Title:                  Proposal for MeasurementSet units convention
Person responsible:     Bob Garwood (bgarwood@nrao.edu)
Originator of proposal: Bob Garwood (bgarwood@nrao.edu)
Exploders targeted:     aips2-astro-design, aips2-gbt
Time table:
        Date of issue:          1996 January 18		Done
        Comments due:           1996 January 24         Done
        Revised proposal:       1996 January 26		Done
        Final comments due:     1996 January 29		Done
        Decision date:          1996 January 31		Done

Statement of goals:

To adopt a convention so that a Quantum can be easily constructed
from a value in a MeasurementSet.

Proposed changes:

	- A UNIT keyword will be associated with each column in
	  a MeasurementSet.  The value of this keyword will be a 	
	  String.  This String will one of the expected inputs
	  to the Quantum constructor.  
	- This UNIT will apply to all values in that column.
	  For Array columns, this UNIT will apply to all
	  values within each row.
	- All columns must have a UNIT keyword (an empty string
	  will imply a unitless column).
	- All scalar and array PredefinedColumns will have required
	  values for UNIT that will be checked by MS::validate()
	  (validate() is used at construction and destruction of a
	  MeasurementSet) (i.e. for scalar and array PredefinedColumns
	  there is no choice on the UNIT for that column).
	- It is strongly recommended that UNIT be a "simple" unit (no 
	  scale factors, just the basic SI unit and standard
	  astronomical units such as Jy).  An empty string is a valid 
	  value for UNIT.
	- An optional DISPLAY_UNIT keyword maybe used to indicate
	  appropriate units display utilities may use when 
	  displaying these values.  This is keyword is mearly a
	  hint and an application may choose to ignore this hint.

	To aid the user in constructing a Quantum (the aips++ class
	that joins the aips++ class encapsulating the SI unit system,
	the Unit class, with a value), the following member functions
	will be added to the MeasurementSet class:

	String& MS::unit(MS::PredefinedColumn whichColumn) const;
	String& MS::unit(const String& whichColumn) const;

	Both of these return the value of the UNIT keyword for
	the indicated column (either expressed as a predefined column
	or as a string).  They will return an empty string if
	the specified column does not exist.

	This enables the following:

	MS ms(...); // normal MeasurementSet construction, many steps not shown
	// construct the data column
	ROTableColumn data(tab,MS::columnName(MS::FLOAT_DATA));
	// construct a Quantum from the first value in the column
	// I need to verify that Quantum supports empty strings
	// as the unit.  I think it should.  Alternatively,
	// the user could check here.
	Quantum<Float> dataQuantum(data.asFloat(0), 
                                    ms.unit(MS::FLOAT_DATA));
	// you could then set the value of dataQuantum using the
	// value from another row of the same column
	dataQuantum.setValue(data.asFloat(rownr));


Expected Impact:


Since we're doing nothing now to indicate units, I think the
expected impact is rather low.  On the other hand, I don't think
very many folks have used Quantum yet and so that class might
conceivably change in a way which would require some change
to MS.