<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<p>Mark,</p>
<p>I just checked the Chandra documentation:
<a class="moz-txt-link-freetext" href="http://cxc.harvard.edu/csc2/columns/master.html">http://cxc.harvard.edu/csc2/columns/master.html</a> and
<a class="moz-txt-link-freetext" href="http://cxc.harvard.edu/csc2/columns/positions.html#ra_dec">http://cxc.harvard.edu/csc2/columns/positions.html#ra_dec</a></p>
<p>In my opinion, the current model can't accurately describe the
Chandra positional errors because "The radii of the semi-major and
semi-minor axes correspond to the 95% confidence intervals along
these axes" while ellipses obtained from the eigenvalue
decomposition of covariance matrices provide 68.xx% confidence
intervals (and 39.xx% if we consider the inside of the ellipse
instead of each individual axis).<br>
</p>
<p>fx</p>
<p><br>
</p>
<div class="moz-cite-prefix">Le 02/10/2019 à 19:36, Francois-Xavier
PINEAU a écrit :<br>
</div>
<blockquote type="cite"
cite="mid:13af7519-3e56-d5ec-15e8-2e9e5229ec94@astro.unistra.fr">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr"><b>Expéditeur:</b> "CresitelloDittmar, Mark"
<<a href="mailto:mdittmar@cfa.harvard.edu"
moz-do-not-send="true">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" moz-do-not-send="true">dm@ivoa.net</a>><br>
<b>Objet:</b> <b>[meas] RFC comments - FXP</b><br>
<br>
</div>
</blockquote>
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div>Francois-Xavier,<br>
<br>
</div>
Thanks for posting your comments on
the measurement model RFC page.<br>
</div>
I suppose this comment/request applies
to both yours and Markus' comments...<br>
</div>
<br>
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>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
<p>Mark,<br>
</p>
<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>
</p>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div>
<div>
<div>
<div>
<div>
<div><br>
</div>
If I read your points correctly it looks
like:<br>
</div>
+ the error matrix component is very
likely to be problematic as currently
modeled (seconding Markus' comments).<br>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
<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>
</p>
<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>
<p><br>
</p>
<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>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div>
<div>
<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>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
But we have to ensure that the 'next' phase will be compatible
with the current one, right?<br>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div>
<div> + Elliptical, Ellipsoid are compatible,
but could be enhanced by the addition of
'confidence level'. Laurent has also mentioned
this.<br>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
'Confidence level' or multiplication factor to get '1 sigma'.<br>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div>
<div> This I would also prefer to push to a
'next' phase working a use-case which contains
them.<br>
</div>
<br>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
<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>
</p>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div>The others are more
straightforward/clarifications:<br>
</div>
+ drop one of 'stat' and 'rand'.. I'm fine with
that.<br>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
Ok<br>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div> + definition of Ellipse.posAngle<br>
</div>
- 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>
</div>
</div>
</blockquote>
</blockquote>
<p>Well, "counter-clockwise" depends if one looks from the
interior or from the exterior of the unit sphere. <br>
</p>
<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>
</p>
<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>
</p>
<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>
</p>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div> + re: Equatorial, Galactic, Ecliptic
positions..<br>
</div>
<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>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
<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>
</p>
<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>
</p>
<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>
</p>
<blockquote type="cite"
cite="mid:F442E35D-6F94-481B-8AF0-19D0FC2E1882@astro.unistra.fr">
<blockquote type="cite">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div>
<div>
<div><br>
</div>
<div>Mark<br>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</blockquote>
fx<br>
</blockquote>
</body>
</html>