# General documentation¶

general_api_design

General API design.

The design of the class hierarchy is strongly derived from ISO_19111_2019.

Classes for which the constructors are not directly accessible have their instances constructed with create() methods. The returned object is a non-null shared pointer. Such objects are immutable, and thread-safe.

TODO

general_properties

General properties.

All classes deriving from IdentifiedObject have general properties that can be defined at creation time. Those properties are:

standards

Applicable standards.

ISO_19111

ISO:19111 / OGC Topic 2 standard.

Topic 2 - Spatial referencing by coordinates.

This is an Abstract Specification describes the data elements, relationships and associated metadata required for spatial referencing by coordinates. It describes Coordinate Reference Systems (CRS), coordinate systems (CS) and coordinate transformation or coordinate conversion between two different coordinate reference systems.

ISO_19111_2019

ISO 19111:2019.

This is the revision mostly used for PROJ C++ modelling.

ISO_19111_2007

ISO 19111:2007.

The precedent version of the specification was: OGC 08-015r2, 2010-04-27, ISO 19111:2007

WKT2

WKT2 standard.

Well-known text representation of coordinate reference systems.

Well-known Text (WKT) offers a compact machine- and human-readable representation of the critical elements of coordinate reference system (CRS) definitions, and coordinate operations. This is an implementation of ISO_19111

PROJ implements the two following revisions of the standard:

WKT2_2018

WKT2:2018.

OGC 18-010r1, 2018-06-08, WKT2-2018 (not yet adopted, at time of writing)

WKT2_2015

WKT2:2015.

WKT1

WKT1 specification.

Older specifications of well-known text representation of coordinate reference systems are also supported by PROJ, mostly for compatibility with legacy systems, or older versions of GDAL.

GDAL v2.3 and earlier mostly implements:

OGC 01-009, 2001-01-12, OpenGIS Coordinate Transformation Service Implementation Specification

The GDAL documentation, OGC WKT Coordinate System Issues discusses issues, and GDAL implementation choices.

An older specification of WKT1 is/was used by some software packages:

ISO_19115

GeoAPI