[TAP] data type for column metadata

[Patrick Dowler]

I have been mulling things over and also talked with Arnold a bit, and this 
was the result:

I think a simple extension to the column metadata would be as follows. The 
name and datatype would be mandatory and all others optional (as they don't 
apply in many cases, or are simply not important; if a service doesn't 
specify that should be OK).

http://www.ivoa.net/Documents/latest/STC.html
http://tycho.usno.navy.mil/systime.html

name: the column name
datatype: the SQL-ish datatype as discussed last week
units: units the column values are in
refsystem: reference system or scale
 - values taken from STC, Table 2 (time), Table 3 (space), 
 - values from FITS WCS Paper 3 Table 1 (energy): spectral CTYPE?
refposition: reference position
 - values taken from STC, Table 1
flavor: what kind of a value is it? 
 - could be taken from STC, section 4.4.1.2.2 (CARTESIAN, SPHERICAL, etc)
 - could be taken from STC section 4.4.1.4.2 (for doppler/velocity)?
 - includes differentiating JD vs MJD
 - could be taken from FITS WCS style generally
 
Note: In STC one differentiates energy vs frequency vs wavelength somewhat 
implicitly via the units. In FITS WCS Paper 3 Table 1 there are explicit 
CTYPE values for these as well as the different velocity definitions. The 
text in STC section 4.4.1.4.2 says that a future version will be compliant 
with the FITS WCS std. 

Examples: name, datatype, units, refsystem, refposition, flavor

* a column containing spatial positions:
  pos, POINT, deg, ICRS, TOPOCENTER, UNITSPHERE

* separate columns containing spatial coordinates:
  ra, DOUBLE, deg, ICRS, TOPOCENTER, UNITSPHERE
  dec, DOUBLE, deg, ICRS, TOPOCENTER, UNITSPHERE
note: nothing really says which is LON and which is LAT

* a column containing an instant of time
  some_date, DOUBLE, d, UTC, TOPOCENTER, MJD

* another column containing an instant in time
  lastModified, TIMESTAMP,  <null>, UTC, <null>, <null>
note: reference position not important for this sort of time value

* a pair of columns containing energy bounds of an obervation
  energy1, DOUBLE, m, WAVE, BARYCENTER, <null>
  energy2, DOUBLE, m, WAVE, BARYCENTER, <null>
note: using FITS WCS CTYPE for refsystem, no flavor needed for 1D

* a column containing a  measured redshift
  z, DOUBLE, <null>, ZOPT, BARYCENTER, <null>
note: using FITS WCS CTYPE for refsystem

* a column containing a measured velocity
  v, DOUBLE, m s-1, VOPT, BARYCENTER, <null>
note: using FITS WCS CTYPE for refsystem

For many columns in a table, the values for refsystem, refposition, and flavor 
will be null (not applicable), but for coordinate values they are necessary.

Comments on this? 

-- 

Patrick Dowler
Tel/Tél: (250) 363-0044
Canadian Astronomy Data Centre
National Research Council Canada
5071 West Saanich Road
Victoria, BC V9E 2M7

Centre canadien de donnees astronomiques
Conseil national de recherches Canada
5071, chemin West Saanich
Victoria (C.-B.) V9E 2M7