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### B.42.1 Usage

The usage of `tskymap` is

       stilts <stilts-flags> tskymap ifmt=<in-format> istream=true|false
                                     icmd=<cmds> ocmd=<cmds>
                                     omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|plastic|tosql|gui
                                     out=<out-table> ofmt=<out-format>
                                     lon=<expr/deg> lat=<expr/deg>
                                     tiling=hpx<K>|healpixnest<K>|healpixring<K>|htm<K>
                                     count=true|false
                                     cols=<expr>[;<combiner>[;<name>]] ...
                                     combine=sum|sum-per-unit|count|count-per-unit|mean|median|Q1|Q3|min|max|stdev|stdev_pop|hit
                                     perunit=steradian|degree2|arcmin2|arcsec2|mas2|uas2
                                     complete=true|false
                                     runner=sequential|parallel|parallel<n>|partest
                                     [in=]<table>

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.SkyDensityMap`.

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

`cols = <expr>[;<combiner>[;<name>]] ...` (String[])
  Defines the quantities to be calculated. The value is a space-separated list of items, one for each aggregated column in the output table.

  Each item is composed of one, two or three tokens, separated by semicolon ("`;`") characters:

  - `<expr>`: (required) column name or expression using the expression language for the quantity to be aggregated.
  - `<combiner>`: (optional) combination method, using the same options as for the `combine` parameter. If omitted, the value specified for that parameter will be used.
  - `<name>`: (optional) name of output column; if omitted, the `<expr>` value (perhaps somewhat sanitised) will be used.
  It is often sufficient just to supply a space-separated list of input table column names for this parameter, but the additional syntax may be required for instance if it's required to calculate both a sum and mean of the same input column.
`combine = sum|sum-per-unit|count|count-per-unit|mean|median|Q1|Q3|min|max|stdev|stdev_pop|hit` (Combiner)
  Defines the default way that values contributing to the same density map bin are combined together to produce the value assigned to that bin. Possible values are:
  - `sum`: the sum of all the combined values per bin
  - `sum-per-unit`: the sum of all the combined values per unit of bin size
  - `count`: the number of non-blank values per bin (weight is ignored)
  - `count-per-unit`: the number of non-blank values per unit of bin size (weight is ignored)
  - `mean`: the mean of the combined values
  - `median`: the median
  - `Q1`: first quartile
  - `Q3`: third quartile
  - `min`: the minimum of all the combined values
  - `max`: the maximum of all the combined values
  - `stdev`: the sample standard deviation of the combined values
  - `stdev_pop`: the population standard deviation of the combined values
  - `hit`: 1 if any values present, NaN otherwise (weight is ignored)
  - `Q.nnn`: quantile nnn (e.g. Q.05 is the fifth percentile)

  For density-like values (`count-per-unit`, `sum-per-unit`) the scaling is additionally influenced by the `perunit` parameter.

  Note this value may be overridden on a per-column basis by the `cols` parameter.

  [Default: `mean`]

`complete = true|false` (Boolean)
  Determines whether the output table contains a row for every pixel in the tiling, or only the rows for pixels in which some of the input data fell.

  The value of this parameter may affect performance as well as output. If you know that most pixels on the sky will be covered, it's probably a good idea to set this true, and if you know that only a small patch of sky will be covered, it's better to set it false.

  [Default: `false`]

`count = true|false` (Boolean)
  Controls whether a COUNT column is added to the output table along with any other columns that may have been requested. If included, this reports the number of rows from the input table that fell within the corresponding bin.

  [Default: `true`]

`icmd = <cmds>` (ProcessingStep[])
  Specifies processing to be performed on the input table as specified by parameter `in`, before any other processing has taken place. The value of this parameter is one or more of the filter commands described in Section 6.1. If more than one is given, they must be separated by semicolon characters (";"). This parameter can be repeated multiple times on the same command line to build up a list of processing steps. The sequence of commands given in this way defines the processing pipeline which is performed on the table.

  Commands may alternatively be supplied in an external file, by using the indirection character '`@`'. Thus a value of "`@filename`" causes the file `filename` to be read for a list of filter commands to execute. The commands in the file may be separated by newline characters and/or semicolons, and lines which are blank or which start with a '`#`' character are ignored. A backslash character '`\`' at the end of a line joins it with the following line.

`ifmt = <in-format>` (String)
  Specifies the format of the input table as specified by parameter `in`. 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)`]

`in = <table>` (StarTable)
  The location of the 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 `ifmt` 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.
`istream = true|false` (Boolean)
  If set true, the input table specified by the `in` parameter will be read as a stream. It is necessary to give the `ifmt` parameter in this case. Depending on the required operations and processing mode, this may cause the read to fail (sometimes it is necessary to read the table more than once). It is not normally necessary to set this flag; in most cases the data will be streamed automatically if that is the best thing to do. However it can sometimes result in less resource usage when processing large files in certain formats (such as VOTable). This parameter is ignored for scheme-specified tables.

  [Default: `false`]

`lat = <expr/deg>` (String)
  Latitude in degrees for the position of each row in the input table. This may simply be a column name, or it may be an algebraic expression as explained in Section 10. The sky system used here will determine the grid on which the output map is built.
`lon = <expr/deg>` (String)
  Longitude in degrees for the position of each row in the input table. This may simply be a column name, or it may be an algebraic expression as explained in Section 10. The sky system used here will determine the grid on which the output map is built.
`ocmd = <cmds>` (ProcessingStep[])
  Specifies processing to be performed on the output table, after all other processing has taken place. The value of this parameter is one or more of the filter commands described in Section 6.1. If more than one is given, they must be separated by semicolon characters (";"). This parameter can be repeated multiple times on the same command line to build up a list of processing steps. The sequence of commands given in this way defines the processing pipeline which is performed on the table.

  Commands may alternatively be supplied in an external file, by using the indirection character '`@`'. Thus a value of "`@filename`" causes the file `filename` to be read for a list of filter commands to execute. The commands in the file may be separated by newline characters and/or semicolons, and lines which are blank or which start with a '`#`' character are ignored. A backslash character '`\`' at the end of a line joins it with the following line.

`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: `-`]

`perunit = steradian|degree2|arcmin2|arcsec2|mas2|uas2` (SolidAngleUnit)
  Defines the unit of sky area used for scaling density-like combinations (e.g. `combine`=`count-per-unit` or `sum-per-unit`). If the combination mode is calculating values per unit area this configures the area scale in question. For non-density-like combination modes (e.g. `combine`=`sum` or `mean`) it has no effect.

  Possible values are:

  - `steradian`: steradian
  - `degree2`: square degree
  - `arcmin2`: square arcminute
  - `arcsec2`: square arcsecond
  - `mas2`: square milliarcsec
  - `uas2`: square microarcsec

  [Default: `degree2`]

`runner = sequential|parallel|parallel<n>|partest` (RowRunner)
  Selects the threading implementation, i.e. to what extent processing is done in parallel. The options are currently:
  - `sequential`: runs using only a single thread
  - `parallel`: runs using multiple threads for large tables, with parallelism given by the number of available processors
  - `parallel<n>`: runs using multiple threads for large tables, with parallelism given by the supplied value `<n>`
  - `partest`: runs using multiple threads even when tables are small (only intended for testing purposes)

  Using parallel processing can speed up execution considerably; however, depending on the I/O operations required, it can also slow it down by disrupting patterns of disk access. If the content of a file is on a solid state disk, or is already in cache for instance because a similar command has been run recently, then `parallel` will probably be faster. However, if the data is being read directly from a spinning disk, for instance because the file is too large to fit in RAM, then `sequential` or `parallel<n>` with a small `<n>` may be faster.

  The value of this parameter should make only very tiny differences to the output table. If you notice significant discrepancies please report them.

  [Default: `parallel`]

`tiling = hpx<K>|healpixnest<K>|healpixring<K>|htm<K>` (SkyTiling)
  Describes the sky tiling scheme that is in use. One of the following values may be used:
  - `hpxK`: alias for `healpixnestK`
  - `healpixnestK`: HEALPix using the Nest scheme at order `K`
  - `healpixringK`: HEALPix using the Ring scheme at order `K`
  - `htmK`: Hierarchical Triangular Mesh at level `K`
  So for instance `hpx5` or `healpixnest5` would both indicate the HEALPix NEST tiling scheme at order 5.

  At level K, there are 12*4^K HEALPix pixels, or 8*4^K HTM pixels on the sky. More information about these tiling schemes can be found at the HEALPix and HTM web sites.

  [Default: `hpx5`]

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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