# How to test whether two countries are adjacent using ArcGIS Desktop?

I have an interesting question:
Is there a dataset by which I can use to test whether two countries are adjacent? For example, if inputs are Mexico and U.S., they are adjacent; if inputs are England and Russia, they are not. Is it easy to produce an adjacency matrix for all the countries?

Edited: This question might be extended as, if we get a political boundary map, is there any ways to produce an adjacency matrix for all the political entities (e.g. Given a map of Florida, can a counties matrix be built to indicate the adjacency of all the counties)

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In ArcGIS 10.1 you can use the Geometry on Geometry Check Tool

and use the 'Spatial Relation Check Touches'

Touches—A part of the feature from feature class 1 comes into contact with the boundary of a feature from feature class 2. The interiors of the features do not intersect.

http://resources.arcgis.com/en/help/main/10.1/index.html#/Finding_geometries_with_spatial_relationships/01020000001n000000/

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what if I have a world map and I want to produce the adjacency matrix for all the country? – Seen Aug 17 '12 at 21:21
A couple of Questions: Are England and France adjacent? Are Greenland and Canada? United States and Russia? – mhoran_psprep Aug 18 '12 at 4:46
I don't think all of them should be considered as adjacent. @mhoran_psprep – Seen Aug 18 '12 at 13:04

I've created an ArcGIS 10.0 Add-in that will list adjacent features of a dataset. It also allows you to show adjacent features that share the same attributes.

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To do this I think you should use the Polygon Neighbors tool which became available at 10.1 for all license levels:

Creates a table with statistics based on polygon contiguity (overlaps, coincident edges, or nodes).

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Look at the correlates of war dataset. There is an entire army of International Relations Scholars that produce the Correlates of war (COW) dataset. The COW data conceives of the world in terms of "dyadic" or "bilateral" relations. So for example, there is one data point for U.S.-Canada, one for U.S.-Mexico, One for U.K.-France, etc. They have long obsessed over whether Britain and France are considered contiguous or not. Anyway, if you download the data, you should be able to filter it by country name and then join it up with vector data in ArcMap.

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We provided an easy to use JSON format for country adjacency based on COW dataset, have a look and fork it on github :)