Fwd: [meas] RFC comments - FXP

[Francois-Xavier PINEAU]

>> *Expéditeur:* "CresitelloDittmar, Mark" <mdittmar at cfa.harvard.edu 
>> <mailto:mdittmar at cfa.harvard.edu>>
>> *Date:* 30 septembre 2019 à 22:06:49 UTC+2
>> *Destinataire:* Data Models mailing list <dm at ivoa.net 
>> <mailto:dm at ivoa.net>>
>> *Objet:* *[meas] RFC comments - FXP*
>>
>> Francois-Xavier,
>>
>> Thanks for posting your comments on the measurement model RFC page.
>> I suppose this comment/request applies to both yours and Markus' 
>> comments...
>>
>> Can you make a statement about whether you see a fundamental conflict 
>> in the model with the applications you are considering (eg: Gaia), or 
>> if the content is compatible, but would need enhancing/refining to 
>> accommodate your case better or more fully.

Mark,

See here after: with the current version of the model, I don't know how 
to describe the XMM (or GALEX) positional error.

I also have the feeling that it does not cover the "complexity" of the 
Gaia (or Hipparcos) astrometry.

>>
>> If I read your points correctly it looks like:
>>   + the error matrix component is very likely to be problematic as 
>> currently modeled (seconding Markus' comments).

I am not sure if the problem comes from the model or from the mapping of 
the model to catalogue data.

For example, in "Matrix2x2":

1 - m11 and m22 are variances while in catalogues we usually find 
standard deviations.

2 - m12(=m21) are co-variances while in practice the co-variance is 
often the product of 3 columns (2 std dev + correlation): rho * sigma1 * 
sigma2.

Is the model trying to describe all the possible mappings?

1 - if yes, standard deviation, correlation, co-sigma are missing

2-  if not, Ellipse and Ellipsoid may be redundant with 
CovarianceMatrix2x2 and 3x3 if we consider standard 1 sigma errors in 
each dimension.

(Why not a generic CovarianceMatrix with a dimension having the 
dimension of the measurement?)


May a measurement be a composition of measurements?

Considering Gaia (letting aside plx and Vr so far to simplify):

- like it is already possible with the current model, we could describe 
a Pos(+err) measurement and a PM(+err) measurement: a lot of tools will 
be happy just with Pos, Aladin will also use PM with an epoch slider.

- increasing the complexity for more advanced/precise tools, we could 
additionally describe and Astrometric (?) measurement being made of the 
composition of Pos(+err) + PM(+err)  + correlations between Pos and PM 
errors

>>      So, I fully expect to push that element to a 'next' phase where 
>> the Gaia usage might be a good thread to exercise it more directly.
But we have to ensure that the 'next' phase will be compatible with the 
current one, right?
>>    + Elliptical, Ellipsoid are compatible, but could be enhanced by 
>> the addition of 'confidence level'.  Laurent has also mentioned this.
'Confidence level' or multiplication factor to get '1 sigma'.
>>       This I would also prefer to push to a 'next' phase working a 
>> use-case which contains them.
>>
In this case, the document should probably state what can/can't be 
describe in its current version.

Is a symmetrical uncertainty allowed to describe a radial error?

Explanations: I see a possible confusion naming 'radius' the Symmetrical 
uncertainty value:

A - is it the 1 sigma error of the distribution on the polar 'r' coordinate?

B - is it the 1 sigma value of the distributions on each independent 
('x', 'y', ...) coordinate?

I understand from the definition in 10.2(.1) that the document describe 
case B, but some catalogues (like XMM) provide a value corresponding to 
case A.

So I don't know how to describe the XMM (or GALEX) positional error in 
the current version of the model.

>> The others are more straightforward/clarifications:
>>    + drop one of 'stat' and 'rand'.. I'm fine with that.
Ok
>>    + definition of Ellipse.posAngle
>>       - I'd like the definition to match the common usage.. but I was 
>> under the impression that "East of North" was a counter-clockwise 
>> direction.  Pulling from wikipedia (yeah.. I know);  "The 
>> International Astronomical Union defines it as the angle measured 
>> relative to the north celestial pole (NCP), turning positive into the 
>> direction of the right ascension. In the standard (non-flipped) 
>> images this is a counterclockwise measure relative to the axis into 
>> the direction of positive declination."

Well, "counter-clockwise" depends if one looks from the interior or from 
the exterior of the unit sphere.

Taking the East as the x-axis and the North as the y-axis in a regular 
frame representation (x increasing in the right direction and y 
increasing in the top direction), East of North is clockwise.

The Meas document definition is: "counter-clockwise from the positive 
direction of the FIRST axis of the associated Coordinate"

I may miss-interpret, but I consider the FIRST axis to be the local East 
while the position angle is defined from the North axis).

(By-the-way, do we agree that the errors associated to Lon and Lat are 
actually the errors in the local frame, i.e., to make it simple, the 
ones associated to Lon*cos(Lat) and Lat?).

>>    + re: Equatorial, Galactic, Ecliptic positions..
>>       Yes, this is a bit orthogonal to Cartesian.  The motivation is 
>> that at this level, I'm trying to expose the 'properties' that occur 
>> most often in our data which we would want to identify quickly and 
>> easily.  Setting a basis which can be expanded on in the Source 
>> properties thread. I have expected that "(ra,dec)" vs "(l,b)" might 
>> be an important distinction despite them both being Spherical.  If I 
>> open a cube, catalog or whatever and find SphericalPosition-s, then 
>> still have to go to the Frame to determine if it's an ra/dec or l/b 
>> or lon/lat of an observatory.  That's extra steps. Maybe that isn't 
>> so important?

When I see (l,b), I think: those are the longitude and latitude in 
galactic (with the difference that in galactic the longitude may be in 
]-180, 180] instead of [0, 360[).

The column names (l, b), (ra, dec), (elon, elat) all implicitly describe 
longitude and latitude (in degrees or in sexagesimal) providing 
additional information on the coordinate system (with ambiguities in the 
case of (ra, dec): FK5, ICRS, ... ?).

As far as the model provides the coordinate system associated to a 
position, I (so far) do not understand the need to explicitly have the 
different column names in duplicated classes.

(Like mentioned by Markus, I also prefer to prevent possible incoherence 
by replacing the duplicated classes EquatorialPos, GalacticPos and 
EclipticPos by a single class).

>>
>> Mark
fx
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