Provided by: stilts_3.4.7-4_all 
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
in=<table>
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.
ifmt=<in-format>
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.
icmd=<cmds>
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 alteratively 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.
pixdata=<pix-table>
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.
pfmt=<in-format>
File format for the HEALPix pixel data table. This is usually, but not necessarily,
FITS.
pcmd=<cmds>
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 alteratively 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.
ocmd=<cmds>
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 alteratively 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.
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".
pixorder=nested|ring|(auto)
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.
stat=point|mean
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.
lon=<expr>
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.
lat=<expr>
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.
insys=icrs|fk5|fk4|galactic|supergalactic|ecliptic
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)
pixsys=icrs|fk5|fk4|galactic|supergalactic|ecliptic
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)
radius=<expr>
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 3.4.7-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-PIXSAMPLE(1)