Dear All,

The forthcoming meeting in Cambridge could be a good opportunity
to discuss how  "column groups"  can be introduced in VOTable.

The proposition submitted below 
It came out in many discussions that
The question of how to has been raised 
This is a recurrent question
Iis would like to 

The  "column groups"  try to answer questions frequently asked about
column associations, typically:
--> error (or standard deviation) associated to a column, e.g. a flux
    consists of two numbers: the measured value + the mean error
--> qualities or weights associated to values
--> source or origin (e.g. telescope, or bibliographical reference) of a value
--> individual components e.g. x,y position of a CCD
--> etc...

This "column grouping" has obviously the same role as defining structures
made of columns; defining structures made of structures can also be viewed
as grouping groups of columns.

I see essentially two ways of defining such "column groups" in VOTable:

a. generalize the <COOSYS> method currently used to describe the coordinate 
   systems. This kind of "by reference" method defines a structure,
   and any <FIELD> can declare to be a member of that structure.
   As an illustration of a group of columns containing a flux value
   and its error, the XML code coold look like:

   1. define a structure:
      <STRUCTURE ID="Flux1" name="FluxParameters">
	<PARAMETER ID="Freq1" name="Frequency" value="8.6" datatype="float"
	  unit="GHz" />
      </STRUCTURE>
   
   2. within the <TABLE> definition, columns belonging to this structure
      refer to it:
      <FIELD name="flux" datatype="float" ref="Flux1" unit="mJy" />
      <FIELD name="e_flux" datatype="float" ref="Flux1" unit="mJy" />

b. introduce a new element e.g. <MFIELD> (multiple field) in the <TABLE> 
   description which would contain the fields. The same example of a
   flux + its associated error would be coded as:

   <MFIELD name="Flux" ucd="PHOT_FLUX_RADIO_8.4GHZ">
    <FIELD ID="Flux1" name="fluxValue" datatype="float" unit="mJy">
     <DESCRIPTION>Value of the flux at 8.4GHz</DESCRIPTION>
    </FIELD>
    <FIELD ID="e_Flux1" name="errFlux1" ucd="ERROR" datatype="float" unit="mJy">
     <DESCRIPTION>Error on flux value</DESCRIPTION>
    </FIELD>
    <PARAMETER ID="Freq1" name="Frequency" value="8.6" datatype="float"
      unit="GHz" />
   </MFIELD>

There could be a third way which would introduce new tags within each
table element like e.g. <VAL> and <ERR> to give
  <TD><VAL>11.35</VAL><ERR>1.12</ERR></TD>
but it would be against the current philosophy of VOTable which is to
define all metadata first followed by the data alone in order to
keep the  efficiency of large data sets (and the FITS compatibility); 
it could also require frequent modifications of the schema (XMLSchema) -- 
which could harm working applications.

The way