sexagesimal

[Doug Tody]

I'm not sure I want to start this discussion up again, but having
scanned the emails after returning from travel I'm not sure I saw the
obvious mentioned, which is to use a similar HMS format to ISO 8601
time format.  We want to do that right, so that we can be consistent?
This also happens to be identical to the existing IRAF format for
sexagesimal numbers, which is in wide use.

Basically this is [+-]HH:MM:SS.SSS; whether we have hours or degrees
is a unit and need not affect the format.  A sexagesimal number in
this format can easily be parsed as a number - this is already done
in IRAF for example, which can equivalently accept either decimal
degrees or DMS, automatically recognizing the format while parsing.

Any astronomy standard for sexagesimal should specify a simple, fixed
format such as this as the "blessed" format.  Of course, for legacy data
and user input a smarter parser is needed, but that is a separate
problem and we don't need to accept a wide range of formats in a
formatting standard.

 	- Doug


On Sat, 16 Sep 2006, Roy Williams wrote:

> Dear Data Model Working Group
>
> I believe that the IVOA should bite the bullet and standardize the 
> sexagesimal format. Some say it is impossible, that we have to take whatever 
> they give us. But this is a horrible place to be, it takes hundreds of lines 
> of code to understand something like 5h:23m,35d:12m:23".23 and all its 
> tortured friends.
>
> There is no reason to allow this level of complexity to continue.  If we can 
> standardize something as complex as spectrum formats, why are we still 
> getting tripped up the sexagesimal format? Whenever I write code to read 
> sexagesimal I feel I am in an uncertain world that makes me fall, I am like 
> tripping over my own trousers! Do not be afraid that they will laugh, I 
> believe this the sort of thing that real astronomers really want from us!
>
> My suggestion is to say that the IVOA understands a sexagesimal position if 
> and only if it fits the definition as "six numbers separated by non-numbers". 
> It is a very simple defintion and yet accepts almost all the formats that are 
> used.
>
> I suggest that an IVOA sexagesimal position must consist of the numbers RAh, 
> RAm, RAs, Decd, Decm, Decs, and there can be arbitrary "non-number" 
> characters separating them. In other words, all characters that are not 0-9 
> or + or - or . are considered separators, and what is left must parse as six 
> numbers, with RAh, RAm, Decd, Decm as integers, and RAh and Decs as floats. 
> Note that the seconds is a required field for both RA and Dec. Once this is 
> done, everything else is straightforward.
>
> If there is general agreement from the DM group, I volunteer to make a *very 
> short* writeup and get it to IVOA Recommendation quickly.
>
> Roy Williams
> -------------------------
>
> Examples of valid IVOA sexagsimal positions, these parse to the same thing:
> 5h 23m 0s, 35d 12m 23.23s
> 5h 23m 0s, 35°  12' 23.23"
> 5 23 0 35 12 23.22
> 5 23 0 35 +12 23.22
> 5dfds23,,,    ,,,,0;;;;;35kjdshhgf-12,    23.22
>
>
> California Institute of Technology
> 626 395 3670
>