As a representative of the SIM mission I should mention that SIM will be
regularly be making measurements of positions with precision order 0.1
microarcsec (although accuracy may be limited by fiducial grid accuracy).
1 microarcsec = 1E-6/3600 = 2.778E-10.
So to express, say 243 degrees plus 9.13 microarcsec or
243.00000000025361 which has 17 digits, is not expressable in double
precision (where the significand holds a bit over 15 digits).
Therefore we need to introduce quad precision if VOTable is to be used
in SIM and GAIA preparatory work.
Although I would prefer to use hh mm ss.ssssssss since quad precision is way overkill.
Ed
Thomas McGlynn wrote:
> Mark Taylor wrote:
>> On Thu, 20 Apr 2006, Thomas McGlynn wrote:
>>
>>
>>> While Roy and Dave may have hinted at this, let me say very
>>> plainly that I think this is a bad idea. Coordinates in
>>> VOTables should be respresented in decimal degrees and
>>> any other usage should be strongly discouraged. Allowing
>>> other formats makes the job of software which is to
>>> read and write VOTables much harder and more error prone.
>>
>>
>> I should clarify that I'm *not* suggesting that understanding these
>> formats should be a requirement for compliant VOTable I/O software.
>> The representation type would simply be an extra bit of information
>> which software could use or ignore at its option. Thus it is not
>> true that it makes the job of reading or writing VOTables any harder.
>> It makes it easier for software which wishes to accommodate
>> data in, e.g. ISO-8601 format, and imposes no additional burden on
>> software which does not so wish. I would say introducing a new
>> "representation" attribute is rather analogous to the introduction of
>> the "utype" attribute in this respect.
>>
>> Mark
>>
>
> I don't think I agree. If I wish to be able to read compliant
> coordinates
> from VOTables (the most basic of operations), then my reader
> would need to be far more complex than it currently is. I'll grant you
> that I can copy the bytes of the VOTable onto my machine, but I can't
> do anything useful with them. There might be occasional programs that
> can ignore this -- one doesn't alway use coordinates or time-- but the
> vast
> majority of programs would either need to be cognizant of this practice
> or will have to ignore the tables that use it.
>
> Tom
>
> P.S., I'm not so keen about utypes either but that's another story.
> P.P.S., At least you've gotten this newsgroup going again!
Received on 2006-04-20Z23:38:09