PostGIS is an open source, easy to install, and freely distributed software project, an extension for the PostgreSQL object-relational database server. It has been created from the ground up to implement support for geographic objects in PostgreSQL.
Allows PostgreSQL to be used as a backend spatial database for GIS
In other words, PostGIS "spatially enables" the PostgreSQL database engine, allowing it to be used as a backend spatial database for GIS (Geographic Information Systems), similar to Oracle's Spatial extension or ESRI's SDE.
There are now several open source tools which work with the PostGIS software. The uDig project is working on a full read and write desktop environment that can work with PostGIS directly.
Installing PostGIS on a PostgreSQL server
To install the PostGIS extension on your PostgreSQL database server, download the latest release from Softpedia, save the archive on a location of your choice and unpack it in the ‘contrib’ directory of your PostgreSQL source tree.
Open a Terminal app, navigate to the extracted folder in the contrib directory using the ‘cd’ command (e.g. cd contrib/postgis-0.9.0), run the ‘make’ command to compile the extension, then run the ‘make install’ command as root or the ‘sudo make install’ command as a privileged user to install it.
Additionally, you must run the ‘createlang plpgsql yourtestdatabase’, the ‘psql -d yourtestdatabase -f postgis.sql’ and ‘psql -d yourtestdatabase -f spatial_ref_sys.sql’ commands, where “yourtestdatabase” should be the name of a test database.
Under the hood and supported OSes
While taking a quick look under the hood of the PostGIS software, we’ve noticed that its components where written in either SQL or Perl programming languages. The software can be installed on any Linux kernel-based operating system where a supported PostgreSQL server is available. Both 32 and 64-bit computer platforms are supported at this time.