Dear Jim and Sebastien The original intention of axis was to label the independent variables as 1D xyzt axes of the data variables. This can be deduced from other attributes, but it's more effort. It's partly a plotting hint, but also it's because you might reasonable want to tell software, "give me the z-axis coordinates", or "calculate a mean over the x-direction". The latter is often a zonal mean, but it isn't with a rotated-pole or tripolar grid, yet the operation is still performed sometimes.
It's useful that you've pointed out the confusion of purpose. If it were regarded as an acceptable backwards-incompatibility, which I'm nervous about, I'd be happy if we returned "axis" to its original purpose of identifying 1D axes, and also for scalar coordinate variables (which are equivalent to axes of size one), and provided another attribute to label aux coords as horizontal. I agree that if we have 1D x and y, with 2D lat and lon, the 1D variables are the axes. That's consistent with the original purpose of the axis attribute. > I also find the units of latitude and longitude confusing: it looks like it > was a way to squeeze the direction of the coordinate inside the units. I have > the same observation for the time coordinate that has its origin in the units! This convention was kept in CF for backwards-compatibility with COARDS. CF does not use units in any other case to identify the quantity or sense. > It was done correctly for z coordinate using "units" and "positive", probably > because there are many types of z coordinates with various origin and > directions, and no real consensus. I note however that often the origin is > not always clearly defined. The positive attribute was also kept for backwards-compatibility with COARDS. It has the advantage of being useful to identify the vertical axis, but this can also be done with axis="Z". CF standard names provide information which indicates the sign convention. If coordinate_index is confusing, I think standard_names containing x_index or y_index would be OK, provided we change the existing standard names magnitude_of_derivative_of_position_wrt_x_coordinate_index magnitude_of_derivative_of_position_wrt_y_coordinate_index to remove "_coordinate". Best wishes Jonathan _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
