Complex sexagesimal angles

The TBLFMT item has additional options for reading more complex sexagesimal angles from STL catalogues. These options cover most of the formats used in practice to represent angles in astronomical catalogues held as text files. They should only be used in fixed-format STLs; if they are used in free-format STLs the results are unpredictable. For complex sexagesimal angles the TBLFMT item has the form:

TBLFMT=units{element_descriptors}

units is the units of the angle, as for simple sexagesimal angles. Again the permitted values are as listed in Table . element_descriptors is a series of Fortran-like descriptors for the individual components of the sexagesimal angle. A sexagesimal angle is allowed to comprise up to four components:

• an optional separate sign,

• the `main component'; the integer part of the angle in the specified units. Here this component is called the quotient (following more-or-less the usual usage of the term). This component is mandatory,

• either one or two sexagesimal subdivisions. These components are optional; the format for a given tabulated angle may contain zero, one or two sexagesimal subdivisions.

The descriptors used to read these components are very similar to the descriptors used in Fortran FORMAT statements. In the following $n$ is the total number of characters occupied by the item and $m$ the number of decimal places. Both $n$ and $m$ are positive integers. The following rules apply.

• The descriptor for a separate sign is of the form An.

• All the numeric components (the quotient and any sexagesimal subdivisions) have a descriptor of the form In for INTEGER values or Fn.m for REAL ones.

• Spaces, or any other separator characters, can be skipped by descriptors of the form nX.

• All components are separated by a comma (`,'), again cf Fortran FORMAT statements.

You simply assemble an appropriate set of descriptors to describe a given angular format. The only real restriction is that the quotient and any sexagesimal subdivisions must occur in order of decreasing size (that is, quotient first, least significant subdivision last). However, it is very unusual for sexagesimal angles to be tabulated in any other order. The following additional points apply to the optional separate sign.

• It can occur anywhere in the format; it does not have to be the first component.

• Positive angles are indicated by any of the following characters: +, n or N (N for north). Negative angles are indicated by any of the following characters: -, s or S (S for south).

• The separate sign is optional. Negative values can also be indicated by including a minus sign (`-') as the first character of the quotient (cf the usual rules for reading numbers in Fortran).

Figure  shows an example of an STL format catalogue containing several columns of complex sexagesimal angles. This catalogue is available as file:

/star/share/cursa/angles.TXT

Figure: An example STL format catalogue containing columns of complex sexagesimal angles

The interpretation of the TBLFMT items for these angles is quite straightforward. For example, column ANGLE1 starts in the third character of each record and has units of degrees. It has a separate sign as its first character. The quotient degrees, minutes and seconds are all two-character INTEGER values and are separated by one space (or rather by any single character).