Find default data in NXdata vGroup

[c.m.moreton-smith at rl.ac.uk]

If my memory serves me right, signal=1 was to identify the primary
plotting axis, not necessarily the main data content?  i.e. it would
indicate for T-O-F data - say, whether to plot by detector angle or by
time channel when both are present.

It would probably be less ambiguous to _require_ that anything put in
the NXdata group has a unit attribute specified (just a UD units
string).  The fitting program could simply scan (very few) elements in
the data group, looking for an item with the type of unit attribute it
was expecting, check the rank when it found it, then scan for other
items as necessary.  To some extent this would reduce problems with
arbitrary naming too.

This should always work as long as we are careful to ensure that we
don't store multiple items in the data group with identical units (it
would be better to keep them in a single SDS with a higher rank).

Hopefully this would avoid the risk of spots before the eyes :-)

	Chris

PS - I've just tried sending this to NEXUS at anl.gov and had it bounce?

-----Original Message-----
From: Mark.Koennecke at psi.ch [mailto:Mark.Koennecke at psi.ch]
Sent: 23 March 1998 13:16
To: cmm at rapostie.nd.rl.ac.uk
Subject: Find default data in NXdata vGroup




  High there,

  a colleague of mine is working on a fitting program and wants to read
  NeXus data. He found something with the data vGroup which might be a
real
  issue: How to automatically decide what is the main data content? This
is not
  completely clear to me. I can think of two options to resolve this:
  - We prescribe, that the data vGroup may contain only data + axis
attributes
    with NO auxiliaries. 
  - We put an attribute signal=1 on to the main data content. This is
reviving
    an idea from John Tischler.

  I'll opt for the second option because it leaves us the freedom to
have
  auxilliary data, for instance temperature or certain angles, in the
  data vGroup. Such data can make sense there because sometimes such
data
  is necessary even for a first glance. For example: On a four circle
  diffractometer equipped with an area-PSD a detector frame without
information
  about setting angles is utterly useless. Except if all you want  is to
look 
  at boring pictures with spots.

  What does everybody else thinks?

                                            Mark Koennecke