Provided by: stilts_3.4.9-5_all 
NAME
stilts-tskymap - Calculates sky density maps
SYNOPSIS
stilts tskymap [ifmt=<in-format>] [istream=true|false] [in=<table>] [icmd=<cmds>] [ocmd=<cmds>]
[omode=out|meta|stats|count|checksum|cgi|discard|topcat|samp|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]
DESCRIPTION
tskymap calculates a weighted density map (or, to put it another way, a histogram) on the sky from
columns of an input table. The sky is divided up into some discrete set of tiles according to a specified
tessellation scheme (currently HEALPix or HTM are supported), and the required quantities are aggregated
into bins corresponding to these tiles. The output table has a column giving the pixel index identifying
each tile, plus one or more columns each representing an aggregation of a quantity from the input table.
By default the number of rows from the input table falling within each tile is included as the first col‐
umn in the output table. But by specifying the cols and combine parameters you can add more columns giv‐
ing the sum, mean, median or other statistics of input table columns or expressions as well.
The output table can then, for instance, be plotted using plot2sky's healpix layer type (though an alter‐
native is to do that plot directly using a skydensity layer).
In the case of HEALPix tiling, the result can also be output in a FITS file suitable for use by external
applications that understand the semi-standard FITS-Healpix convention. Note in this case, for maximum
compatibility, the fits-healpix output format should in general be used.
See also tgridmap, which does the same thing for N-dimensional grid geometry.
OPTIONS
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 auto‐
matically. 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.
istream=true|false
If set true, the input table specified by the in parameter will be read as a stream. It is neces‐
sary 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 au‐
tomatically if that is the best thing to do. However it can sometimes result in less resource us‐
age when processing large files in certain formats (such as VOTable). This parameter is ignored
for scheme-specified tables.
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 ex‐
plicitly 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.
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 com‐
mands described in SUN/256. If more than one is given, they must be separated by semicolon charac‐
ters (";"). 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 pa‐
rameters. 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".
lon=<expr/deg>
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 SUN/256. The sky system used here will
determine the grid on which the output map is built.
lat=<expr/deg>
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 SUN/256. The sky system used here will
determine the grid on which the output map is built.
tiling=hpx<K>|healpixnest<K>|healpixring<K>|htm<K>
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.
count=true|false
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.
cols=<expr>[;<combiner>[;<name>]] ...
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 parame‐
ter. If omitted, the value specified for that parameter will be used.
* <name>: (optional) name of output column; if omitted, the <expr> value (perhaps somewhat sani‐
tised) 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
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)
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.
perunit=steradian|degree2|arcmin2|arcsec2|mas2|uas2
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
complete=true|false
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.
runner=sequential|parallel|parallel<n>|partest
Selects the threading implementation, i.e. to what extent processing is done in parallel. The op‐
tions 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 sup‐
plied 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 oper‐
ations 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 di‐
rectly from a spinning disk, for instance because the file is too large to fit in RAM, then se‐
quential 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 no‐
tice significant discrepancies please report them.
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 3.4.9-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-TSKYMAP(1)