Documentation about units

[jetesdal]

This is mostly a question. I just started looking into using parcels. Going over the examples, I wondered how the reading functions in parcels know what units the velocities (U, V, W) have. Does FieldSet.from_netcdf() read the metadata of the netcdf files (e.g., "units: m s-1") or is it assumed that the velocities are given in m/s? I couldn't find any info about required units in the documentation, so I wanted to check if velocities (U, V, W) should always be in m/s?

[delandmeterp]

No, we do not read the units of the fields such that we don't do any automated rescaling. If your data is not in m/s, you can use the function Field.set_scaling_factor()

It is noteworthy that we do the scaling between m/s and °/s, such that it is important that you set if your mesh is spherical or flat. You can check it by printing field.grid.mesh
When you use Field.from_netcdf, the default value is spherical

[erikvansebille]

Thanks @jetesdal, for this question. I've now added a tutorial that further explains how UnitConverter objects are used 'under the hood' in Parcels; see also #515. Hope that this clarifies your question further

[jetesdal]

Thank you for your answers and adding the tutorial! This tutorial will be very useful. One small suggestion would be to add that U and V are assumed to be in m/s into the documentation of the loading functions (e.g., for FieldSet.from_data). This way users can see this info in the docstring of the relevant functions (e.g., by calling "FieldSet.from_data?").

[erikvansebille]

Good idea @jetesdal to also put some more information in the docstring of the relevant functions. I'm reopening this issue so that we can work on that