Thursday, December 10, 2009

Thursday Data: GADM Database of Global Administrative Areas

Even if you have access to GIS software, you may not have access to relevant vector data.  This is especially true when mapping those parts of the world that don't hold the corporate headquarters of companies that make the GIS software. Vector data of any quality can be hard to find for free online, and even items of data that aren't free aren't necessarily good.

Enter the GADM Database of Global Administrative Areas. GADM is attempting to compile high-resolution spatial data for every country down to the smallest administrative boundaries. Oh yeah, and it's free.

It's also not nearly complete, and there are some errors, but the amount of data they have is as impressive as its quality, and users publicly comment to let GADM know what errors they've found in the data so they can be ironed out. Take the following spatial data:

Both shapefiles are of Jamaica.  The one on top--the one with very little coastal detail--is part of the countries shapefile that comes with the ArcGIS 9 ESRI Data & Maps 9.3 release in 2008.  This data isn't exactly free.  The bottom shapefile comes free from GADM.  And look, it even has tiny little islands!  And it isn't just a difference in coastlines, as GADM is attempting to provide equally detailed boundaries within each country to the smallest administrative levels.