Clarification needed on the units

[astronerdF]

While loading the Coordinates of the gas particles using the Illustris package, the units given on the website is ckpc/h https://www.tng-project.org/data/docs/specifications/#parttype0
The units for GroupPos variable as well are ckpc/h as well.

When looking at the same variables using the scida package, the units for Coordinates are centimeter

coords = gas["Coordinates"]
coords.to_base_units().units
----------------------
</> centimeter

When looking at the GroupPos variable, the units are dimensionless

import dask.array as da
ds = load(basePath+"/snapdir_099", units=True)
data = ds.return_data(haloID=42)
grp = data["Group"]
grp["GroupPos"].units
----------------------
</> dimensionless

Additionally, for the variable Coordinates, the maximum value for the same snapshot (TNG100-3 snap 99) is the same whether the data is read from the Illustris package or the Scida package:

gas_il = il.snapshot.loadHalo(basePath,99,0,'gas',fields="Coordinates")
print("gas_il -  max Coordinate: ", np.max(np.array(gas_il))) #UNITS ckpc/h

ds = load(basePath+"/snapdir_099", units=True)
print("gas_scida - max Coordinate: ", ds.data["Group"]["Coordinates"].max().compute())
-----------------------------------------------------------------------------------------
</> gas_il -  max Coordinate:  74999.99978918549
 gas_scida - max Coordinate:  74999.8984375 dimensionless

[cbyrohl]

Thanks for pointing this out. "dimensionless" is currently also used if units cannot automatically be detected. TNG100-2/TNG100-3 were apparently not correctly detected. The "Coordinates" field has metadata allowing us to determine the units nonetheless. This metadata is unavailable for the group catalogs. Thus the missing data.

Upon load() there should have been a "WARNING:scida.interfaces.mixins.units: [...]" point this out?

In the future, if "a few" fields cannot have their units determined, we should set the unit to a custom "unknown" rather than "dimensionless". If all/more than "a few" fields do not have units, we should fall back to no units at all.

[cbyrohl]

If not earlier, then with PR #69 TNG100-2/TNG100-3 are correctly detected. Furthermore, when we do not have unit information for a field, then the units are "unknown" rather than "dimensionless" from now on.