Storing and representing columns of angles

CAT provides special facilities for representing columns which contain angles. Usually angular columns are used to store celestial coordinates such as Right Ascension and Declination, though they can contain any angular measure. There are two requirements for the treatment of angles stored in astronomical catalogues:

• inside a program angles should be expressed in radians and stored in DOUBLE PRECISION or REAL variables for ease of processing,

• usually they are best displayed to a user in units of hours or degrees, subdivided into sexagesimal minutes and seconds. Alternatively, small angles might be best displayed as minutes or seconds of arc or time and represented to some number of places of decimals.

CAT provides both these facilities, as follows:

• when the value of a field in an angular column is obtained using CAT_EGT0D or CAT_EGT0R it is returned as DOUBLE PRECISION or REAL variable in radians,

• when the value of a field in an angular column is obtained formatted for display using CAT_EGT0F it is returned as a CHARACTER string containing the value expressed in hours, degrees, minutes or seconds and optionally formatted as a sexagesimal value.

In order for CAT to know that a column contains an angle it must satisfy the following two requirements.

• The data type must be DOUBLE PRECISION or REAL¹⁶,

• The units attribute of the column should be set to `RADIANS' followed by an angular format specifier enclosed in curly brackets (`{}'). The simplest forms of this angular format specifier are simply `HOURS' and ` DEGREES' for hours and degrees respectively. Thus, examples of the UNITS attribute are:

  RADIANS{HOURS}

  to display the column in hours,

  RADIANS{DEGREES}

  to display the column in degrees.

  The angular format specifiers are described in full in the following section.

If the external display format attribute of the column, EXFMT, is explicitly set it should be set to a normal Fortran 77 format specifier corresponding to the data type of the column, for example `D16.8' for a DOUBLE PRECISION column or `E14.6' for a REAL column¹⁷.

When the catalogue is written fields should be written to angular columns using CAT_PUT0D or CAT_PUT0R with the angles expressed in radians.