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 column in the output table. But by
specifying the cols and combine parameters you can add more
columns giving 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 alternative 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.
- 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.
- istream=true|false
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.
- 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.
- 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.
- 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".
- 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 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|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
- 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 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.