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Methodology
Collection of freely available geographical data
If possible, data that were compiled by other organisations were used as a basis for the database. These data were
freely downloadable from the respective websites. There were 3 major geographical datasets available:
- Australian Maritime Boundaries Information System (AMBIS) 2001 Version 1.1 (link). This information system (developed by Geoscience
Australia) 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.
- Different shapefiles of the Exclusive Economic Zone of the United States and its overseas territories
(freely available from the website of
NOAA; downloaded on 02/03/2005 and in 2008) were joined and included in the VLIZ geodatabase.
- The Maritime Boundaries layer of the Eurosion GIS Database
contains maritime boundaries (territorial sea, exclusive economic zone, fishing zone) of several European countries and
their overseas territories. The EEZ-limits were extracted from this layer (downloaded on 28/04/2005) and included in the VLIZ geodatabase.
- 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 of the MarBound polygon layer from version 3 on.
Conversion of the information in digital documents to a GIS
The geographical coordinates of boundaries, negotiated in multilateral treaties, could be found in digital documents
available on the website of the United Nations. A major problem is that those coordinates are only available as flat text.
Moreover, the coordinates were written in a lot of different formats (decimal degrees, degrees-minutes-seconds,
degrees-minutes,...). Therefore, all the coordinates were converted to decimal degrees and imported in a database.
From that format, the coordinates were imported in ArcGIS 9 and the different points were connected as a polyline.
For this operation, a function ('Make one polyline from points') of an extension
(XTools Pro version 2.01) was used.
The figure below shows an example (the negotiated boundary between Greenland and Iceland).
Calculation of the 200 Nautical Mile limit of a country
To this end, 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 on the National Geophysical Data Centre (NGDC; downloaded on 23/02/2005). An example of both alternatives is shown below.
Calculation of the median line between two countries
When the distance between two countries is less than 400 nautical miles, the maritime boundary is the median line. On
that median line, each point is equidistant to the baselines (or shoreline if the baseline is not available) of both
countries. For the calculation of the median line, a tool in ArcView 3.2 was created based on the extension 'Create
Thiessen Polygons - v2.6'. The first step is the creation of points out of the baseline or shoreline. When this is done,
Thiessen Polygons can be drawn around the points (to reduce computation time, only those points that are closest to the
other country were selected manually). A Thiessen Polygon is a polygon in which every place is closest to the specific center
point. The places on the boundary of a Thiessen Polygon are equidistant to two points
(More Info). When those two points
are part of the baseline or shoreline of a different country, a segment of the boundary is drawn. In the figure below
there is an example of the maritime boundary calculation in the western part of the English Channel.
Remarks
When nations are still disputing their maritime boundaries, the median line was calculated. When nations are still
disputing claims over land surface or islands (for example the Southern Kuriles or the Paracel Islands), those areas
were considered as a separate area with a link to the nation that is disputing it.
Conversion of the boundary lines to EEZ polygons
When all the boundary lines were constructed, the lines could be aggregated and a EEZ-polygon per country was derived.
For the land-side part of the EEZ-polygons, the boundaries of the ESRI-world cover were used. Seperate polygons were
created for islands (ex.: Azores) and dependencies (ex.: Réunion) but those polygons are linked with their corresponding
sovereign nation.
Sources of error
When the global polygon layer was finished, quality was checked. Therefore, 66700 random locations were georeferenced
based on the polygon layer. The locations were also georeferenced based on the commercial Veridian (now GD-AIS) database. Comparing the
results lead to a 99% parallel. Possible sources of error are:
- Lack of information about baselines: mostly, boundaries were calculated based on the coastline because no information
was available about the baselines.
- Inaccurate information of land boundaries: as the ESRI-world cover is strongly generalised, land boundaries
between countries are not exact. Because of this, some calculations could be inaccurate. Another consequence of the
generalisation of the ESRI-cover is that some islands may be assigned to the wrong country.
- Generalisation of the land-side boundary: due to the generalisation of the ESRI-cover, locations that are
in fact on the land, could be in the sea and opposite.
Users who come across errors, inaccuracies and/or omissions are requested to get in touch, either through the
forum or through email, and describe the problem. We hope that this will allow us to improve the database.
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