.. _glossary: Glossary ======== Short definitions of terms used throughout the pyproj documentation. For a longer, narrative introduction see :ref:`concepts`. The PROJ project also maintains its own :doc:`glossary `. .. glossary:: :sorted: Area of use The geographic region a :term:`Coordinate Reference System (CRS)` or :term:`coordinate operation` is intended for. Outside of it the projection may be badly distorted or the transformation inaccurate. Available as :attr:`pyproj.crs.CRS.area_of_use`. Authority code A short identifier for a predefined CRS, datum, ellipsoid, or operation in a registry, written as ``AUTHORITY:CODE``, for example ``EPSG:4326`` or ``ESRI:54009``. The most common authority is :term:`EPSG`. :func:`pyproj.database.get_authorities` lists the authorities known to PROJ. Axis order The order in which a CRS lists its coordinates. Many geographic CRSes, including ``EPSG:4326``, define latitude first and longitude second, whereas many software packages assume x/y (longitude/latitude). Check :attr:`pyproj.crs.CRS.axis_info` or use ``always_xy=True`` when creating a :class:`pyproj.transformer.Transformer`. See also the :ref:`gotchas` page. Bound CRS A CRS bundled with an explicit :term:`transformation` to a target CRS (usually WGS 84), for example a PROJ string containing ``+towgs84=``. Represented by :class:`pyproj.crs.BoundCRS`. Compound CRS A CRS made of a horizontal CRS plus a :term:`vertical CRS`, so that coordinates carry a height as well as a position. Represented by :class:`pyproj.crs.CompoundCRS`. Conversion In the strict sense used by PROJ and the ISO 19111 standard, a :term:`coordinate operation` defined purely by mathematics, with no change of :term:`datum`, such as applying a map :term:`projection`. Conversions are exact. Compare with :term:`transformation`. Coordinate operation Any process that takes coordinates in one CRS and produces coordinates in another. Either a :term:`conversion` or a :term:`transformation`, or a chain of both. Represented in pyproj by :class:`pyproj.transformer.Transformer`. Coordinate Reference System (CRS) The full description needed to tie coordinates to positions on the Earth: a :term:`datum`, a coordinate system (axes, units, and :term:`axis order`), and for a :term:`projected CRS` a :term:`projection`. Represented by :class:`pyproj.crs.CRS`. Datum A model of the Earth's shape (an :term:`ellipsoid`) together with how that model is positioned and oriented relative to the real Earth. Two CRSes with different datums can report different coordinates for the same physical location. Available as :attr:`pyproj.crs.CRS.datum`. Ellipsoid The slightly flattened sphere used to approximate the Earth's shape, defined by a semi-major axis and a flattening. Examples: WGS 84, GRS 1980, Clarke 1866. Available as :attr:`pyproj.crs.CRS.ellipsoid`. EPSG The most widely used registry of CRS, datum, ellipsoid, and coordinate operation definitions, originally created by the European Petroleum Survey Group and now maintained by the International Association of Oil & Gas Producers (IOGP) at `epsg.org `__. PROJ ships a copy of it as its database, so ``EPSG:4326`` style :term:`authority codes ` work offline. Geodesic The shortest path between two points on the surface of an :term:`ellipsoid`. Geodesic calculations give distances, azimuths, and areas on the curved Earth without using a projection. Provided by :class:`pyproj.Geod`. Geographic CRS A CRS whose coordinates are angles on the :term:`ellipsoid`, i.e. longitude and latitude, usually in degrees. Example: WGS 84 (``EPSG:4326``). Represented by :class:`pyproj.crs.GeographicCRS`. OGC The Open Geospatial Consortium, the standards body that (jointly with ISO) publishes the specifications PROJ implements, including :term:`Well-Known Text (WKT)` and the ``urn:ogc:def:...`` :term:`URN` scheme for identifying CRSes. Prime meridian The line of zero longitude for a CRS. Almost always Greenwich, but some historical CRSes use others (Paris, Ferro, ...). PROJ The C/C++ library (https://proj.org) that pyproj wraps. PROJ implements the projections and transformations and ships the CRS database. PROJ string PROJ's compact ``+key=value`` syntax for describing a CRS or a pipeline, for example ``+proj=utm +zone=17 +datum=NAD83`` or ``+proj=latlon``. Convenient, but it cannot express everything in a CRS, so prefer an :term:`authority code` or :term:`Well-Known Text (WKT)` for storing a CRS. See :ref:`gotchas`. Projected CRS A CRS in which a map :term:`projection` has been applied to a :term:`geographic CRS`, so that coordinates are distances (usually meters) on a plane. Examples: :term:`UTM` zones, Web Mercator (``EPSG:3857``). Represented by :class:`pyproj.crs.ProjectedCRS`. Projection The mathematical recipe for flattening the curved surface of the Earth onto a plane, such as Transverse Mercator or Lambert Conformal Conic. Every projection distorts some combination of shape, area, distance, or direction. PROJ lists all supported projections in :ref:`proj:projections`. Transformation In everyday use, and in the names of :class:`pyproj.transformer.Transformer` and its ``transform`` method, any :term:`coordinate operation` from one CRS to another. In the stricter sense used by PROJ and the ISO 19111 standard, only an operation between two different :term:`datums `; because such an operation is based on measurements rather than pure mathematics it is approximate and has an accuracy, and the most accurate ones often need a :term:`transformation grid`. Compare with :term:`conversion`. Transformation grid A data file of measured corrections that PROJ uses to make some :term:`transformations ` more accurate. Grids are not bundled with pyproj wheels; see :ref:`transformation_grids`. URN Uniform Resource Name: a standard syntax for persistent identifiers. The :term:`OGC` defines a URN form of :term:`authority codes `, for example ``urn:ogc:def:crs:EPSG::4326`` is the same CRS as ``EPSG:4326``, and several codes can be combined into a :term:`compound CRS` with ``urn:ogc:def:crs,crs:EPSG::2393,crs:EPSG::5717``. :class:`pyproj.crs.CRS` accepts both forms. UTM Universal Transverse Mercator: a projection system that divides the Earth into 60 longitude zones, each 6° wide, with a northern and southern hemisphere variant of each zone. Each zone/hemisphere/datum combination is a separate :term:`projected CRS ` with coordinates in meters, for example ``EPSG:26917`` (NAD83 / UTM zone 17N) and ``EPSG:32617`` (WGS 84 / UTM zone 17N). Use :func:`pyproj.database.query_utm_crs_info` to find the zone for a location. Vertical CRS A CRS describing heights only, for example height above a particular sea-level model. Usually combined with a horizontal CRS in a :term:`compound CRS`. Represented by :class:`pyproj.crs.VerticalCRS`. Well-Known Text (WKT) A standard text format that spells out the complete definition of a CRS (datum, ellipsoid, axes, units, projection parameters, identifiers). Lossless, and the preferred way to store a CRS that has no :term:`authority code`. Two versions exist; WKT2 is current and preferred. Produced by :meth:`pyproj.crs.CRS.to_wkt`. WGS 84 World Geodetic System 1984, the :term:`datum` (and :term:`ellipsoid`) used by GPS and most web mapping. As a :term:`geographic CRS` it is ``EPSG:4326``.