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### B.43.1 Usage

The usage of `tskymatch2` is

       stilts <stilts-flags> tskymatch2 ifmt1=<in-format> ifmt2=<in-format>
                                        omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|plastic|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
                                        [in1=]<table1> [in2=]<table2>

If you don't have the `stilts` script installed, write "`java -jar stilts.jar`" instead of "`stilts`" - see Section 3. The available `<stilts-flags>` are listed in Section 2.1. For programmatic invocation, the Task class for this command is `uk.ac.starlink.ttools.task.SkyMatch2`.

Parameter values are assigned on the command line as explained in Section 2.3. They are as follows:

`dec1 = <expr>` (String)
  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 Section 10. If left blank, an attempt is made to guess from UCDs, column names and unit annotations what expression to use.
`dec2 = <expr>` (String)
  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 Section 10. If left blank, an attempt is made to guess from UCDs, column names and unit annotations what expression to use.
`error = <value/arcsec>` (Double)
  The maximum separation permitted between two objects for them to count as a match. Units are arc seconds.
`find = all|best|best1|best2` (PairMode)
  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.

  [Default: `best`]

`ifmt1 = <in-format>` (String)
  Specifies the format of the first input table as specified by parameter `in1`. The known formats are listed in Section 5.1.1. 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.

  [Default: `(auto)`]

`ifmt2 = <in-format>` (String)
  Specifies the format of the second input table as specified by parameter `in2`. The known formats are listed in Section 5.1.1. 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.

  [Default: `(auto)`]

`in1 = <table1>` (StarTable)
  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.
`in2 = <table2>` (StarTable)
  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.
`join = 1and2|1or2|all1|all2|1not2|2not1|1xor2` (JoinType)
  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

  [Default: `1and2`]

`ofmt = <out-format>` (String)
  Specifies the format in which the output table will be written (one of the ones in Section 5.1.2 - 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`".

  [Default: `(auto)`]

`omode = out|meta|stats|count|checksum|cgi|discard|topcat|samp|plastic|tosql|gui` (ProcessingMode)
  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`
  - `plastic`
  - `tosql`
  - `gui`
  Use the `help=omode` flag or see Section 6.4 for more information.

  [Default: `out`]

`out = <out-table>` (TableConsumer)
  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`".

  [Default: `-`]

`ra1 = <expr>` (String)
  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 Section 10. If left blank, an attempt is made to guess from UCDs, column names and unit annotations what expression to use.
`ra2 = <expr>` (String)
  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 Section 10. If left blank, an attempt is made to guess from UCDs, column names and unit annotations what expression to use.
`runner = parallel|parallel<n>|parallel-all|sequential|classic|partest` (RowRunner)
  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.

  [Default: `parallel`]

`tuning = <healpix-k>` (Integer)
  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.

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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk