UCDs Use cases

This page presents good practice use cases for assigning a relevant Unified Content Descriptor (UCD) to each frequently used quantity.

It is focusing on practical use cases of tabular data, and each table field (field meaning attribute, or column) is described by :

• A column name
• A plain english description of the column
• The corresponding UCD

Coordinates

• A simple table, containing one set of coordinates :

The UCD are the same whether the values are sexagesimal or decimal values.

Note that both UCDs could be complemented with the ;meta.main qualifier to indicate that the columns are the main values, but as there is only one right ascension and declination in the table, this is not mandatory (unless this is explicitly required by some protocol).

• A table containing several coordinate values for each tuple :

In this case, there are two right ascensions (and declinations), and so it can be useful to append ;meta.main to one of the values: this will indicate that if a tool has to pick one right ascension value, the column described by pos.eq.ra;meta.main is to be preferred ( alpha2 here). Therefore, one should avoid to attach identical UCDs containing meta.main to several column within a single table, as it would create ambiguities.

• A table containing many position information :

Column name	X_det	Y_det	X_offset	Y_offset	alpha1	delta1	alpha2	delta2	l	b	...
UCD	pos.cartesian.x;instr.det	pos.cartesian.y;instr.det	pos.cartesian.x;arith.diff;instr.det	pos.cartesian.y;arith.diff;instr.det	pos.eq.ra	pos.eq.dec	pos.eq.ra;meta.main	pos.eq.dec;meta.main	pos.galactic.lon;meta.main	pos.galactic.lat;meta.main	...
Description	Pixel value of source peak along first axis	Pixel value of source peak along second axis	Offset between peak and center along first axis	Offset between peak and center along second axis	Right ascension of peak luminosity	Declination of peak luminosity	Right ascension of photocenter	Declination of photocenter	Galactic longitude	Galactic latitude	...

The positions in pixel values are described as cartesian coordinates. We use the secondary word instr.det to indicate the kind of frame these coordinates are related to. Note that the instr.det is optional: the first word carries most of the meaning.

The position offset in pixel is a difference between two coordinates that would be described by the same UCD primary word, so we use the arith.diff word to describe it.

We considered here that the coordinates of the photocenter were the main coordinates to use for each source.

Photometry

coming soon