[Nexus] unit_cell in NXsample

[Tobias.Richter at diamond.ac.uk]

I not like to pollute the namespace with stuff like "a". unit_cell_a would be a possibility. Having all 6 values under the same umbrella however does have some appeal. We could split the angles and lengths, maybe? 2 umbrellas.

To illustrate my earlier unit idea - this is what I am looking at now:

NX/entry/sample> read unit_cell
  unit_cell[6] (NX_FLOAT64) = [ 5.144700 5.144700 5.144700 ...]
    angle_units = deg
    length_units = angstrom
NX/entry/sample>

An obvious alternative would be to prescribe the units. There are at most two sensible choices for the two units each. Possibly fewer.

Tobias


> -----Original Message-----
> From: NeXus [mailto:nexus-bounces at nexusformat.org] On Behalf Of Pete R.
> Jemian
> Sent: 17 September 2014 18:17
> To: nexus at nexusformat.org
> Subject: Re: [Nexus] unit_cell in NXsample
> 
> It would be more sensible to describe each term separately rather than
> rely on this arcane coding of the terms.  If this definition is to be
> revised, I suggest creating fields for each of these terms:
> a
> b
> c
> alpha
> beta
> gamma
> 
> Otherwise, we need to relax the units attribute.  A value of "NX_LENGTH
> or NX_NX_ANGLE" (or syntax that can be properly coded in XML) looks
> sensible to the eye but I'm not sure if we have the XML Schema support
> for this.  Yet.  For now, NX_ANY is the only existing value that spans
> what is proper.  An alternative is to introduce a new unit type
> NX_LATTICE_PARAMETER which could be either length or angle.
> 
> No good answer for now.
> 
> Pete
> 
> On 09/17/2014 11:48 AM, Tobias.Richter at diamond.ac.uk wrote:
> > The unit_cell in NXsample is defined like this:
> >
> > <field name="unit_cell" type="NX_FLOAT" units="NX_LENGTH">
> >
> > <doc>Unit cell parameters (lengths and angles)</doc>
> >
> > <dimensions rank="2">
> >
> > <dim index="1" value="n_comp"/>
> >
> > <dim index="2" value="6"/>
> >
> > </dimensions>
> >
> > </field>
> >
> > Since it already says "lengths and angles" the fact that the required
> > units are NX_LENGTH is a bit puzzling. No units for the angles?
> >
> > Would it be sensible to have "length_units" and "angle_units"
> attributes?
> >
> > Tobias
> >
> >
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