PROJ 6 has undergone extensive changes to increase its functional scope from a
cartographic projection engine with so-called “early-binding” geodetic datum
transformation capabilities to a more complete library supporting coordinate
transformations and coordinate reference systems.
As a foundation for other enhancements, PROJ now includes a C++ implementation
of the modelisation propopsed by the ISO-19111:2019 standard / OGC Abstract
Specification Topic 2: “Referencing By Coordinates”, for geodetic reference
frames (datums), coordinate reference systems and coordinate operations.
Construction and query of those geodetic objects is available through a new C++
API, and also accessible for the most part from bindings in the C API.
Those geodetic objects can be imported and exported from and into the OGC
Well-Known Text format (WKT) in its different variants: ESRI WKT, GDAL WKT 1,
WKT2:2015 (ISO 19162:2015) and WKT2:2018 (ISO 19162:2018). Import and export of
CRS objects from and into PROJ strings is also supported. This functionality
was previously available in the GDAL software library (except WKT2 support
which is a new feature), and is now an integral part of PROJ.
A unified database of geodetic objects, coordinate reference systems and their
metadata, and coordinate operations between those CRS is now available in a
SQLite3 database file, proj.db. This includes definitions imported from the
IOGP EPSG dataset (v9.6.0 release), the IGNF (French national mapping agency)
geodetic registry and the ESRI projection engine database. PROJ is now the
reference software in the “OSGeo C stack” for this CRS and coordinate operation
database, whereas previously this functionality was spread over PROJ, GDAL and
libgeotiff, and used CSV or other adhoc text-based formats.
Late-binding coordinate operation capabilities, that takes metadata such as
area of use and accuracy into account, has been added. This can avoid in a
number of situations the past requirement of using WGS84 as a pivot system,
which could cause unneeded accuracy loss, or was not doable at all sometimes
when transformation to WGS84 was not available. Those late-binding capabilities
are now used by the proj_create_crs_to_crs() function and the cs2cs utility.
A new command line utility, projinfo, has been added to query information about
a geodetic object of the database, import and export geodetic objects from/into
WKT and PROJ strings, and display coordinate operations available between two