Determining whether one row represents the same item as another is done by comparing the values in certain of their columns to see if they are the same or similar. The most common astronomical case is to say that two rows match if their celestial coordinates (right ascension and declination) are within a given small radius of each other on the sky. There are other possibilities; for instance the coordinates to compare may be in a Cartesian space, or have a higher (or lower) dimensionality than two, or the match may be exact rather than within an error radius....
If you just need to match two tables according to sky position
with fixed errors you are recommended to use the simplified
tskymatch2
task.
For other cases, this section describes how to specify much more
flexible match criteria for use with
tmatch1
, tmatch2
or tmatchn
by setting the following parameters:
matcher
params
values*
tmatch2
you must specify both
values1
and values2
.
tuning
For example, suppose we wish to locate objects in two tables which are
within 3 arcseconds of each other on the sky. One table has columns
RA and DEC which give coordinates in degrees, and the other has columns
RArad and DECrad which give coordinates in radians. These are the
arguments which would be used to tell tmatch2
what the match
criteria are:
matcher=sky params=3 values1='RA DEC' values2='radiansToDegrees(RArad) radiansToDegrees(DECrad)'It is clearly important that corresponding values are comparable (in the same units) between the tables being matched, and in geometrically sensitive cases such as matching on the sky, it's important that they are the units expected by the matcher as well. To determine what those units are, either consult the roster below, or run the following command:
stilts tmatch2 help=matcherwhich will tell you about all the known matchers and their associated
params
, values*
and tuning
parameters.
The following subsections list the basic matcher
types and the
requirements of their associated params
,
values*
and tuning
parameters.
The units of the required values are given where significant.
sky
: Sky Matching
skyerr
:
Sky Matching with Per-Object Errors
skyellipse
:
Sky Matching of Elliptical Regions
sky3d
: Spherical Polar Matching
exact
: Exact Matching
1d
, 2d
, ...:
Isotropic Cartesian Matching
2d_anisotropic
, ...:
Anisotropic Cartesian Matching
2d_cuboid
, ...:
Cuboid Cartesian Matching
1d_err
, 2d_err
, ...:
Cartesian Matching with Per-Object Errors
2d_ellipse
:
Cartesian Matching of Elliptical Regions