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.

The various representations listed are:

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

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

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

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[“", ...] N/A
geographic_crs_name GEOGCS[“", ...] N/A
horizontal_datum_name GEOGCS[“", DATUM["", ...] ] +datum
reference_ellipsoid_name GEOGCS[“", DATUM["", SPHEROID["", ...] ] ] +ellps
towgs84 GEOGCS[“", DATUM["", TOWGS84[<7 params>] ] +towgs84
prime_meridian_name GEOGCS[“",PRIMEM["",] + 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 2+^   ProjCenterLong    
    latitude_of_projection_origin latitude_of_center +lat_0 2+^   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  
    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  
    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 2+^ CT_CylindricalEqualArea (3) CF says EPSG 9834/9835 ?  
    longitude_of_central_meridian central_meridian +lon_0 2+^   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 2+^ CT_Stereographic EPSG 9809 is “Oblique Stereographic” or “Double Stereographic”  
    longitude_of_projection_origin central_meridian +lon_0 2+^   ProjCenterLong    
    latitude_of_projection_origin latitude_of_origin +lat_0 2+^   ProjCenterLat    
    scale_factor_at_projection_origin scale_factor +k_0 2+^   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:

  1. Multiple CF/WKT translations, see projection-specific translations.
  2. Parameter used in every projection.
  3. 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, 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 ‘

Mapping from CF Grid Mapping Attributes to CRS WKT Elements (previous table by Phil Bentley, Oct 2011)

These provisional mappings have been compiled to support, among other things, CF proposal 69. If you spot any errors or omissions, please email the author or the CF mailing list, or else update this wiki page.

Mappings are only listed for the current set of CF grid mapping attributes – there are a number of WKT elements, and many CRS PARAMETERs, for which there are no corresponding CF attributes.

The order of attributes follows Table F.1 in the CF conventions document.

The names of WKT PARAMETER elements follow the names of coordinate operation parameters defined in the EPSG geodetic parameter registry.

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

Attachments (3) (work in progress)