence continuous updates are necessary. Due to new agreements, errors in the actual database, change of the political landscape, etc updates are necessary. Therefore we publish regular new updates of the database through the release of new versions of the Maritime Boundaries database. Since the launch of the database in 2006, five updates have been created. An overview of the changes can be found in the log which can be found on the website, but is also attached to the zip file to be downloaded.
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 reverse. 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). All polygons have the same legend, but for the boundaries, a legend, based on the type of the line has been used, where the following five categories are used: (i) 200 nm, (ii) disputed, (iii) median line, (iv) treaty, (v) treaty – digitized.
In order to optimize the dissemination of the global maritme 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).