Most of the Geotk library can work out of the box without any installation or setup. However they are some exceptions to this general rule. Some coordinate operations require a large amount of data, and not all those data are distributed with Geotk. For example the EPSG database is bundled in Geotk because it is considered a fundamental piece of referencing service applicable to many Coordinate Reference Systems in use around the world. On the other hand, NADCON grids are large (about 2 Mb, including approximatively 1 Mb for Alaska alone) while applicable to only one country.
Optional data like NADCON need to be saved in some specific directory. By default Geotk searches for the "Geotoolkit.org" sub-directory in the application directory. This is always a sub-directory of the user home directory, but the exact location is platform dependent. For example this is under "Application Data" on Windows, "Library" on MacOS and the home directory on Linux.
The Geotoolkit.org directory can be modified at user convenience through a graphical user interface provided by this module. To launch the application, run the following command from a directory which contain at least the following JAR files (omitting version numbers):
java -jar geotk-setup.jar