[meas] RFC comment - MD #8,9,10,11

[CresitelloDittmar, Mark]

These comments are all about the Error model.. there is a fair amount of
text with these comments, so I'll let the reader get the details from the twiki
page <https://wiki.ivoa.net/twiki/bin/view/IVOA/MeasRFC>.

8) would like to remove Bounds?D, Ellipse, Ellipsoid, and CovarianceMatrix
<https://wiki.ivoa.net/twiki/bin/edit/IVOA/CovarianceMatrix?topicparent=IVOA.MeasRFC;nowysiwyg=0>

These are in the model as basic forms of multi-dimensional error types
which do occur in our data.
I admit that the specific products that I'm working with do use them, so
they could be removed
without directly affecting that.

However, the risk of removing the multi-d types is that it then becomes
very tempting to move the relation between the coordinate value(s) and
their Error (as you do in the alternate syntax in #11).  This is a topic
we've gone through more than once I think.. and I strongly resist changing
that relation.  As soon as you need to incorporate the multi-d errors (like
Gaussian), it needs to be associated with the coordinate pair.

9) does not like the Matrix representation; and usage for Covariance in
particular

If the CovarianceMatrix representation is wrong.. it should be corrected or
removed.  This isn't really in my wheelhouse, but I thought I looked into
it enough to have it properly represented.  If not, I'd need someone to
provide the corrected model.

As for MatrixMxN itself.. as a DataType, I can't model it as an open array
of cells like I do in the Transform model (where it is an ObjectType).  I
could model it as a list of cells, but would still need 2 types with fixed
lengths (#cells).  That is an easy switch, but more bulky in direct
serializations.

10) points out that Gaussian, and other distributions, are missing, but
also seems to indicate that we don't want to get into these at this point.

So, we're not adding distributions yet.. please.

11) general issue with the manner that correlation is modeled, suggests a
      ra   = GenericMeasure
<https://wiki.ivoa.net/twiki/bin/edit/IVOA/GenericMeasure?topicparent=IVOA.MeasRFC;nowysiwyg=0>(value=20
Error(id=err-ra, statError=1e-7))
      dec = GenericMeasure
<https://wiki.ivoa.net/twiki/bin/edit/IVOA/GenericMeasure?topicparent=IVOA.MeasRFC;nowysiwyg=0>(value=30,
Error(id=err-ra, statError=1e-7))
      Correlation(err1=err-ra, err2=err-dec, coeff=0.5)

The problem with this sort of ad hoc serialization representation is that
I'm not sure what you mean in terms of the model.  It looks like maybe
this..

[image: correlation.png]
Which I don't quite understand... really implies only 1D errors.
Anyway.. if the correlation isn't done right in this model, it is probably
best to skip it until we have a concrete example to work.  Perhaps with the
catalogue/Source properties thread which is getting input from Gaia (I
think).

Mark
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