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STILTS-PIXSAMPLE(1) Stilts commands STILTS-PIXSAMPLE(1)

NAME

stilts-pixsample - Samples from a HEALPix pixel data file

SYNOPSIS

stilts pixsample [in=<table>] [ifmt=<in-format>] [icmd=<cmds>] [pixdata=<pix-table>] [pfmt=<in-format>] [pcmd=<cmds>] [ocmd=<cmds>] [omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|tosql|gui] [out=<out-table>] [ofmt=<out-format>] [pixorder=nested|ring|(auto)] [stat=point|mean] [lon=<expr>] [lat=<expr>] [insys=icrs|fk5|fk4|galactic|supergalactic|ecliptic] [pixsys=icrs|fk5|fk4|galactic|supergalactic|ecliptic] [radius=<expr>]

DESCRIPTION

pixsample samples data at the sky position represented by each row from an all-sky map contained in a HEALPix-format pixel data file. Such files are actually tables (usually in FITS format) in which the row number corresponds to a HEALPix pixel index, and the pixel values are cell contents; one or more columns may be present containing values for one or more all-sky maps. The result of this command is to add a column to the input table representing the pixel data at the position of each input row for each of the data columns in the HEALPix table.

This command does not attempt to convert between coordinate systems except as instructed, so it is important to know what coordinate system the HEALPix file is in, and ensure that the coordinates supplied from the input table match this. You may need to examine the documentation or headers of the HEALPix file in question to find out. See the Examples section for some examples.

There is a choice of how the sampling is done; the simplest way is just to use the value of the pixel covering the indicated position. An alternative is to average over a disc of given radius (perhaps a function of the input row). Other options (e.g. max/min) could easily be added.

Although HEALPix is not a common format for storing image data in general, it is used for storing a number of important all-sky data sets such as the WMAP results and Schlegel dust maps. The NASA LAMBDA (Legacy Archive for Microwave Background Data Analysis) archive has a number of maps in a suitable format, including foreground data like predicted reddening as well as CMB maps.

Note at present this command only supports all-sky, not partial, HEALPix maps. Partial map support may be added at some point in the future if there is demand.

OPTIONS

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.

Specifies the format of the input table as specified by parameter in. 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.

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 SUN/256. 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 '\fR' at the end of a line joins it with the following line.

The location of the table containing the pixel data. The data must be in the form of a HEALPix table, with one pixel per row in HEALPix order. These files are typically, but not necessarily, FITS tables. A filename or URL may be used, but a local file will be more efficient.

Some HEALPix format FITS tables seem to have rows which contain 1024-element arrays of pixels instead of single pixel values. This (rather perverse?) format is not currently supported here, but if there is demand support could be added.

File format for the HEALPix pixel data table. This is usually, but not necessarily, FITS.

Specifies processing to be performed on pixel data table as specified by parameter pixdata, before any other processing has taken place. The value of this parameter is one or more of the filter commands described in SUN/256. 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 '\fR' at the end of a line joins it with the following line.

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 SUN/256. 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 '\fR' at the end of a line joins it with the following line.

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.

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".

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".

Selects the pixel ordering scheme used by the pixel data file. There are two different ways of ordering pixels in a HEALPix file, "ring" and "nested", and the sampler needs to know which one is in use. If you know which is in use, choose the appropriate value for this parameter; if (auto) is used it will attempt to work it out from headers in the file (the ORDERING header). If no reliable ordering scheme can be determined, the command will fail with an error.

Determines how the pixel values will be sampled to generate an output value. The options are:

  • point: Uses the value at the pixel covering the supplied position. In this case the radius parameter is not used.
  • mean: Averages the values over all the pixels within a radius given by the radius parameter. This averaging is somewhat approximate; all pixels which are mostly within the radius are averaged with equal weights.

Expression which evaluates to the longitude coordinate in degrees in the input table at which positions are to be sampled from the pixel data table. This will usually be the name or ID of a column in the input table, or an expression involving one. If this coordinate does not match the coordinate system used by the pixel data table, both coordinate systems must be set using the insys and pixsys parameters.

Expression which evaluates to the latitude coordinate in degrees in the input table at which positions are to be sampled from the pixel data table. This will usually be the name or ID of a column in the input table, or an expression involving one. If this coordinate does not match the coordinate system used by the pixel data table, both coordinate systems must be set using the insys and pixsys parameters.

Specifies the sky coordinate system in which sample positions are provided by the lon/lat parameters. If the sample positions are given in the same coordinate system as that given by the pixel data table, both the insys and pixsys parameters may be set null.

The available coordinate systems are:

  • icrs: ICRS (Right Ascension, Declination)
  • fk5: FK5 J2000.0 (Right Ascension, Declination)
  • fk4: FK4 B1950.0 (Right Ascension, Declination)
  • galactic: IAU 1958 Galactic (Longitude, Latitude)
  • supergalactic: de Vaucouleurs Supergalactic (Longitude, Latitude)
  • ecliptic: Ecliptic (Longitude, Latitude)

Specifies the sky coordinate system used for the HEALPix data in the pixdata file. If the sample positions are given in the same coordinate system as that given by the pixel data table, both the insys and pixsys parameters may be set null.

The available coordinate systems are:

  • icrs: ICRS (Right Ascension, Declination)
  • fk5: FK5 J2000.0 (Right Ascension, Declination)
  • fk4: FK4 B1950.0 (Right Ascension, Declination)
  • galactic: IAU 1958 Galactic (Longitude, Latitude)
  • supergalactic: de Vaucouleurs Supergalactic (Longitude, Latitude)
  • ecliptic: Ecliptic (Longitude, Latitude)

Determines the radius in degrees over which pixels will be sampled to generate the output statistic in accordance with the value of the stat parameter. This will typically be a constant value, but it may be an algebraic expression based on columns from the input table.

Not used if stat=point.

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