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### B.37.1 Usage

The usage of `tgridmap` is

       stilts <stilts-flags> tgridmap 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>
                                      coords=<expr> ... logs=true|false ...
                                      bounds=[<lo>]:[<hi>] ... binsizes=<size> ...
                                      nbins=<num> ...
                                      cols=<expr>[;<combiner>[;<name>]] ...
                                      combine=sum|sum-per-unit|count|count-per-unit|mean|median|Q1|Q3|min|max|stdev|stdev_pop|hit
                                      sparse=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.GridDensityMap`.

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

`binsizes = <size> ...` (Double[])
  Gives the extent of of the data bins in each dimension in data coordinates. The form of the value is a space-separated list of values, giving a list of extents for the first, second, ... dimension. Either this parameter or the `nbins` parameter must be supplied.

  If supplied, this parameter must have the same number of words as the `coords` parameter.

`bounds = [<lo>]:[<hi>] ...` (double[][])
  Gives the bounds for each dimension of the cube in data coordinates. The form of the value is a space-separated list of words, each giving an optional lower bound, then a colon, then an optional upper bound, for instance "1:100 0:20" to represent a range for two-dimensional output between 1 and 100 of the first coordinate (table column) and between 0 and 20 for the second. Either or both numbers may be omitted to indicate that the bounds should be determined automatically by assessing the range of the data in the table. A null value for the parameter indicates that all bounds should be determined automatically for all the dimensions.

  If any of the bounds need to be determined automatically in this way, two passes through the data will be required, the first to determine bounds and the second to calculate the map.

  If supplied, this parameter must have the same number of words as the `coords` parameter.

`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.

  The default value is "`1;count;COUNT`" which simply provides an unweighted histogram, i.e. a count of the rows in each bin (aggregation of the value "`1`" using the combination method "`count`", yielding an output column named "`COUNT`").

  [Default: `1;count;COUNT`]

`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)

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

  [Default: `mean`]

`coords = <expr> ...` (String[])
  Defines the dimensions of the grid over which accumulation will take place. The form of this value is a space-separated list of words each giving a column name or algebraic expression defining one of the dimensions of the output grid. For a 1-dimensional histogram, only one value is required.
`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`]

`logs = true|false ...` (Boolean[])
  Determines whether each coordinate axis is linear or logarithmic. By default the grid axes are linear, but if this parameter is supplied with one or more true values, the bins on the corresponding axes are assigned logarithmically instead.

  If supplied, this parameter must have the same number of words as the `coords` parameter.

`nbins = <num> ...` (Integer[])
  Gives the approximate number of bins in each dimension. The form of the value is a space-separated list of integers, giving the number of bins for the output histogram in the first, second, ... dimension. An attempt is made to use round numbers for bin sizes so the bin counts may not be exactly as specified. Either this parameter or the `binsizes` parameter must be supplied.

  If supplied, this parameter must have the same number of words as the `coords` parameter.

`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: `-`]

`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`]

`sparse = true|false` (Boolean)
  Determines whether a row is written for every cell in the defined grid, or only for those cells in which data appears in the input. The result will usually be more compact if this is set false, but if you want to compare results from different runs it may be convenient to set it true.

  [Default: `true`]

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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