Add support for building coordinate system with from_cf/to_cf
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Since WKT (or PROJ JSON) is the backend for the CF conversions, additional support for different units and axis order are now possible. Currently the x/y axis order and units of meters for projected degrees for geographic are assumed.
http://cfconventions.org/cf-conventions/cf-conventions.html#coordinate-system
Examples of input units can be accessed from the coordinates:
http://cfconventions.org/cf-conventions/cf-conventions.html#coordinate-types
double y(y);
y:units = "km";
y:long_name = "y coordinate of projection";
y:standard_name = "projection_y_coordinate";
double x(x);
x:units = "km";
x:long_name = "x coordinate of projection";
x:standard_name = "projection_x_coordinate";
double lat(y, x);
lat:units = "degrees_north";
lat:long_name = "latitude coordinate";
lat:standard_name = "latitude";
double lon(y, x);
lon:units = "degrees_east";
lon:long_name = "longitude coordinate";
lon:standard_name = "longitude";
Additionally, the vertical component can be supported:
http://cfconventions.org/cf-conventions/cf-conventions.html#vertical-coordinate
axis_name:units = "meters" ;
axis_name:positive = "down" ;
See also: cf-convention/cf-conventions#223
Horizontal dimensions are those whose coordinate variables have an axis attribute of X or Y, or a units attribute indicating latitude and longitude.
Axis example:
{'units': 'rad',
'axis': 'X',
'long_name': 'GOES Projection x-Coordinate',
'standard_name': 'projection_x_coordinate'}
Issues that would help with this:
This looks useful: https://github.com/SciTools/cf-units
Using: m.convert(1, "m") on any unit should get the unit conversion factor.
Not going to include in pyproj as it adds unnecessary dependencies for a piece of functionality that is not really a main PROJ feature. Instead, I am leaning towards documentation for how to do it yourself if you so desire.
Also, here is the file/database for the unit name mappings:
- https://github.com/nco/nco/blob/master/data/udunits.dat
- https://www.unidata.ucar.edu/software/udunits/udunits-current/doc/udunits/udunits2.html#Database
More specifically:
Can do fraction of unit - this can be used for unit conversion factor:
>>> from cf_units import Unit
>>> u = Unit("0.5 m")
>>> u.convert(1, "m")
0.5