Glossary

Short definitions of terms used throughout the pyproj documentation. For a longer, narrative introduction see Concepts. The PROJ project also maintains its own glossary.

Area of use

The geographic region a Coordinate Reference System (CRS) or coordinate operation is intended for. Outside of it the projection may be badly distorted or the transformation inaccurate. Available as 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 EPSG. 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 pyproj.crs.CRS.axis_info or use always_xy=True when creating a pyproj.transformer.Transformer. See also the Gotchas/FAQ page.

Bound CRS

A CRS bundled with an explicit transformation to a target CRS (usually WGS 84), for example a PROJ string containing +towgs84=. Represented by pyproj.crs.BoundCRS.

Compound CRS

A CRS made of a horizontal CRS plus a vertical CRS, so that coordinates carry a height as well as a position. Represented by pyproj.crs.CompoundCRS.

Conversion

In the strict sense used by PROJ and the ISO 19111 standard, a coordinate operation defined purely by mathematics, with no change of datum, such as applying a map projection. Conversions are exact. Compare with transformation.

Coordinate operation

Any process that takes coordinates in one CRS and produces coordinates in another. Either a conversion or a transformation, or a chain of both. Represented in pyproj by pyproj.transformer.Transformer.

Coordinate Reference System (CRS)

The full description needed to tie coordinates to positions on the Earth: a datum, a coordinate system (axes, units, and axis order), and for a projected CRS a projection. Represented by pyproj.crs.CRS.

Datum

A model of the Earth’s shape (an 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 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 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 authority codes work offline.

Geodesic

The shortest path between two points on the surface of an ellipsoid. Geodesic calculations give distances, azimuths, and areas on the curved Earth without using a projection. Provided by pyproj.Geod.

Geographic CRS

A CRS whose coordinates are angles on the ellipsoid, i.e. longitude and latitude, usually in degrees. Example: WGS 84 (EPSG:4326). Represented by pyproj.crs.GeographicCRS.

OGC

The Open Geospatial Consortium, the standards body that (jointly with ISO) publishes the specifications PROJ implements, including Well-Known Text (WKT) and the urn:ogc:def:... 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 authority code or Well-Known Text (WKT) for storing a CRS. See Gotchas/FAQ.

Projected CRS

A CRS in which a map projection has been applied to a geographic CRS, so that coordinates are distances (usually meters) on a plane. Examples: UTM zones, Web Mercator (EPSG:3857). Represented by 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 Projections.

Transformation

In everyday use, and in the names of pyproj.transformer.Transformer and its transform method, any 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 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 transformation grid. Compare with conversion.

Transformation grid

A data file of measured corrections that PROJ uses to make some transformations more accurate. Grids are not bundled with pyproj wheels; see Transformation Grids.

URN

Uniform Resource Name: a standard syntax for persistent identifiers. The OGC defines a URN form of 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 compound CRS with urn:ogc:def:crs,crs:EPSG::2393,crs:EPSG::5717. 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 projected CRS with coordinates in meters, for example EPSG:26917 (NAD83 / UTM zone 17N) and EPSG:32617 (WGS 84 / UTM zone 17N). Use 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 compound CRS. Represented by 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 authority code. Two versions exist; WKT2 is current and preferred. Produced by pyproj.crs.CRS.to_wkt().

WGS 84

World Geodetic System 1984, the datum (and ellipsoid) used by GPS and most web mapping. As a geographic CRS it is EPSG:4326.