PROJ

PROJ is an open-source software library and command-line toolkit for performing cartographic projections and coordinate transformations. It converts coordinates between different coordinate reference systems (CRS), including geographic coordinates (latitude and longitude) and projected coordinates (x and y), while applying datum shifts and grid-based corrections when required. The library is data-driven, relying on a database of CRS definitions and transformation operations.

Origins and development of PROJ trace back to PROJ.4, a widely used library created to support coordinate transformations in GIS. In the 2010s the project was taken forward under the Open Source Geospatial Foundation (OSGeo) and evolved into the modern PROJ, with a focus on modular design, expanded CRS coverage, and a robust API. A major refactor around PROJ 6 introduced a new internal architecture, improved handling of coordinate operation pipelines, and tightened integration with the EPSG dataset and grid-shift files. Subsequent releases have continued to enhance performance, accuracy, and data formats, including better support for JSON and modern CRSs.

Functionality centers on transforming coordinates between CRSs, including the ability to chain multiple operations (datum shifts, projections, and grid-based corrections) into a pipeline. It supports definitions in PROJ strings, Well-Known Text (WKT), and modern CRS databases, and can utilize grid files such as NADCON and NTv2 for high-precision transformations. PROJ provides a C library (libproj) with bindings to other languages and is embedded in many GIS applications and libraries, including GDAL/OGR, QGIS, and PostGIS, as well as being usable directly via a command-line tool. It is released under an open-source permissive license.