Provided by: stilts_3.5.1-2_all 
NAME
stilts-tskymatch2 - Crossmatches 2 tables on sky position
SYNOPSIS
stilts tskymatch2 [in1=<table1>] [ifmt1=<in-format>] [in2=<table2>] [ifmt2=<in-format>]
[omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|tosql|gui]
[out=<out-table>] [ofmt=<out-format>] [ra1=<expr>] [dec1=<expr>]
[ra2=<expr>] [dec2=<expr>] [error=<value/arcsec>] [tuning=<healpix-k>]
[join=1and2|1or2|all1|all2|1not2|2not1|1xor2]
[find=all|best|best1|best2] [runner=parallel|parallel<n>|parallel-
all|sequential|classic|partest]
DESCRIPTION
tskymatch2 performs a crossmatch of two tables based on the proximity of sky positions.
You specify the columns or expressions giving right ascension and declination in degrees
for each input table, and a maximum permissible separation in arcseconds, and the
resulting joined table is output.
If you omit expressions for the RA and Dec, an attempt is made to identify the columns to
use using column Unified Content Descriptors (UCDs) or names. First columns bearing
appropriate UCD1 or UCD1+ values (POS_EQ_RA, POS_EQ_RA_MAIN, pos.eq.ra or
pos.eq.ra;meta.main and their equivalents for declination) are sought. If these cannot be
found, columns named something like "RA" or "RA2000" are sought. If either is found, the
column units are consulted and radian->degree conversions are performed if necessary
(degrees are assumed if no unit value is given). If nothing likely can be found, then the
command will fail with an error message. This search logic is intended as a convenience
only; it is somewhat ad hoc and subject to change. To make sure that the correct angle
values are being used, specify the ra and dec position parameters explicitly.
tskymatch2 is simply a cut-down version, provided for convenience, of the more general
matching task tmatch2. If you want more match options or otherwise more configurability,
you can probably find it by using tmatch2.
OPTIONS
in1=<table1>
The location of the first input table. This may take one of the following forms:
* A filename.
* A URL.
* The special value "-", meaning standard input. In this case the input format
must be given explicitly using the ifmt1 parameter. Note that not all formats
can be streamed in this way.
* A scheme specification of the form :<scheme-name>:<scheme-args>.
* A system command line with either a "<" character at the start, or a "|"
character at the end ("<syscmd" or "syscmd|"). This executes the given pipeline
and reads from its standard output. This will probably only work on unix-like
systems.
In any case, compressed data in one of the supported compression formats (gzip,
Unix compress or bzip2) will be decompressed transparently.
ifmt1=<in-format>
Specifies the format of the first input table as specified by parameter in1. The
known formats are listed in SUN/256. This flag can be used if you know what format
your table is in. If it has the special value (auto) (the default), then an attempt
will be made to detect the format of the table automatically. This cannot always be
done correctly however, in which case the program will exit with an error
explaining which formats were attempted. This parameter is ignored for scheme-
specified tables.
in2=<table2>
The location of the second input table. This may take one of the following forms:
* A filename.
* A URL.
* The special value "-", meaning standard input. In this case the input format
must be given explicitly using the ifmt2 parameter. Note that not all formats
can be streamed in this way.
* A scheme specification of the form :<scheme-name>:<scheme-args>.
* A system command line with either a "<" character at the start, or a "|"
character at the end ("<syscmd" or "syscmd|"). This executes the given pipeline
and reads from its standard output. This will probably only work on unix-like
systems.
In any case, compressed data in one of the supported compression formats (gzip,
Unix compress or bzip2) will be decompressed transparently.
ifmt2=<in-format>
Specifies the format of the second input table as specified by parameter in2. The
known formats are listed in SUN/256. This flag can be used if you know what format
your table is in. If it has the special value (auto) (the default), then an attempt
will be made to detect the format of the table automatically. This cannot always be
done correctly however, in which case the program will exit with an error
explaining which formats were attempted. This parameter is ignored for scheme-
specified tables.
omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|tosql|gui
The mode in which the result table will be output. The default mode is out, which
means that the result will be written as a new table to disk or elsewhere, as
determined by the out and ofmt parameters. However, there are other possibilities,
which correspond to uses to which a table can be put other than outputting it, such
as displaying metadata, calculating statistics, or populating a table in an SQL
database. For some values of this parameter, additional parameters (<mode-args>)
are required to determine the exact behaviour.
Possible values are
* out
* meta
* stats
* count
* checksum
* cgi
* discard
* topcat
* samp
* tosql
* gui
Use the help=omode flag or see SUN/256 for more information.
out=<out-table>
The location of the output table. This is usually a filename to write to. If it is
equal to the special value "-" (the default) the output table will be written to
standard output.
This parameter must only be given if omode has its default value of "out".
ofmt=<out-format>
Specifies the format in which the output table will be written (one of the ones in
SUN/256 - matching is case-insensitive and you can use just the first few letters).
If it has the special value "(auto)" (the default), then the output filename will
be examined to try to guess what sort of file is required usually by looking at the
extension. If it's not obvious from the filename what output format is intended, an
error will result.
This parameter must only be given if omode has its default value of "out".
ra1=<expr>
Right ascension in degrees for the position of each row of table 1. This may simply
be a column name, or it may be an algebraic expression calculated from columns as
explained in SUN/256. If left blank, an attempt is made to guess from UCDs, column
names and unit annotations what expression to use.
dec1=<expr>
Declination in degrees for the position of each row of table 1. This may simply be
a column name, or it may be an algebraic expression calculated from columns as
explained in SUN/256. If left blank, an attempt is made to guess from UCDs, column
names and unit annotations what expression to use.
ra2=<expr>
Right ascension in degrees for the position of each row of table 2. This may simply
be a column name, or it may be an algebraic expression calculated from columns as
explained in SUN/256. If left blank, an attempt is made to guess from UCDs, column
names and unit annotations what expression to use.
dec2=<expr>
Declination in degrees for the position of each row of table 2. This may simply be
a column name, or it may be an algebraic expression calculated from columns as
explained in SUN/256. If left blank, an attempt is made to guess from UCDs, column
names and unit annotations what expression to use.
error=<value/arcsec>
The maximum separation permitted between two objects for them to count as a match.
Units are arc seconds.
tuning=<healpix-k>
Tuning parameter that controls the pixel size used when binning the rows. The legal
range is from 0 (corresponding to pixel size of about 60 degrees) to 20 (about 0.2
arcsec). The value of this parameter will not affect the result but may affect the
performance in terms of CPU and memory resources required. A default value will be
chosen based on the size of the error parameter, but it may be possible to improve
performance by adjusting the default value. The value used can be seen by examining
the progress output. If your match is taking a long time or is failing from lack of
memory it may be worth trying different values for this parameter.
join=1and2|1or2|all1|all2|1not2|2not1|1xor2
Determines which rows are included in the output table. The matching algorithm
determines which of the rows from the first table correspond to which rows from the
second. This parameter determines what to do with that information. Perhaps the
most obvious thing is to write out a table containing only rows which correspond to
a row in both of the two input tables. However, you may also want to see the
unmatched rows from one or both input tables, or rows present in one table but
unmatched in the other, or other possibilities. The options are:
* 1and2: An output row for each row represented in both input tables (INNER JOIN)
* 1or2: An output row for each row represented in either or both of the input
tables (FULL OUTER JOIN)
* all1: An output row for each matched or unmatched row in table 1 (LEFT OUTER
JOIN)
* all2: An output row for each matched or unmatched row in table 2 (RIGHT OUTER
JOIN)
* 1not2: An output row only for rows which appear in the first table but are not
matched in the second table
* 2not1: An output row only for rows which appear in the second table but are not
matched in the first table
* 1xor2: An output row only for rows represented in one of the input tables but
not the other one
find=all|best|best1|best2
Determines what happens when a row in one table can be matched by more than one row
in the other table. The options are:
* all: All matches. Every match between the two tables is included in the result.
Rows from both of the input tables may appear multiple times in the result.
* best: Best match, symmetric. The best pairs are selected in a way which treats
the two tables symmetrically. Any input row which appears in one result pair is
disqualified from appearing in any other result pair, so each row from both
input tables will appear in at most one row in the result.
* best1: Best match for each Table 1 row. For each row in table 1, only the best
match from table 2 will appear in the result. Each row from table 1 will appear
a maximum of once in the result, but rows from table 2 may appear multiple
times.
* best2: Best match for each Table 2 row. For each row in table 2, only the best
match from table 1 will appear in the result. Each row from table 2 will appear
a maximum of once in the result, but rows from table 1 may appear multiple
times.
The differences between best, best1 and best2 are a bit subtle. In cases where
it's obvious which object in each table is the best match for which object in the
other, choosing betwen these options will not affect the result. However, in
crowded fields (where the distance between objects within one or both tables is
typically similar to or smaller than the specified match radius) it will make a
difference. In this case one of the asymmetric options (best1 or best2) is usually
more appropriate than best, but you'll have to think about which of them suits your
requirements. The performance (time and memory usage) of the match may also differ
between these options, especially if one table is much bigger than the other.
runner=parallel|parallel<n>|parallel-all|sequential|classic|partest
Selects the threading implementation. The options are currently:
* parallel: uses multithreaded implementation for large tables, with default
parallelism, which is the smaller of 6 and the number of available processors
* parallel<n>: uses multithreaded implementation for large tables, with
parallelism given by the supplied value <n>
* parallel-all: uses multithreaded implementation for large tables, with a
parallelism given by the number of available processors
* sequential: uses multithreaded implementation but with only a single thread
* classic: uses legacy sequential implementation
* partest: uses multithreaded implementation even when tables are small
The parallel* options should normally run faster than sequential or classic (which
are provided mainly for testing purposes), at least for large matches and where
multiple processing cores are available.
The default value "parallel" is currently limited to a parallelism of 6 since
larger values yield diminishing returns given that some parts of the matching
algorithms run sequentially (Amdahl's Law), and using too many threads can
sometimes end up doing more work or impacting on other operations on the same
machine. But you can experiment with other concurrencies, e.g. "parallel16" to run
on 16 cores (if available) or "parallel-all" to run on all available cores.
The value of this parameter should make no difference to the matching results. If
you notice any discrepancies please report them.
SEE ALSO
stilts(1)
If the package stilts-doc is installed, the full documentation SUN/256 is available in
HTML format:
file:///usr/share/doc/stilts/sun256/index.html
VERSION
STILTS version 3.5.1-debian
This is the Debian version of Stilts, which lack the support of some file formats and
network protocols. For differences see
file:///usr/share/doc/stilts/README.Debian
AUTHOR
Mark Taylor (Bristol University)
Mar 2017 STILTS-TSKYMATCH2(1)