Axes and Coordinates¶

One powerful feature of cf_xarray is the ability to refer to named dimensions by standard axis or coordinate names in Dataset or DataArray methods.

For example, one can call ds.cf.mean("latitude") instead of ds.mean("lat")

Tip

Most xarray methods are wrapped by cf-xarray. Simply access them as DataArray.cf.method(dim="latitude") for example and try it! If something does not work, please raise an issue.

Coordinate Criteria¶

How does this work? cf_xarray has an internal table of criteria (mostly copied from MetPy) that lets it identify specific coordinate names "latitude", "longitude", "vertical", "time".

Tip

See Custom Criteria to find out how to define your own custom criteria.

This table lists these internal criteria

	latitude	longitude	time	vertical
_CoordinateAxisType	Lat	Lon	Time
axis			T
cartesian_axis			T
grads_dim			t
long_name	latitude	longitude	time	air_pressure, altitude, depth, geopotential_height, height, height_above_geopotential_datum, height_above_mean_sea_level, height_above_reference_ellipsoid
positive				down, up
standard_name	latitude	longitude	time	air_pressure, altitude, depth, geopotential_height, height, height_above_geopotential_datum, height_above_mean_sea_level, height_above_reference_ellipsoid
units	degreeN, degree_N, degree_north, degreesN, degrees_N, degrees_north	degreeE, degree_E, degree_east, degreesE, degrees_E, degrees_east

Any DataArray that has standard_name: "latitude" or _CoordinateAxisType: "Lat" or "units": "degrees_north" in its attrs will be identified as the "latitude" variable by cf-xarray. Similarly for other coordinate names.

Axis Names¶

Similar criteria exist for the concept of “axes”.

	T	X	Y	Z
_CoordinateAxisType	Time	GeoX	GeoY	GeoZ, Height, Pressure
axis	T	X	Y	Z
cartesian_axis	T	X	Y	Z
grads_dim	t	x	y	z
long_name	time	cell index along first dimension, grid_longitude, projection_x_angular_coordinate, projection_x_coordinate	cell index along second dimension, grid_latitude, projection_y_angular_coordinate, projection_y_coordinate	atmosphere_hybrid_height_coordinate, atmosphere_hybrid_sigma_pressure_coordinate, atmosphere_ln_pressure_coordinate, atmosphere_sigma_coordinate, atmosphere_sleve_coordinate, model_level_number, ocean_double_sigma_coordinate, ocean_s_coordinate, ocean_s_coordinate_g1, ocean_s_coordinate_g2, ocean_sigma_coordinate, ocean_sigma_z_coordinate
positive
standard_name	time	grid_longitude, projection_x_angular_coordinate, projection_x_coordinate	grid_latitude, projection_y_angular_coordinate, projection_y_coordinate	atmosphere_hybrid_height_coordinate, atmosphere_hybrid_sigma_pressure_coordinate, atmosphere_ln_pressure_coordinate, atmosphere_sigma_coordinate, atmosphere_sleve_coordinate, model_level_number, ocean_double_sigma_coordinate, ocean_s_coordinate, ocean_s_coordinate_g1, ocean_s_coordinate_g2, ocean_sigma_coordinate, ocean_sigma_z_coordinate
units

`.axes` and `.coordinates` properties¶

Alternatively use the special properties DataArray.cf.axes or DataArray.cf.coordinates to access the variable names. These properties return dictionaries that map “CF names” to a list of variable names. Note that a list is always returned even if only one variable name matches the name "latitude" (for example).

airds.cf.axes

{'X': ['lon'], 'Y': ['lat'], 'T': ['time']}

airds.cf.coordinates

{'longitude': ['lon'], 'latitude': ['lat'], 'time': ['time']}

Axes or Coordinate?¶

TODO describe latitude vs Y; longitude vs X; vertical vs Z

Checking presence of axis or coordinate¶

Note that a given “CF name” is only present if there is at least one variable that can be identified with that name. The airds dataset has no "vertical" coordinate or "Z" axis, so those keys are not present. So to check whether a "vertical" coordinate or "Z" axis is present, one can

"Z" in airds.cf.axes

False

"vertical" in airds.cf.coordinates

False

Or one can check the dataset as a whole:

"Z" in airds.cf

False

Using the repr¶

It is always useful to check the variables identified by cf-xarray using the repr

airds.cf

Coordinates:
             CF Axes: * X: ['lon']
                      * Y: ['lat']
                      * T: ['time']
                        Z: n/a

      CF Coordinates: * longitude: ['lon']
                      * latitude: ['lat']
                      * time: ['time']
                        vertical: n/a

       Cell Measures:   area: ['cell_area']
                        volume: n/a

      Standard Names: * latitude: ['lat']
                      * longitude: ['lon']
                      * time: ['time']

              Bounds:   n/a

       Grid Mappings:   n/a

Data Variables:
       Cell Measures:   area, volume: n/a

      Standard Names:   air_temperature: ['air']

              Bounds:   n/a

       Grid Mappings:   n/a

Axes and Coordinates¶

Coordinate Criteria¶

Axis Names¶

.axes and .coordinates properties¶

Axes or Coordinate?¶

Checking presence of axis or coordinate¶

Using the repr¶

`.axes` and `.coordinates` properties¶