[NeXus-committee] Linking NXdata to NXdetector groups

[Akeroyd, FA (Freddie)]

> I guess so, but won't you need to provide some way of associating axis
> information within the NXdata group with the parent NXdetector group, such
> as proposed in Proposal 3?
> 
>  e.g., how do you link the polar angle with distance in this example?
> 
>   <NXinstrument>
>    <NXdetector>
>       <NXdata name="Bank1">
>          <data axes="polar_angle(1:i)">
>          <polar_angle>
>       </NXdata>
>       <NXdata name="Bank2">
>          <data axes="polar_angle(i+1:N)">
>          <polar_angle>
>       </NXdata>
>       <distance(1:N)>
>    </NXdetector>
> </NXinstrument>
> 
Ray,

A fair point. When I posted the question about multiple NXdata in a single
NXdetector on the SWIKI http://www.neutron.anl.gov.:8080/NeXus/32 I couldn't
initially think of a good example of it myself, but Peter posted a summary
of one that maybe he can clarify. From reading his text, it would appear the
second NXdata was information from the same detector bank, but which did not
share axes with the first NXdata and so a "splitting mechanism" was not
needed. If such a splitting up of a bank was needed then one way would be to
store separate polar_angle arrays in each NXdata etc. and not have such
arrays in the NXdetector - you would still know to what detector bank they
referred to as the NXdata is within the NXdetector. However if you are
starting to split up polar_angle then there are other arrays that need
splitting up as well so you should probably be looking at having another
NXdetector.  

> > * Proposal 1 is currently the only one that addresses non-detector axes
> in
> > an NXdata group, but the same attribute linking scheme could be applied
> to
> > all the other proposals for non-detector axes. The choice then is still
> > about how and where you store the detector information.
> 
> That's true, but then we are explicitly using links to associate data, as
> with Proposal 1.  Proposals 2 and 4 seem to be designed to work without
> links.
> 
The SWIKI discussions neglected the issue of non-detector axes in NXdata,
but I think people can be forgiven a bit as the title of the page was
"connecting NXdata with NXdetector" rather than "NXdata with NX*" :-) Though
proposal 4 does state "links are used strictly to save space and not to
convey any information", I was assuming that this referred to the NXdata <->
NXdetector association only and not to all forms of linking; maybe this
could be clarified? As you say, we do need a mechanism that allows
non-detector axes to be linked into NXdata and their source determined.

Regards,

Freddie