The match criteria box allows you to specify what counts as a match
between two rows. The selection you make in this box will determine
which columns you have to fill in for the table(s) being matched
in the rest of the window. In most cases what you are selecting here
is the coordinate space in which rows will be compared against each other,
and a numerical value or values to determine how close two rows have to be
in terms of a metric on that space to count as a match.
The following match types are offered:
-
Sky
- Comparison of positions on the celestial sphere.
In this case you will need to specify columns giving
Right Ascension and Declination
for each table participating in the match.
The Max Error value you must fill in is the
maximum separation of matched points around a great circle.
-
Sky with Errors
- The matching is like that for the Sky option above,
but an error radius (positional uncertainty) can be given for each
row in the input tables, rather than just a single value for the whole
match.
You need to specify a single Max Error value, which gives the
global maximum separation applying to all matches, and for each
of the input tables, along with the Right Ascension and
Declination columns, you also specify an Error column
which gives the error radius corresponding to that position.
Two rows are considered to match when the separation between the
two RA,Dec positions is smaller than both the
Max Error value and the sum of the two
Error values for the corresponding rows.
If either of the per-row Error values is blank, then any separation
up to the Max Error is considered to match.
According to these rules, you might decide to set the Max Error
to an arbitarily large number so that only the sum of per-row Errors
will determine the actual match criteria.
However please don't do this, since the Max Error also
functions as a tuning parameter for the matching algorithm,
and ought to be reasonably close to the actual maximum acceptable
separation -
if necessary use the Statistics Window
to determine the actual maximum uncertainty.
-
Sky 3D
- Comparison of positions in the sky taking account of
distance from the observer.
In this case you will need to specify columns giving
Right Ascension and Declination in angular units,
as well as distance along the line of sight in arbitrary units
for each table participating in the match.
The Error value is a maximum separation in Cartesian space
of matched points in the same units as the radial distance.
-
Exact Value
- Requires exact matching of values.
In this case you will need to specify the column containing the match key
for each table participating in the match;
this might typically be an object name or index number.
Two rows count as matching if they have exactly the same entry in
the specified field, except rows with a null value in that column,
which don't match any other row.
-
N-dimensional Cartesian
- Comparison of positions in an isotropic N-dimensional Cartesian space.
In this case you will need to specify N columns giving
coordinates for each table participating in the match.
The Error value is the maximum spatial separation of matched points.
Currently the highest dimensionality you can select is 3-d -
does anyone want a higher number?
-
N-dimensional Cartesian (anisotropic)
- Comparison of positions in an N-dimensional Cartesian space
with an anisotropic metric.
In this case you will need to specify N columns giving coordinates
for each table participating in the match,
and an error radius for each of these dimensions.
Points P1 and P2 are considered to match if P2 falls within
the ellipsoid defined by the error radii centered on P1.
This kind of match will typically be used for non-'spatial' spaces,
for instance (magnitude,redshift) space, in which the metrics in
different dimensions are not related to each other.
Currently the highest dimensionality you can select is 4-d -
does anyone want a higher number?
-
Sky + X
- Comparison of positions on the celestial sphere with an additional
numeric constraint.
This is a combination of the Sky and
1-d Cartesian matches above, so the columns you need
to supply are RA, Dec and one extra, and the errors are
angular separation on the sky and the error in the extra column.
A match is registered if it matches in both of the constituent tests.
You could use this for instance to match objects which are both close
on the sky and have similar luminosities.
-
Sky + XY
- Comparison of positions on the celestial sphere with an additional
2-d anisotropic Cartesian constraint.
This is a combination of the Sky and 2-d Anisotropic Cartesian
matches above, so the columns you need to supply are
RA, Dec and two extra, and the errors are
angular separation on the sky and the error radii corresponding to
the extra columns.
A match is registered if it matches in both of the constituent tests.
You could use this for instance to match objects which are both close
on the sky and have similar luminosities and redshifts.
-
HTM
- Performs sky matching in just the same way as the Sky
option above, but using a different algorithm (pixelisation of the
celestial sphere is performed using the Hierarchical Triangular Mesh
rather than the HEALPix scheme). The results in both cases
should be identical, but HTM is much slower. Hence, this option
is only useful for debugging. It may be withdrawn in future releases.
Depending on the match type, the units of the error value(s) you enter
may be significant. In this case, there will be a unit selector
displayed alongside the entry box. You must choose units which
are correct for the number you enter.