[yt-dev] [Bug] Halo Catalog units

[Nathan Goldbaum]

On Wed, Jun 1, 2016 at 11:40 AM, Rasmi Elasmar <re2300 at columbia.edu> wrote:

> Hi all,
>
> Greg and I found a bug involving halo catalog unit handling:
>
> >>> halo.quantities['particle_position_x']
>> 0.495074982741 cm
>> >>> halo.quantities['particle_position_x'].in_units('code_length')
>> 0.495074982741 code_length
>> >>> halo.quantities['particle_position_x'].in_units('cm')
>> 0.495074982741 cm
>> >>> ds.unit_registry['code_length']
>> (9.195880139956267e+25, (length))
>> >>> halos_ds.unit_registry['code_length']
>> (1.0, (length))
>
>
> The halos_ds mixes up cm and code_length units when the HaloCatalog object
> is created from a saved halo catalog. The halo catalog values are saved in
> code_length, but the HaloCatalog object assumes they are in cm.
>
> Here
> <https://bitbucket.org/yt_analysis/yt/src/b8a09cd382dd34f386ce3634e7f78df3f5d9401d/yt/analysis_modules/halo_analysis/halo_catalog.py?at=yt&fileviewer=file-view-default#halo_catalog.py-453>
> is where the code_length units are written out after halo-finding is done
> (this is confirmed with an h5ls).
> Here is where the halos_ds
> <https://bitbucket.org/yt_analysis/yt/src/b8a09cd382dd34f386ce3634e7f78df3f5d9401d/yt/frontends/halo_catalog/data_structures.py?at=yt&fileviewer=file-view-default#data_structures.py-78> for
> the HaloCatalog is created. The length unit is set in cm -- the catalog is
> assumed to be in cgs.
> The HaloCatalog fields
> <https://bitbucket.org/yt_analysis/yt/src/b8a09cd382dd34f386ce3634e7f78df3f5d9401d/yt/frontends/halo_catalog/fields.py?at=yt&fileviewer=file-view-default#fields.py-21>
> also assume cgs.
>
> In theory, the HaloCatalog could just parse the code_length units of the
> halos_ds, but this isn't necessarily known at the time of creation, so the
> ideal fix may be to save the halo catalog length units in cm instead of in
> code_length. Then the assumptions that are made about length being in cm
> when creating a HaloCatalog object from a halo catalog would be correct.
> Any thoughts on this approach or other approaches?
>
>
It would probably be better just to save the unit registry into the hdf5
file. You might find the UnitRegistry.to_json() and
UnitRegistry.from_json() to be useful here - the json data could be saved
in the HDF5 output file as a string dataset.

https://bitbucket.org/yt_analysis/yt/src/yt/yt/units/unit_registry.py?at=yt&fileviewer=file-view-default#unit_registry.py-120


> Thanks,
>
> Rasmi
>
>
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>
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