OpenStreetMap is a popular collaborative project for creating a free map of the world. Every user participating in the project can edit data; at the same time, it is possible for everyone to download those datasets in .osm datafiles (an XML format) under the terms of the Open Data Commons Open Database License (ODbL) at the time of writing.

The osm2pgsql command is a command-line tool that can import .osm datafiles (eventually zipped) to the PostGIS database. To use the command, it is enough to give the PostgreSQL connection parameters and the .osm file to import.

It is possible to import only features that have certain tags in the spatial database, as defined in the default.style configuration file. You can decide to comment in or out the OSM tagged features that you would like to import, or not, from this file. The command by default exports all the nodes and ways to linestring, point, and geometry PostGIS geometries.

It is highly recommended to enable hstore support in the PostgreSQL database and use the -hstore option of osm2pgsql when importing the data. Having enabled this support, the OSM tags for each feature will be stored in a hstore PostgreSQL data type, which is optimized for storing (and retrieving) sets of key/values pairs in a single field. This way, it will be possible to query the database as follows:

SELECT way, tags FROM planet_osm_polygon WHERE (tags -> 'landcover') = 'trees';