Steve Allen wrote:
> On Fri 2005-06-24T14:53:46 +0200, Andrea Preite Martinez hath writ:
...
>>S | pos.earthop | Earth orientation parameters
>>Q | pos.earthop.nutation | Earth nutation
>
> These specifications seem incomplete, but I'm not sure what
> they are supposed to be communicating.
open corresponding items ( pos.eop, pos.eop.nutation, pos.eop.pole )
at http://www.imcce.fr/fr/expert/ssvo/wgovp/ucdtree.html to see more possible elements.
Passing the mouse over the proposed UCD display a few words meaning in the upper part
of the window ;-)
>
> As a starter for discussion, does nutation refer to
> IAU 2000A model, IAU 2000B model, or the models resulting from
> the FK5-based revision performed between 1976/1984, or
> the nutation based on the older Newcomb models of precession, or what?
>
> Is polar motion and UT1 relevant? Do they use the IAU 2000 models, or
> one of the older forms used in previous versions of the IERS
> conventions, or the even older FK5 forms, or what?
>
>
>>Q | pos.eq.ha | Hour-angle
>
>
>>Q | pos.az | Position in alt-azimutal frame
>>Q | pos.az.alt | Alt-azimutal altitude
>>Q | pos.az.azi | Alt-azimutal azimut
>>Q | pos.az.zd | Alt-azimutal zenith distance
>
>
> If I understand the work of the general relativists who have been
> contributing to various IAU initiatives over the past two decades then
> all of the above quantities do not have precisely-defined meanings.
> There is no mathematical framework for a rigorous mapping of celestial
> sphere to observation angles which is valid at the microarcsecond
> level.
Perhaps, but a lot of measurements with much less accuracy exist.
UCD has not the goal to replace STC or any equivalent,
only give some (not all) informations about used quantity.
>
> The meaning of hour angle has been called into question at
> a precision much less tight by the IAU 2000 redefinition of UT1
> and the introduction of the CIO/CEO- and TIO/TEO- based
> expressions. There is an active IAU committee trying to work
> out nomenclature for the "classical" vs. the "CIO-based"
> versions of these sorts of quantities.
>
> Should the definitions of these admit that they may
> not be meaningful at the milliarcsecond level and give
> references to the draft documents which explain why?
>
> On Tue 2005-06-28T15:16:26 +0200, Pierre Didelon hath writ:
>
>
>>A.3.1 pos.earth
>> Q | pos.earth.lon | Longitude on Earth
>>exists but not latitude!
>>Q | pos.earth.altitude | Altitude on Earth
>>exists but not distance which give the distance to the center
>>
>>A.3.2 lon/lat/alt/dist
>
>
> In the context of FITS WCS Paper III we decided that it was not a good idea
> to admit the concepts of latitude, longitude, and height into the
> standard.
>
> (Note, by the way that I strongly prefer to use the term "height" rather
> than "altitude" or "elevation". The geodetic community also uses "height"
> at least in part to avoid confusion with the other two, which may be angles.)
Whatever is the choosen term, it is needed, even in STC, at least to specify
the altitude of the observing place.
>
> Unless there is also a UCD word something like pos.earth.geodatum it is not
> possible to associate a precise meaning with .lon, .lat, and .alt
>
> As we point out in FITS WCS Paper III, there are something like 1000
> different geodetic datums in use, and some of their positions differ
> from each other by kilometers. Rather than burden FITS with the need
> to recognize even a small subset of those datums we decided to omit
> angular body coordinates and stick to Cartesian coordinates only.
>
> True, even Cartesian coordinates technically require the specification
> of a datum, but in current usage all the Cartesian terrestrial datums
> differ by only about 0.1 m.
>
> I recognize that there is a very strong tradition for the use of
> angular body coordinates, but if precise meaning is required they
> are not a good idea.
>
> --
> Steve Allen <sla-at-ucolick.org> WGS-84 (GPS)
> UCO/Lick Observatory Natural Sciences II, Room 165 Lat +36.99858
> University of California Voice: +1 831 459 3046 Lng -122.06014
> Santa Cruz, CA 95064 http://www.ucolick.org/~sla/ Hgt +250 m
^
|
perhaps not precise enough, but nevertheless usefull quantities ;-)
--
Pierre
------------------------------------------------------------------
DIDELON :@: pdidelon_at_cea.fr Phone : 33 (0)1 69 08 58 89
CEA SACLAY - Service d'Astrophysique 91191 Gif-Sur-Yvette Cedex
------------------------------------------------------------------
Aidez les enfants Tibétains : http://www.a-e-t.org/jcparrain.htm
ou d'autres : http://www.sosesf.org/
------------------------------------------------------------------
Puissé-je toujours marcher avec la beauté devant moi.
Dicton Navajo Tony Hillerman - Les clowns sacrés
------------------------------------------------------------------
Received on 2005-06-29Z10:15:45