Geocoordinates

CARLA parses a georeference from metadata provided in the OpenDRIVE header that enables the conversion of a CARLA location given in X, Y and Z coordinates (in meters) into a geocoordinate given in latitude (degrees), longitude (degrees) and altitude (meters). The reverse transformation is also possible from latitude, longitude and altitude into a CARLA coordinate.

A map projection is used to convert between CARLA coordinates and geocoordinates. This projection is used in the GNSS sensor and the transform_to_geolocation() and geolocation_to_transform() methods of the Map object. The type of projection, a reference ellipsoid along with the geolocation of the center of the map (i.e. X=Y=0) and other parameters associated with the projection are defined in the OpenDRIVE header information in the <geoReference> tag in a PROJ.4-style string.

The following is an example of a <geoReference> tag provided in an OpenDRIVE file header. Details on the meaning of each parameter are given in the following sections:

<geoReference><![CDATA[+proj=tmerc +lat_0=0 +lon_0=0 +k=1 +x_0=0 +y_0=0 +ellps=WGS84]]></geoReference>

The georeference defined in the OpenDRIVE and geoprojection can be queried through the CARLA Python API using the get_georeference() and get_geoprojection() methods of the Map object.

Map projection

CARLA supports 4 different map projection types. The projection type is denoted in the +proj field of the georeference:

Map projection	OpenDRIVE header
Transverse Mercator (default)	`+proj=tmerc`
Universal Transverse Mercator	`+proj=utm`
Web Mercator	`+proj=merc`
Lambert Conformal Conic - 2SP version	`+proj=lcc`

Each map projection requires a specific set of parameters defined in the OpenDRIVE georeference:

Transverse mercator

Parameter	Description
`+proj`	`tmerc`
`+ellps`	Reference ellipsoid used to model the Earth's shape
`+lat_0`	Latitude value for the origin of the map
`+lon_0`	Longitude value for the origin of the map
`k`	Scale factor at origin
`+a`	Semi-major axis of the ellipsoid model
`+b`	Semi-major axis of the ellipsoid model
`+f`	Flattening of the ellipsoid model (a-b)/a
`+f_inv`	Inverse flattening of the ellipsoid model 1/f
`x_0`	False Easting offset
`y_0`	False Northing offset

Universal transverse mercator

Parameter	Description
`+proj`	`utm`
`+ellps`	Reference ellipsoid used to model the Earth's shape
`+zone`	Defines the longitudinal zone for the UTM projection (1-60)
`+north`	Defines the geolocation as in the Northern hemisphere (default)
`+south`	Defines the geolocation as in the Southern hemisphere
`+a`	Semi-major axis of the ellipsoid model
`+b`	Semi-major axis of the ellipsoid model
`+f`	Flattening of the ellipsoid model (a-b)/a
`+f_inv`	Inverse flattening of the ellipsoid model 1/f

Web mercator

The Web mercator assumes a spherical Earth, therefore only the a parameter, the semi-major axis of the ellipsoid is used.

Parameter	Description
`+proj`	`merc`
`+ellps`	Reference ellipsoid used to model the Earth's shape
`+a`	Semi-major axis of the ellipsoid model

Lambert conformal conic 2SP

Parameter	Description
`+proj`	`lcc`
`+ellps`	Reference ellipsoid used to model the Earth's shape
`+lat_0`	Latitude value for the origin of the map
`+lat_1`	Latitude value for the upper latitude limit
`+lat_2`	Latitude value for the lower latitude limit
`+lon_0`	Longitude value for the origin of the map
`+a`	Semi-major axis of the ellipsoid model
`+b`	Semi-major axis of the ellipsoid model
`+f`	Flattening of the ellipsoid model (a-b)/a
`+f_inv`	Inverse flattening of the ellipsoid model 1/f
`x_0`	False Easting offset
`y_0`	False Northing offset

Reference ellipsoid

The reference ellipsoid is the ellipsoid model used to model the Earth's surface. Since the Earth is closer to ellipsoidal than spherical a simple spherical model may cause inaccuracies which are not acceptable for certain applications. The reference ellipsoid can be defined using several parameters:

Parameter	Description
a	Semi major axis
b	Semi minor axis
f	Flattening (a-b)/a
f_inv	Inverse flattening 1/f

CARLA supports numerous commonly used reference ellipsoids. The type of ellipsoid to use is defined in the +ellps field of the <geoReference> tag:

Ref. ellipsoid	a	f_inv
`wgs84` (default)	6378137.0	298.257223563
`grs80`	6378137.0	298.257222101
`intl`	6378388.0	297.0
`bessel`	6377397.155	299.1528128
`clrk66`	6378206.4	294.9786982138
`airy`	6377563.396	299.3249646
`wgs72`	6378135.0	298.26
`wgs66`	6378145.0	298.25
`sphere`	6370997.0	inf

If a reference ellipsoid is defined in the georeference tag and then parameters are also given, the given parameters will override those for the chosen reference ellipsoid.

Geocoordinates and the Python API

The Python API provides the facility to convert between CARLA coordinates and geocoordinates and also to define projections with custom parameters. The CARLA Map object maintains the information about the georeference gathered from the OpenDRIVE header and provides methods for conversion:

# Retrieve the map object
carla_map = world.get_map()

# Convert a carla.Location to a carla.Geolocation
geolocation = carla_map.transform_to_geolocation(carla.Location(10,10,1))

# Convert a carla.Geolocation to a carla.Location
geolocation = carla.GeoLocation(latitude=41.400779, longitude=2.188103, altitude=0)
location = carla_map.geolocation_to_transform(geolocation)

The Map object provides methods to inspect the current georeference and geoprojection:

# Get the georeference
carla_map.get_georeference() # returns a carla.GeoLocation

# Get the geoprojection
carla_map.get_geoprojection() # returns a carla.GeoProjection

Custom projection

If an alternative georeference is desired, different from that provided in the OpenDRIVE metadata, one can be created using additional API objects.

To define a custom ellipsoid use the carla.GeoEllipsoid object:

geoellipsoid = carla.GeoEllipsoid(a=6378137.0, f_inv=297.0)

Then to define a projection, use the relevant GeoProjection object:

Projection type	CARLA object
Transverse Mercator	carla.GeoProjectionTM
Universal Transverse Mercator	carla.GeoProjectionUTM
Web Mercator	carla.GeoProjectionWebMerc
Lambert Conformal Conic 2SP	carla.GeoProjectionLCC2SP

geoprojection = carla.GeoProjectionTM(lat_0=41.400779, lon_0=2.188103, k=1.0, ellps=geoellipsoid)

This geoprojection can then be used to transform the coordinates using the geotransform methods:

# Retrieve the map object
carla_map = world.get_map()

# Convert a carla.Location to a carla.Geolocation
geolocation = carla_map.transform_to_geolocation(carla.Location(10,10,1), geoprojection)

# Convert a carla.Geolocation to a carla.Location
geolocation = carla.GeoLocation(latitude=41.400779, longitude=2.188103, altitude=0)
location = carla_map.geolocation_to_transform(geolocation, geoprojection)