Mapping from CF Grid Mapping Attributes to CRS WKT Elements

CF-1.6 ←→ WKT/PROJ.4 translation

Mapping from CF Grid Mapping Attributes to CRS WKT Elements

These provisional mappings have been compiled to support, among other things, CF proposals 69 and 80; which have been incorporated in the CF Conventions document version 1.7. If you spot any errors or omissions, please email the authors (Phil Bentley and Etienne Tourigny) or the CF mailing list, or else update this wiki page.

In Tables 1 and 2 the names of WKT PARAMETER elements follow the names of coordinate operation parameters defined in the EPSG geodetic parameter registry.

The following tables list translations of projection parameter names between CF and various other representations (such as OGC WKT, EPSG and PROJ.4). The information was obtained from various sources, including http://www.remotesensing.org/geotiff/proj_list/ .

The various representations listed are:

CF The CF-1.6 (and possibly CF-1.7) parameter names
OGC WKT The names following OGC WKT specification (used by GDAL/OGR and CadCorp(?))
PROJ.4 The names used in the PROJ.4 software (http://trac.osgeo.org/proj/)
EPSG The names and codes used in the EPSG Geodetic Parameter Dataset (http://www.epsg.org/ and http://www.epsg-registry.org/)
GeoTIFF ID The names used in the GeoTIFF raster format (http://trac.osgeo.org/geotiff/)

The following files are provided to list the various possible Datum, Ellipsoid and Prime Meridian names (in support of CF proposal 80)

horiz_datum.csv - horizontal datum parameters for use as reference for CF-1.7 (source: GDAL/OGR, modified by Etienne Tourigny)
ellipsoid.csv - Ellipsoid parameters from the EPSG database (source: GDAL/OGR)
prime_meridian.csv - Prime Meridian parameters from the EPSG database (source: GDAL/OGR)

Table 1 - Existing (CF-1.6) CF Grid Mapping Attributes

CF Grid Mapping Attribute	Corresponding WKT Syntax (1)
earth_radius	SPHEROID["<reference_ellipsoid_name>", <semi_major_axis>, 0.0, ...]
false_easting	PARAMETER["False easting", <value>]
false_northing	PARAMETER["False northing", <value>]
grid_mapping_name	no known equivalent
grid_north_pole_latitude	no known equivalent
grid_north_pole_longitude	no known equivalent
inverse_flattening	SPHEROID["<reference_ellipsoid_name>", <semi_major_axis>, <inverse_flattening>, ...]
latitude_of_projection_origin	PARAMETER["Latitude of natural origin", <value>]
longitude_of_central_meridian	PARAMETER["Longitude of natural origin", <value>]
longitude_of_prime_meridian	PRIMEM["<prime_meridian_name>", <longitude_of_prime_meridian>, ...]
longitude_of_projection_origin	PARAMETER["Longitude of natural origin", <value>]
north_pole_grid_longitude	no known equivalent
perspective_point_height	PARAMETER["Viewpoint height", <value>]
scale_factor_at_central_meridian	PARAMETER["Scale factor at natural origin", <value>]
scale_factor_at_projection_origin	PARAMETER["Scale factor at natural origin", <value>]
semi_major_axis	SPHEROID["<reference_ellipsoid_name>", <semi_major_axis>, <inverse_flattening>, ...]
semi_minor_axis	no direct equivalent, the SPHEROID element is used to define ellipsoid geometry
standard_parallel (one parallel)	PARAMETER["Latitude of standard parallel", <value>]
standard_parallel (two parallels)	PARAMETER["Latitude of 1st standard parallel", <value>], PARAMETER["Latitude of 2nd standard parallel", <value>]
straight_vertical_longitude_from_pole	PARAMETER["Longitude of origin", <value>]

Table 2 - Future (CF-1.7) CF Grid Mapping Attributes

CF Grid Mapping Attribute	Corresponding WKT Syntax (1)	PROJ.4 Syntax
projected_crs_name	PROJCS["<projected_coordinate_system_name>", ...]	N/A
geographic_crs_name	GEOGCS["<geographic_coordinate_system_name>", ...]	N/A
horizontal_datum_name	GEOGCS["<name>", DATUM["<horizontal_datum_name>", ...] ]	+datum
reference_ellipsoid_name	GEOGCS["<name>", DATUM["<name>", SPHEROID["<reference_ellipsoid_name>", ...] ] ]	+ellps
towgs84	GEOGCS["<name>", DATUM["<name>", TOWGS84[<7 params>] ]	+towgs84
prime_meridian_name	GEOGCS["<name>",PRIMEM["<prime_meridian_name>",<value>]	+ pm

Table 3 - Common projection parameter names

CF	OGC WKT	PROJ.4	EPSG name	Note

false_easting	false_easting	+x_0	False easting / Easting at false origin	(2)
false_northing	false_northing	+y_0	False northing / Northing at false origin	(2)
scale_factor_at_projection_origin	scale_factor	+k_0	Scale factor at natural origin	(1)
scale_factor_at_central_meridian	scale_factor	+k_0	Scale factor at natural origin	(1)
standard_parallel![1]	standard_parallel_1	+lat_1	Latitude of 1st standard parallel
standard_parallel![2]	standard_parallel_2	+lat_2	Latitude of 2nd standard parallel
longitude_of_central_meridian	longitude_of_center / central_meridian	+lon_0	(1)	(1)
longitude_of_projection_origin	longitude_of_center / central_meridian	+lon_0	(1)	(1)
latitude_of_projection_origin	latitude_of_origin / latitude_of_center	+lat_0	(1)	(1)
straight_vertical_longitude_from_pole	central_meridian	+lon_0	(1)	(1)

Table 4 - Specific projection parameter names

Projection	Name	CF	OGC WKT	PROJ.4	EPSG name	EPSG code	GeoTIFF ID	Note

AEA	Albers Equal Area	albers_conical_equal_area	Albers_Conic_Equal_Area	+proj=aea	Albers Equal Area	9822	CT_AlbersEqualArea
		standard_parallel![1]	standard_parallel_1	+lat_1	Latitude of 1st standard parallel	8823	!StdParallel1
		standard_parallel![2]	standard_parallel_2	+lat_2	Latitude of 2nd standard parallel	8824	!StdParallel2
		longitude_of_central_meridian	longitude_of_center	+lon_0	Longitude of false origin	8822	NatOriginLong
		latitude_of_projection_origin	latitude_of_center	+lat_0	Latitude of false origin	8821	NatOriginLat
false_easting			false_easting	+x_0	Easting at false origin	8826	FalseEasting
false_northing			false_northing	+y_0	Northing at false origin	8827	FalseNorthing

AE	Azimuthal equidistant	azimuthal_equidistant	Azimuthal_Equidistant	+proj=aeqd	N/A	N/A	CT_AzimuthalEquidistant	EPSG:9832 is 'Modified Azimuthal Equidistant'
		longitude_of_projection_origin	longitude_of_center	+lon_0			ProjCenterLong
		latitude_of_projection_origin	latitude_of_center	+lat_0			ProjCenterLat
false_easting			false_easting	+x_0	FalseEasting
false_northing			false_northing	+y_0	FalseNorthing

LAEA	Lambert azimuthal equal area	lambert_azimuthal_equal_area	Lambert_Azimuthal_Equal_Area	+proj=laea	Lambert Azimuthal Equal Area	9820	CT_LambertAzimEqualArea
		longitude_of_projection_origin	longitude_of_center	+lon_0	Longitude of natural origin	8802	ProjCenterLong
		latitude_of_projection_origin	latitude_of_center	+lat_0	Latitude of natural origin	8801	ProjCenterLat
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

LCC-1SP	Lambert conformal	lambert_conformal_conic	Lambert_Conformal_Conic_1SP	+proj=lcc	Lambert Conic Conformal (1SP)	9801	CT_LambertConfConic_1SP	(3) link
		standard_parallel	latitude_of_origin	+lat_1	Latitude of natural origin	8801	NatOriginLat	standard_parallel=latitude_of_projection_origin
		longitude_of_central_meridian	central_meridian	+lon_0	Longitude of natural origin	8802	NatOriginLong
		latitude_of_projection_origin	latitude_of_origin	+lat_0	Latitude of natural origin	8801	NatOriginLat
		N/A	scale_factor	+k_0	Scale factor at natural origin	8805	ScaleAtNatOrigin	(3) always 1
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

LCC-2SP	Lambert conformal	lambert_conformal_conic	Lambert_Conformal_Conic_2SP	+proj=lcc	Lambert Conic Conformal (2SP)	9802	CT_LambertConfConic_2SP	(3) link
		standard_parallel![1]	standard_parallel_1	+lat_1	Latitude of 1st standard parallel	8823	!StdParallel1
		standard_parallel![2]	standard_parallel_2	+lat_2	Latitude of 2nd standard parallel	8824	!StdParallel2
		longitude_of_central_meridian	central_meridian	+lon_0	Longitude of false origin	8822	FalseOriginLong
		latitude_of_projection_origin	latitude_of_origin	+lat_0	Latitude of false origin	8821	FalseOriginLat
false_easting			false_easting	+x_0	Easting at false origin	8826	FalseEasting
false_northing			false_northing	+y_0	Northing at false origin	8827	FalseNorthing

CEA	Lambert cylindrical equal area	lambert_cylindrical_equal_area	Cylindrical_Equal_Area	+proj=cea	N/A	CT_CylindricalEqualArea		(3) CF says EPSG 9834/9835 ?
		longitude_of_central_meridian	central_meridian	+lon_0			NatOriginLong
		standard_parallel![1]	standard_parallel_1	+lat_ts	!StdParallel1			default to 0
false_easting			false_easting	+x_0	FalseEasting
false_northing			false_northing	+y_0	FalseNorthing

M-1SP	Mercator (1SP)	mercator	Mercator_1SP	+proj=merc	Mercator (variant A)	9804	Mercator_1SP	(3)
		longitude_of_projection_origin	central_meridian	+lon_0	Longitude of natural origin	8801	NatOriginLong
		scale_factor_at_projection_origin	scale_factor	+k_0	Scale factor at natural origin	8805	ScaleAtNatOrigin
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

M-2SP	Mercator (2SP)	mercator	Mercator_2SP	+proj=merc	Mercator (variant B)	9805	N/A	(3)
		longitude_of_projection_origin	central_meridian	+lon_0	Longitude of natural origin	8801
		standard_parallel![1]	standard_parallel_1	+lat_ts	Latitude of 1st standard parallel	8823
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

Ortho	Orthographic	orthographic	Orthographic	+proj=ortho	Orthographic	9840	CT_Orthographic
		longitude_of_projection_origin	central_meridian	+lon_0	Longitude of natural origin	8802	ProjCenterLong
		latitude_of_projection_origin	latitude_of_origin	+lat_0	Latitude of natural origin	8801	ProjCenterLat
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

PS-A	Polar stereographic	polar_stereographic	Polar_Stereographic	+proj=stere	Polar Stereographic (variant A)	9810	CT_PolarStereographic	(3)
		straight_vertical_longitude_from_pole	central_meridian	+lon_0	Longitude of natural origin	8802	StraightVertPoleLong
latitude_of_projection_origin			N/A	+lat_0	Latitude of natural origin	8801	NatOriginLat	+90/-90
		scale_factor_at_projection_origin	scale_factor	+k_0	Scale factor at natural origin	8805	ScaleAtNatOrigin
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

PS-B	Polar stereographic	polar_stereographic	Polar_Stereographic	+proj=stere	Polar Stereographic (variant B)	9829	N/A	(3)
		straight_vertical_longitude_from_pole	central_meridian	+lon_0	Longitude of natural origin	8802
latitude_of_projection_origin			N/A	+lat_0	Longitude of origin	8801	+90/-90
standard_parallel			latitude_of_origin	+lat_ts	Latitude of standard parallel	8832
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

St	Stereographic	stereographic	Stereographic	+proj=stere	N/A	CT_Stereographic		EPSG 9809 is “Oblique Stereographic” or “Double Stereographic”
		longitude_of_projection_origin	central_meridian	+lon_0			ProjCenterLong
		latitude_of_projection_origin	latitude_of_origin	+lat_0			ProjCenterLat
		scale_factor_at_projection_origin	scale_factor	+k_0			ScaleAtNatOrigin
false_easting			false_easting	+x_0	FalseEasting
false_northing			false_northing	+y_0	FalseNorthing

TM	Transverse Mercator	transverse_mercator	Transverse_Mercator	+proj=tmerc	Transverse Mercator	9807	CT_TransverseMercator	(3)
		scale_factor_at_central_meridian	scale_factor	+k_0	Scale factor at natural origin	8805	ScaleAtNatOrigin
		longitude_of_central_meridian	central_meridian	+lon_0	Longitude of natural origin	8802	NatOriginLong
		latitude_of_projection_origin	latitude_of_origin	+lat_0	Latitude of natural origin	8801	NatOriginLat
false_easting			false_easting	+x_0	False easting	8806	FalseEasting
false_northing			false_northing	+y_0	False northing	8807	FalseNorthing

Table Notes:

Multiple CF/WKT translations, see projection-specific translations.
Parameter used in every projection.
See projection-specific notes.

Units Information

Translation of units depends on if the crs is geographic or projected

For a geographic crs (grid_mapping_name = "latitude_longitude"), units are angular. GEOGCS contains a UNIT node which gives the conversion to radians. When the crs is in degrees the following node is used: UNIT["degree",0.0174532925199433]. For a more complete description, see CT 1.0 section "7.3.19 UNIT" CF requires all value to be in degrees, which is specified in the dimension’s units attribute ( lon:units = "degrees_east" / lat:units = "degrees_north" ), the udunits packages allows to transform from radians.

For a projected crs, units are linear. PROJCS must contains a UNIT node which gives the conversion into meters. For example, if the units are in kilometers the following node is used: UNIT["kilometre",1000]]. In CF, units are given in the dimension’s units attribute (xc:units = "m").

Projection-specific notes

EPSG codes below correspond to "EPSG dataset coordinate operation method" codes.

Lambert conformal 1SP / 2SP

The 1SP variant corresponds to EPSG code 9801 - Lambert Conic Conformal (1SP), with CF latitude_of_projection_origin=standard_parallel and WKT scale_factor=1. A scale factor less than 1 means that there are 2 standard parallels (see http://www.remotesensing.org/geotiff/proj_list/lambert_conic_conformal_1sp.html), but it cannot be translated to the CF 1SP variant, therefore the 2SP variant should be used instead. The 2SP variant corresponds to EPSG code 9802 - Lambert Conic Conformal (2SP).

Lambert cylindrical equal area

The scale_factor_at_projection_origin variant is not recommended as it does not translate to and from WKT/PROJ.4. Snyder formulas 10-2b and 10-2 can be used to relate scale_factor_at_projection_origin, standard_parallel1 and latitude_of_projection_origin but the latter is not part of this projection’s parameters. It has been proposed to deprecate or remove this variant from the CF spec (see CF trac ticket 75).

Mercator 1SP / 2SP

The scale_factor_at_projection_origin variant corresponds to EPSG code 9804 - Mercator (1SP) or Mercator (variant A), and the standard_parallel variant corresponds to EPSG code 9805 - Mercator (2SP) or Mercator (variant B).

Polar stereographic

The standard_parallel variant corresponds to EPSG code 9829 - Polar Stereographic (Variant B), while the scale_factor_at_projection_origin variant corresponds to EPSG code 9810 - Polar Stereographic (Variant A). As WKT/PROJ.4 require the standard parallel, [Snyder] formula 21-7 can be used to compute it from scale_factor_at_projection_origin if that variant is used.

Transverse Mercator

Transverse Mercator can be translated to PROJ.4 using either +proj=tmerc (Transverse Mercator) or +proj=utm (Universal Transverse Mercator) by computing zone number from longitude_of_central_meridian. For example, a TM projection with longitude_of_central_meridian=-117 would have the corresponding PROJ.4 string: '+proj=utm +zone=11 +datum=NAD27 +units=m +no_defs '