Class: QgsCoordinateReferenceSystem

class qgis.core.QgsCoordinateReferenceSystem

Bases: sip.wrapper

Constructs an invalid CRS object

QgsCoordinateReferenceSystem(definition: str) Constructs a CRS object from a string definition using createFromString()

It supports the following formats: - “EPSG:<code>” - handled with createFromOgcWms() - “POSTGIS:<srid>” - handled with createFromSrid() - “INTERNAL:<srsid>” - handled with createFromSrsId() - “PROJ4:<proj4>” - handled with createFromProj4() - “WKT:<wkt>” - handled with createFromWkt()

If no prefix is specified, WKT definition is assumed.

Parameters:definition – A String containing a coordinate reference system definition.

QgsCoordinateReferenceSystem(id: int, type: QgsCoordinateReferenceSystem.CrsType = QgsCoordinateReferenceSystem.PostgisCrsId) Constructor a CRS object using a PostGIS SRID, an EPSG code or an internal QGIS CRS ID.

Note

We encourage you to use EPSG code, WKT or Proj4 to describe CRS’s in your code wherever possible. Internal QGIS CRS IDs are not guaranteed to be permanent / involatile.

Parameters:
  • id – The ID valid for the chosen CRS ID type
  • type – One of the types described in CrsType

QgsCoordinateReferenceSystem(srs: QgsCoordinateReferenceSystem) Copy constructor

This class represents a coordinate reference system (CRS).

Coordinate reference system object defines a specific map projection, as well as transformations between different coordinate reference systems. There are various ways how a CRS can be defined: using well-known text (WKT), PROJ string or combination of authority and code (e.g. EPSG:4326). QGIS comes with its internal database of coordinate reference systems (stored in SQLite) that allows lookups of CRS and seamless conversions between the various definitions.

Most commonly one comes across two types of coordinate systems:

1. Geographic coordinate systems - based on a geodetic datum, normally with coordinates being latitude/longitude in degrees. The most common one is World Geodetic System 84 (WGS84). 2. Projected coordinate systems - based on a geodetic datum with coordinates projected to a plane, typically using meters or feet as units. Common projected coordinate systems are Universal Transverse Mercator or Albers Equal Area.

Internally QGIS uses proj4 library for all the math behind coordinate transformations, so in case of any troubles with projections it is best to examine the PROJ representation within the object, as that is the representation that will be ultimately used.

Methods that allow inspection of CRS instances include isValid(), authid(), description(), toWkt(), toProj4(), mapUnits() and others. Creation of CRS instances is further described in ref crs_construct_and_copy section below. Transformations between coordinate reference systems are done using QgsCoordinateTransform class.

For example, the following code will create and inspect “British national grid” CRS:

crs = QgsCoordinateReferenceSystem("EPSG:27700")
if crs.isValid():
    print("CRS Description: {}".format(crs.description()))
    print("CRS PROJ text: {}".format(crs.toProj4()))
else:
    print("Invalid CRS!")

This will produce the following output:

This section gives an overview of various supported CRS definition formats:

1. Authority and Code. Also referred to as OGC WMS format within QGIS as they have been widely used in OGC standards. These are encoded as <auth>:<code>, for example EPSG:4326 refers to WGS84 system. EPSG is the most commonly used authority that covers a wide range of coordinate systems around the world.

An extended variant of this format is OGC URN. Syntax of URN for CRS definition is urn:ogc:def:crs:<auth>:[<version>]:<code>. This class can also parse URNs (versions are currently ignored). For example, WGS84 may be encoded as urn:ogc:def:crs:OGC:1.3:CRS84.

QGIS adds support for “USER” authority that refers to IDs used internally in QGIS. This variant is best avoided or used with caution as the IDs are not permanent and they refer to different CRS on different machines or user profiles.

See authid() and createFromOgcWmsCrs() methods.

2. PROJ string. This is a string consisting of a series of key/value pairs in the following format: +param1=value1 +param2=value2 […]. This is the format natively used by the underlying proj4 library. For example, the definition of WGS84 looks like this:

+proj=longlat +datum=WGS84 +no_defs

See toProj4() and createFromProj4() methods.

3. Well-known text (WKT). Defined by Open Geospatial Consortium (OGC), this is another common format to define CRS. For WGS84 the OGC WKT definition is the following:

GEOGCS[“WGS 84”, DATUM[“WGS_1984”, SPHEROID[“WGS 84”,6378137,298.257223563,AUTHORITY[“EPSG”,”7030”]], AUTHORITY[“EPSG”,”6326”]], PRIMEM[“Greenwich”,0,AUTHORITY[“EPSG”,”8901”]], UNIT[“degree”,0.0174532925199433,AUTHORITY[“EPSG”,”9122”]], AUTHORITY[“EPSG”,”4326”]]

See toWkt() and createFromWkt() methods.

The database of CRS shipped with QGIS is stored in a SQLite database (see QgsApplication.srsDatabaseFilePath()) and it is based on the data files maintained by GDAL project (a variety of .csv and .wkt files).

Sometimes it happens that users need to use a CRS definition that is not well known or that has been only created with a specific purpose (and thus its definition is not available in our database of CRS). Whenever a new CRS definition is seen, it will be added to the local database (in user’s home directory, see QgsApplication.qgisUserDatabaseFilePath()). QGIS also features a GUI for management of local custom CRS definitions.

There are therefore two databases: one for shipped CRS definitions and one for custom CRS definitions. Custom CRS have internal IDs (accessible with srsid()) greater or equal to ref USER_CRS_START_ID. The local CRS databases should never be accessed directly with SQLite functions, instead you should use QgsCoordinateReferenceSystem API for CRS lookups and for managements of custom CRS.

In some cases (most prominently when loading a map layer), QGIS will try to ensure that the given map layer CRS is valid using validate() call. If not, a custom validation function will be called - such function may for example show a GUI for manual CRS selection. The validation function is configured using setCustomCrsValidation(). If validation fails or no validation function is set, the default CRS is assigned (WGS84). QGIS application registers its validation function that will act according to user’s settings (either show CRS selector dialog or use project/custom CRS).

The easiest way of creating CRS instances is to use QgsCoordinateReferenceSystem(const QString&) constructor that automatically recognizes definition format from the given string.

Creation of CRS object involves some queries in a local SQLite database, which may be potentially expensive. Consequently, CRS creation methods use an internal cache to avoid unnecessary database lookups. If the CRS database is modified, then it is necessary to call invalidateCache() to ensure that outdated records are not being returned from the cache.

Since QGIS 2.16 QgsCoordinateReferenceSystem objects are implicitly shared.

There are two different flavors of WKT: one is defined by OGC, the other is the standard used by ESRI. They look very similar, but they are not the same. QGIS is able to consume both flavors.

See also

QgsCoordinateTransform

Enums

Methods

authid Returns the authority identifier for the CRS.
bounds Returns the approximate bounds for the region the CRS is usable within.
createFromId Sets this CRS by lookup of the given ID in the CRS database.
createFromOgcWmsCrs Sets this CRS to the given OGC WMS-format Coordinate Reference Systems.
createFromProj4 Sets this CRS by passing it a PROJ style formatted string.
createFromSrid Sets this CRS by lookup of the given PostGIS SRID in the CRS database.
createFromSrsId Sets this CRS by lookup of internal QGIS CRS ID in the CRS database.
createFromString Set up this CRS from a string definition.
createFromUserInput Set up this CRS from various text formats.
createFromWkt Sets this CRS using a WKT definition.
description Returns the descriptive name of the CRS, e.g., “WGS 84” or “GDA 94 / Vicgrid94”.
ellipsoidAcronym Returns the ellipsoid acronym for the ellipsoid used by the CRS.
findMatchingProj Walks the CRS databases (both system and user database) trying to match stored PROJ string to a database entry in order to fill in further pieces of information about CRS.
fromEpsgId Creates a CRS from a given EPSG ID.
fromOgcWmsCrs Creates a CRS from a given OGC WMS-format Coordinate Reference System string.
fromProj4 Creates a CRS from a proj4 style formatted string.
fromSrsId Creates a CRS from a specified QGIS SRS ID.
fromWkt Creates a CRS from a WKT spatial ref sys definition string.
geographicCrsAuthId Returns auth id of related geographic CRS
hasAxisInverted Returns whether axis is inverted (e.g., for WMS 1.3) for the CRS.
invalidateCache Clears the internal cache used to initialize QgsCoordinateReferenceSystem objects.
isGeographic Returns whether the CRS is a geographic CRS (using lat/lon coordinates)
isValid Returns whether this CRS is correctly initialized and usable
mapUnits Returns the units for the projection used by the CRS.
postgisSrid Returns PostGIS SRID for the CRS.
projectionAcronym Returns the projection acronym for the projection used by the CRS.
readXml Restores state from the given DOM node.
recentProjections Returns a list of recently used projections
saveAsUserCrs Save the proj4-string as a custom CRS.
setValidationHint Set user hint for validation
setupESRIWktFix Make sure that ESRI WKT import is done properly.
srsid Returns the internal CRS ID, if available.
syncDatabase Update proj.4 parameters in our database from proj.4
toProj4 Returns a Proj4 string representation of this CRS.
toWkt Returns a WKT representation of this CRS.
validSrsIds Returns a list of all valid SRS IDs present in the CRS database.
validate Perform some validation on this CRS.
validationHint Gets user hint for validation
writeXml Stores state to the given Dom node in the given document.

Signals

Attributes

EpsgCrsId
InternalCrsId
PostgisCrsId
staticMetaObject
class CrsType

Bases: int

EpsgCrsId = 2
InternalCrsId = 0
PostgisCrsId = 1
authid(self) → str

Returns the authority identifier for the CRS.

The identifier includes both the authority (e.g., EPSG) and the CRS number (e.g., 4326). This is the best method to use when showing a very short CRS identifier to a user, e.g., “EPSG:4326”.

If CRS object is a custom CRS (not found in database), the method will return internal QGIS CRS ID with “QGIS” authority, for example “QGIS:100005”

Returns:the authority identifier for this CRS

See also

description()

bounds(self) → QgsRectangle

Returns the approximate bounds for the region the CRS is usable within.

The returned bounds represent the latitude and longitude extent for the projection in the WGS 84 CRS.

New in version 3.0.

createFromId(self, id: int, type: QgsCoordinateReferenceSystem.CrsType = QgsCoordinateReferenceSystem.PostgisCrsId) → bool

Sets this CRS by lookup of the given ID in the CRS database.

Returns:True on success else False

Note

We encourage you to use EPSG code, WKT or Proj4 to describe CRS’s in your code wherever possible. Internal QGIS CRS IDs are not guaranteed to be permanent / involatile.

createFromOgcWmsCrs(self, crs: str) → bool

Sets this CRS to the given OGC WMS-format Coordinate Reference Systems.

Accepts both “<auth>:<code>” format and OGC URN “urn:ogc:def:crs:<auth>:[<version>]:<code>”. It also recognizes “QGIS”, “USER”, “CUSTOM” authorities, which all have the same meaning and refer to QGIS internal CRS IDs.

Returns:True on success else False

Note

this method uses an internal cache. Call invalidateCache() to clear the cache.

See also

fromOgcWmsCrs()

createFromProj4(self, projString: str) → bool

Sets this CRS by passing it a PROJ style formatted string.

The string will be parsed and the projection and ellipsoid members set and the remainder of the proj4 string will be stored in the parameters member. The reason for this is so that we can easily present the user with ‘natural language’ representation of the projection and ellipsoid by looking them up in the srs.db sqlite database.

We try to match the proj4 string to internal QGIS CRS ID using the following logic:

  • perform a whole text search on proj4 string (if not null)
  • if not match is found, split proj4 into individual parameters and try to find

a match where the parameters are in a different order - if none of the above match, use findMatchingProj()

Parameters:projString – A proj4 format string
Returns:True on success else False

Note

Some members may be left blank if no match can be found in CRS database.

Note

this method uses an internal cache. Call invalidateCache() to clear the cache.

See also

fromProj4()

createFromSrid(self, srid: int) → bool

Sets this CRS by lookup of the given PostGIS SRID in the CRS database.

Parameters:srid – The PostGIS SRID for the desired spatial reference system.
Returns:True on success else False
createFromSrsId(self, srsId: int) → bool

Sets this CRS by lookup of internal QGIS CRS ID in the CRS database.

If the srsid is < USER_CRS_START_ID, system CRS database is used, otherwise user’s local CRS database from home directory is used.

Parameters:srsId – The internal QGIS CRS ID for the desired spatial reference system.
Returns:True on success else False

Note

this method uses an internal cache. Call invalidateCache() to clear the cache.

See also

fromSrsId()

createFromString(self, definition: str) → bool

Set up this CRS from a string definition.

It supports the following formats: - “EPSG:<code>” - handled with createFromOgcWms() - “POSTGIS:<srid>” - handled with createFromSrid() - “INTERNAL:<srsid>” - handled with createFromSrsId() - “PROJ4:<proj4>” - handled with createFromProj4() - “WKT:<wkt>” - handled with createFromWkt()

If no prefix is specified, WKT definition is assumed.

Parameters:definition – A String containing a coordinate reference system definition.
Returns:True on success else False
createFromUserInput(self, definition: str) → bool

Set up this CRS from various text formats.

Valid formats: WKT string, “EPSG:n”, “EPSGA:n”, “AUTO:proj_id,unit_id,lon0,lat0”, “urn:ogc:def:crs:EPSG.n”, PROJ string, filename (with WKT, XML or PROJ string), well known name (such as NAD27, NAD83, WGS84 or WGS72), ESRI.[WKT string] (directly or in a file), “IGNF:xxx”

For more details on supported formats see OGRSpatialReference.SetFromUserInput() ( http://www.gdal.org/ogr/classOGRSpatialReference.html#aec3c6a49533fe457ddc763d699ff8796 )

Parameters:definition – A String containing a coordinate reference system definition.
Returns:True on success else False

Note

this function generates a WKT string using OSRSetFromUserInput() and passes it to createFromWkt() function. / // TODO QGIS3: rename to createFromStringOGR so it is clear it’s similar to createFromString, just different backend

createFromWkt(self, wkt: str) → bool

Sets this CRS using a WKT definition.

If EPSG code of the WKT definition can be determined, it is extracted and createFromOgcWmsCrs() is used to initialize the object. Otherwise the WKT will be converted to a proj4 string and createFromProj4() set up the object.

Parameters:wkt – The WKT for the desired spatial reference system.
Returns:True on success else False

Note

Some members may be left blank if no match can be found in CRS database.

Note

this method uses an internal cache. Call invalidateCache() to clear the cache.

See also

fromWkt()

description(self) → str

Returns the descriptive name of the CRS, e.g., “WGS 84” or “GDA 94 / Vicgrid94”. In most cases this is the best method to use when showing a friendly identifier for the CRS to a user.

Returns:descriptive name of the CRS

Note

an empty string will be returned if the description is not available for the CRS

See also

authid()

ellipsoidAcronym(self) → str

Returns the ellipsoid acronym for the ellipsoid used by the CRS.

Returns:the official proj4 acronym for the ellipoid

Note

an empty string will be returned if the ellipsoidAcronym is not available for the CRS

findMatchingProj(self) → int

Walks the CRS databases (both system and user database) trying to match stored PROJ string to a database entry in order to fill in further pieces of information about CRS.

Note

The ellipsoid and projection acronyms must be set as well as the proj4string!

Returns:long the SrsId of the matched CRS, zero if no match was found
fromEpsgId(epsg: int) → QgsCoordinateReferenceSystem

Creates a CRS from a given EPSG ID.

Parameters:epsg – epsg CRS ID
Returns:matching CRS, or an invalid CRS if string could not be matched

New in version 3.0.

fromOgcWmsCrs(ogcCrs: str) → QgsCoordinateReferenceSystem

Creates a CRS from a given OGC WMS-format Coordinate Reference System string.

Parameters:ogcCrs – OGR compliant CRS definition, e.g., “EPSG:4326”
Returns:matching CRS, or an invalid CRS if string could not be matched

New in version 3.0.

fromProj4(proj4: str) → QgsCoordinateReferenceSystem

Creates a CRS from a proj4 style formatted string.

Parameters:proj4 – proj4 format string
Returns:matching CRS, or an invalid CRS if string could not be matched

New in version 3.0.

fromSrsId(srsId: int) → QgsCoordinateReferenceSystem

Creates a CRS from a specified QGIS SRS ID.

Parameters:srsId – internal QGIS SRS ID
Returns:matching CRS, or an invalid CRS if ID could not be found

See also

validSrsIds()

New in version 3.0.

fromWkt(wkt: str) → QgsCoordinateReferenceSystem

Creates a CRS from a WKT spatial ref sys definition string.

Parameters:wkt – WKT for the desired spatial reference system.
Returns:matching CRS, or an invalid CRS if string could not be matched

See also

createFromWkt()

New in version 3.0.

geographicCrsAuthId(self) → str

Returns auth id of related geographic CRS

hasAxisInverted(self) → bool

Returns whether axis is inverted (e.g., for WMS 1.3) for the CRS.

Returns:True if CRS axis is inverted
invalidateCache()

Clears the internal cache used to initialize QgsCoordinateReferenceSystem objects. This should be called whenever the srs database has been modified in order to ensure that outdated CRS objects are not created.

New in version 3.0.

isGeographic(self) → bool

Returns whether the CRS is a geographic CRS (using lat/lon coordinates)

Returns:True if CRS is geographic, or False if it is a projected CRS
isValid(self) → bool

Returns whether this CRS is correctly initialized and usable

mapUnits(self) → QgsUnitTypes.DistanceUnit

Returns the units for the projection used by the CRS.

postgisSrid(self) → int

Returns PostGIS SRID for the CRS.

Returns:the PostGIS spatial_ref_sys identifier for this CRS (defaults to 0)
projectionAcronym(self) → str

Returns the projection acronym for the projection used by the CRS.

Returns:the official proj4 acronym for the projection family

Note

an empty string will be returned if the projectionAcronym is not available for the CRS

readXml(self, node: QDomNode) → bool

Restores state from the given DOM node. If it fails or if the node is empty, a default empty CRS will be returned.

Parameters:node – The node from which state will be restored
Returns:bool True on success, False on failure
recentProjections() → List[str]

Returns a list of recently used projections

Returns:list of srsid for recently used projections

New in version 2.7.

saveAsUserCrs(self, name: str) → int

Save the proj4-string as a custom CRS.

Returns the new CRS srsid(), or -1 if the CRS could not be saved.

setValidationHint(self, html: str)

Set user hint for validation

setupESRIWktFix()

Make sure that ESRI WKT import is done properly. This is required for proper shapefile CRS import when using gdal>= 1.9.

Note

This function is called by createFromUserInput() and QgsOgrProvider.crs(), there is usually no need to call it from elsewhere.

Note

This function sets CPL config option GDAL_FIX_ESRI_WKT to a proper value, unless it has been set by the user through the commandline or an environment variable. For more details refer to OGRSpatialReference.morphFromESRI() .

srsid(self) → int

Returns the internal CRS ID, if available.

Returns:the internal sqlite3 srs.db primary key for this CRS
staticMetaObject = <PyQt5.QtCore.QMetaObject object>
syncDatabase() → int

Update proj.4 parameters in our database from proj.4

Returns:number of updated CRS on success and negative number of failed updates in case of errors.

Note

This is used internally and should not be necessary to call in client code

toProj4(self) → str

Returns a Proj4 string representation of this CRS.

If proj and ellps keys are found in the parameters, they will be stripped out and the projection and ellipsoid acronyms will be overridden with these.

Returns:Proj4 format string that defines this CRS.

Note

an empty string will be returned if the CRS could not be represented by a Proj4 string

See also

toWkt()

toWkt(self) → str

Returns a WKT representation of this CRS.

Returns:string containing WKT of the CRS

See also

toProj4()

validSrsIds() → object

Returns a list of all valid SRS IDs present in the CRS database. Any of the returned values can be safely passed to fromSrsId() to create a new, valid QgsCoordinateReferenceSystem object.

See also

fromSrsId()

New in version 3.0.

validate(self)

Perform some validation on this CRS. If the CRS doesn’t validate the default behavior settings for layers with unknown CRS will be consulted and acted on accordingly. By hell or high water this method will do its best to make sure that this CRS is valid - even if that involves resorting to a hard coded default of geocs:wgs84.

Note

It is not usually necessary to use this function, unless you are trying to force this CRS to be valid.

See also

setCustomCrsValidation()

validationHint(self) → str

Gets user hint for validation

writeXml(self, node: QDomNode, doc: QDomDocument) → bool

Stores state to the given Dom node in the given document.

Parameters:
  • node – The node in which state will be restored
  • doc – The document in which state will be stored
Returns:

bool True on success, False on failure