NetCDF grid mapping variable

Introduction

When the coordinate variables for a horizontal grid are not longitude and latitude, it is required that the true latitude and longitude coordinates be supplied via the "coordinates" attribute. If in addition it is desired to describe the mapping between the given coordinate variables and the true latitude and longitude coordinates, the attribute "grid_mapping" may be used to supply this description. This attribute is attached to data variables so that variables with different mappings may be present in a single file. The attribute takes a string value which is the name of another variable in the file that provides the description of the mapping via a collection of attached attributes. This variable is called a grid mapping variable and is of arbitrary type since it contains no data. Its purpose is to act as a container for the attributes that define the mapping.

In case the component of a vector is not given in eastward or northward direction the required coordinate transformation must be also supplied using the grid_mapping attribute.

Grid mapping variable

An extensive description is given in appendix F of the CF metadata conventions. Subsequently three different uses cases are going to be presented.

Transverse Mercator projection

This type of transformation can be used for UTM coordinates and Gauß-Krüger coordinates as well.

integer crs ; \\ no value for this container variable must be specified

crs:grid_mapping_name = "transverse_mercator" ;

crs:scale_factor_at_central_merdian = value ;

crs:longitude_of_central_meridian = value ;

crs:latitude_of_projection_origin = value ;

crs:false_easting = value ;

crs:false_northing = value ;

crs:longitude_of_prime_meridian = value ; // optional but recommended

crs:semi_major_axis = value ; // optional but recommended

crs:inverse_flattening = value ; // optional but recommended

crs:comment = "X , Y  : descriptive text\\nLON, LAT : Ellipsoid - European Terrestrial Reference System 1989" ; // additional BAW-DH

crs:epsg_code = "EPSG:epsg-code" ; // see EPSG Geodetic Parameter Registry

Examples for valid EPSG codes are 31466, 31467, 31468, 31469 (all Gauß-Krüger) or 25831, 25832 (all UTM).

Geographic coordinates (ETRS 1989)

integer crs ; \\ no value for this container variable must be specified

crs:grid_mapping_name = "latitude_longitude" ;

crs:semi_major_axis = 6378137.00 ; // optional but recommended

crs:inverse_flattening = 298.257222101 ; // optional but recommended

crs:comment = "X , Y  : descriptive text; // optional

crs:epsg_code = "EPSG:4258" ; // see EPSG Geodetic Parameter Registry

Rotated geographic coordinates

integer crs ; \\ no value for this container variable must be specified

crs:grid_mapping_name = "rotated_latitude_longitude" ;

crs:grid_north_pole_longitude = value ;

crs:grid_north_pole_latitude = value ;

Use cases

Referencing a grid mapping variable

When the coordinate variables for a horizontal grid are not longitude and latitude, it is required that the true latitude and longitude coordinates be supplied via the "coordinates" attribute. Attribute "grid_mapping" is used to describe the mapping between the given coordinate variables and the true latitude and longitude coordinates. Example:

double Mesh0_node_water_depth_2d(time,nMesh0_node) ;

Mesh0_node_water_depth_2d:standard_name = "sea_floor_depth_below_sea_surface" ;

Mesh0_node_water_depth_2d:long_name = "water depth at locations, vertically integrated" ;

Mesh0_node_water_depth_2d:units = "m" ;

Mesh0_node_water_depth_2d:coordinates = "Mesh0_node_lon Mesh0_node_lat" ;

Mesh0_node_water_depth_2d:_FillValue = fillvalue ;

Mesh0_node_water_depth_2d:valid_range = valid minimum, valid maximum ;

Mesh0_node_water_depth_2d:grid_mapping = "crs" ;

Wind defined in rotated pole coordinates

The components of a wind field are given as local wind ("x_wind", "y_wind") in geographic coordinates. Attribute "grid_mapping" references variable "crs" which contains more information about the coordinate transformation used.

float ugrd10m(time, latitude, longitude) ;

ugrd10m:name_id = 557s ;

ugrd10m:grid_mapping = "crs" ;

ugrd10m:standard_name = "x_wind" ;

ugrd10m:units = "m/s" ;

ugrd10m:_FillValue = 9.999e+20 ;

ugrd10m:long_name = "10 m above ground u-component of wind" ;

float vgrd10m(time, latitude, longitude) ;

vgrd10m:name_id = 558s ;

vgrd10m:grid_mapping = "crs" ;

vgrd10m:standard_name = "y_wind" ;

vgrd10m:units = "m/s" ;

vgrd10m:_FillValue = 9.999e+20 ;

vgrd10m:long_name = "10 m above ground v-component of wind" ;

int crs ;

crs:grid_mapping_name = "rotated_latitude_longitude" ;

crs:grid_north_pole_longitude = -170. ;

crs:grid_north_pole_latitude = 40. ;

A generic application will be able to detect that wind is defined locally ("x_wind", "y_wind"). In case "eastward_wind" and "northward_wind" is requested, the required coordinate transformation could be carried through using the parameters from grid mapping variable "crs". In this situation, to perform a correct transformation, standard names as well as grid mapping variable have to be avaluated.

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