UCDsUseCases < IVOA

IVOA Web>UCDsUseCases (2015-06-09, MireilleLouys) (raw view)

---+ UCDs Use cases

This page presents good practice use cases for assigning a relevant<strong> %RED%Unified Content Descriptor%ENDCOLOR%</strong> (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 :
| *Column name* | *alpha* | *delta* | *...* |
| *Description* | The equatorial right ascension | The equatorial declination | ... |
| *UCD* | =%GREEN%pos.eq.ra%ENDCOLOR%= | =%GREEN%pos.eq.dec%ENDCOLOR%= | ... |

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

Note that both UCDs could be complemented with the =%GREEN%;meta.main%ENDCOLOR%= 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 :
| *Column name* | *alpha1* | *delta1* | *alpha2* | *delta2* | *...* |
| *Description* | Raw right ascension | Raw declination | Adjusted right ascension | Adjusted declination | ... |
| *UCD* | =%GREEN%pos.eq.ra%ENDCOLOR%= | =%GREEN%pos.eq.dec%ENDCOLOR%= | =%GREEN%pos.eq.ra;meta.main%ENDCOLOR%= | =%GREEN%pos.eq.dec;meta.main%ENDCOLOR%= | ... |

In this case, there are two right ascensions (and declinations), and so it can be useful to append =%GREEN%;meta.main%ENDCOLOR%= to *one* of the values: this will indicate that if a tool has to pick one right ascension value, the column described by =%GREEN%pos.eq.ra;meta.main%ENDCOLOR%= is to be preferred ( _alpha2_ here). Therefore, one should avoid to attach identical UCDs containing =%GREEN%meta.main%ENDCOLOR%= 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* | *...* |
| *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 | ... |
| *UCD* | =%GREEN%pos.cartesian.x;instr.det%ENDCOLOR%= | =%GREEN%pos.cartesian.y;instr.det%ENDCOLOR%= | =%GREEN%pos.cartesian.x;arith.diff;instr.det%ENDCOLOR%= | =%GREEN%pos.cartesian.y;arith.diff;instr.det%ENDCOLOR%= | =%GREEN%pos.eq.ra%ENDCOLOR%= | =%GREEN%pos.eq.dec%ENDCOLOR%= | =%GREEN%pos.eq.ra;meta.main%ENDCOLOR%= | =%GREEN%pos.eq.dec;meta.main%ENDCOLOR%= | =%GREEN%pos.galactic.lon;meta.main%ENDCOLOR%= | =%GREEN%pos.galactic.lat;meta.main%ENDCOLOR%= | ... |

The positions in pixel values are described as cartesian coordinates. We use the secondary word =%GREEN%instr.det%ENDCOLOR%= to indicate the kind of frame these coordinates are related to. Note that the =%GREEN%instr.det%ENDCOLOR%= 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 =%GREEN%arith.diff%ENDCOLOR%= 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

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Topic revision: r3 - 2015-06-09 - MireilleLouys

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