Edge-matched administrative boundary CODs (COD-EM)
COD-ABs for some countries are available with an alternate edge-matched version (COD-EM) as well as the original definitive version.
COD-ABs remain the definitive, accepted, best-available administrative boundary sets for each country. However, COD-EM layers may be more suitable for cartographic visualizations and might also be useful for topological analysis of lower administrative level international connectivity.
Edge-matched COD-EM layers (administrative level 0 shown) and UN Geo Hub 1:1m boundary (orange)
Edge-matched COD-AB (COD-EM) questions and answers
What are COD-EMs?
COD-EMs are copies of the original definitive COD-AB that have had their external feature polygons extended or shrunk to fit the United Nations Geo Hub 1:1m boundary. They therefore fit the COD-EM of neighbouring countries, when available. They may be suitable for cartographic visualizations and might also be useful for topological analysis of lower administrative level international connectivity.
Where can COD-EMs be found?
COD-EMs are separate datasets on HDX containing the gazetteer from the COD-AB, a zipped folder of shapefiles, a zipped geodatabase, and the ITOS live geoservice.
How can I find which countries have COD-EMs?
The matrix in the “COD-AB Feature Suite” page of the COD Portal has a ‘Edge-matched boundaries’ column that indicates which countries have COD-EMs.
How are the COD-EMs created?
ITOS merges two line layers, one made from the lowest COD administrative level polygons and the other made from the United Nations Geo Hub 1:1m boundary polygons. COD lines are clipped when overlapping the UN boundary and extended when needed. Care is used in this step to not simply extend the line straight forward to the boundary but to determine where the border would logically continue.
A polygon layer is created from the lines after cleaning their topology errors. Single-part points created from exploded multipart polygons in the COD are then spatially joined to the polygons for attribution. All multipart polygons are dissolved to match the number of admin features in the COD-AB. This layer is then dissolved to create the higher administrative boundary layers.
Are edge-matched COD-EMs authoritative?
Yes, for all attribute (name and P-code) information. COD-EMs and the definitive COD-ABs have the same feature sets and gazetteers. However, the geometry of outer polygons may have been compromised.
What are the geometric compromises?
Where countries border each other, one or even both boundaries may be less accurate than the original, definitive boundaries. The UN Geospatial Hub boundary is generally of lower quality than the definitive COD-AB layers.
The size and shape of peripheral polygon features may have been changed. International borders are shifted to the UN boundary, either enlarging or shrinking the polygon area.
Peripheral polygon feature shapes and the relationship of their areas to those of their internal neighbouring features may be distorted, while internal features are untouched.
Peripheral polygon features may artificially appear to touch different, incorrect features belonging the same country. (An example will be illustrated if discovered.)
In theory, peripheral polygon features may be eliminated. This is monitored and will be specially treated if it occurs. (An example will be illustrated if discovered.)
What administrative levels are available?
COD-EMs contain the same administrative levels as the definitive COD-ABs. For instance, the Côte d'Ivoire - Subnational Administrative Boundaries dataset extends to administrative level 3 for both the definitive COD-AB and the COD-EM.