<div dir="ltr">Right,<div><br></div><div>Keep in mind, that source catalogues are not in the use case(s) for this iteration of the models.</div><div>This version is to support the Cube model which is focused on Sparse Cubes (Event list) and Images.</div><div><br></div><div>It is meant to set the framework for exploring catalogues, time series, etc which will feed more </div><div>requirements down to the Meas/Coords models.. like other properties, improved error modeling, etc.</div><div><br></div><div>This is why I'm interested in separating things that are modeled incorrectly (so we can fix or remove them from this version), versus things which are OK, but need refining to be more broadly useful (which can be put in a 'next' category).</div><div><br></div><div>Mark</div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Thu, Oct 3, 2019 at 8:21 AM Francois-Xavier PINEAU <<a href="mailto:francois-xavier.pineau@astro.unistra.fr">francois-xavier.pineau@astro.unistra.fr</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex">
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<p>In my opinion, the current model can't accurately describe the
Chandra positional errors <br>
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<p>Unless the proper conversions are made in the mapping...</p>
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<div>Le 02/10/2019 à 19:36,
Francois-Xavier PINEAU a écrit :<br>
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<div dir="ltr"><b>Expéditeur:</b> "CresitelloDittmar, Mark"
<<a href="mailto:mdittmar@cfa.harvard.edu" target="_blank">mdittmar@cfa.harvard.edu</a>><br>
<b>Date:</b> 30 septembre 2019 à 22:06:49 UTC+2<br>
<b>Destinataire:</b> Data Models mailing list <<a href="mailto:dm@ivoa.net" target="_blank">dm@ivoa.net</a>><br>
<b>Objet:</b> <b>[meas] RFC comments - FXP</b><br>
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<div>Francois-Xavier,<br>
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Thanks for posting your comments
on the measurement model RFC page.<br>
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I suppose this comment/request
applies to both yours and Markus'
comments...<br>
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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.<br>
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<p>Mark,<br>
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<p>See here after: with the current version of the model, I
don't know how to describe the XMM (or GALEX) positional
error.</p>
<p>I also have the feeling that it does not cover the
"complexity" of the Gaia (or Hipparcos) astrometry.<br>
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If I read your points correctly it looks
like:<br>
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+ the error matrix component is very
likely to be problematic as currently
modeled (seconding Markus' comments).<br>
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<p>I am not sure if the problem comes from the model or from the
mapping of the model to catalogue data.</p>
<p>For example, in "Matrix2x2":</p>
<p>1 - m11 and m22 are variances while in catalogues we usually
find standard deviations.<br>
</p>
<p>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.</p>
<p>Is the model trying to describe all the possible mappings?</p>
<p>1 - if yes, standard deviation, correlation, co-sigma are
missing<br>
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<p>2- if not, Ellipse and Ellipsoid may be redundant with
CovarianceMatrix2x2 and 3x3 if we consider standard 1 sigma
errors in each dimension.</p>
<p>(Why not a generic CovarianceMatrix with a dimension having
the dimension of the measurement?)</p>
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<p>May a measurement be a composition of measurements?</p>
<p>Considering Gaia (letting aside plx and Vr so far to
simplify):</p>
<p>- 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.<br>
</p>
<p>- 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</p>
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<div> 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.<br>
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But we have to ensure that the 'next' phase will be compatible
with the current one, right?<br>
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<div> + Elliptical, Ellipsoid are compatible,
but could be enhanced by the addition of
'confidence level'. Laurent has also
mentioned this.<br>
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'Confidence level' or multiplication factor to get '1 sigma'.<br>
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<div> This I would also prefer to push to a
'next' phase working a use-case which contains
them.<br>
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<p>In this case, the document should probably state what
can/can't be describe in its current version.</p>
<p>Is a symmetrical uncertainty allowed to describe a radial
error?</p>
<p>Explanations: I see a possible confusion naming 'radius' the
Symmetrical uncertainty value:</p>
<p>A - is it the 1 sigma error of the distribution on the polar
'r' coordinate?<br>
</p>
<p>B - is it the 1 sigma value of the distributions on each
independent ('x', 'y', ...) coordinate?</p>
<p>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.</p>
<p>So I don't know how to describe the XMM (or GALEX) positional
error in the current version of the model.<br>
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<div>The others are more
straightforward/clarifications:<br>
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+ drop one of 'stat' and 'rand'.. I'm fine with
that.<br>
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Ok<br>
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<div> + definition of Ellipse.posAngle<br>
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- 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."</div>
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<p>Well, "counter-clockwise" depends if one looks from the
interior or from the exterior of the unit sphere. <br>
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<p>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.<br>
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<p>The Meas document definition is: "counter-clockwise from the
positive direction of the FIRST axis of the associated
Coordinate"</p>
<p>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).<br>
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<p>(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?).<br>
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<div> + re: Equatorial, Galactic, Ecliptic
positions..<br>
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<div> 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?<br>
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<p>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[).</p>
<p>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, ... ?).<br>
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<p>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.<br>
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<p>(Like mentioned by Markus, I also prefer to prevent possible
incoherence by replacing the duplicated classes EquatorialPos,
GalacticPos and EclipticPos by a single class). <br>
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fx<br>
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