DESCRIPTION¶
cdsskymatch uses the CDS X-Match service to join a local
table to one of the tables hosted by the Centre de Données
astronomiques de Strasbourg. This includes all of the VizieR tables and the
SIMBAD database. The service is very fast, and in most cases it is the best
way to match a local table against a large external table hosted by a
service. It is almost certainly much better than using coneskymatch,
though it is less flexible than TAP (see the tapquery task for
flexible access to TAP services, and tapskymatch for positional
matches).
The local table is uploaded to the X-Match service in chunks, and
the matches for each chunk are retrieved in turn and eventually stitched
together to form the final result. The tool only uploads sky position and an
identifier for each row of the input table, but all columns of the input
table are reinstated in the result for reference.
For a better understanding of the details of how this service
operates, including exactly what coordinates are matched against the
uploaded positions (roughly: integrated to J2000 using proper motions if
available) and what columns are included in the output (roughly: a subset of
the most commonly used columns), please consult the service
documentation.
Acknowledgement: CDS note that if the use of the X-Match service
is useful to your research, they would appreciate the following
acknowledgement: "This research made use of the cross-match service
provided by CDS, Strasbourg."
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 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".
- ra=<expr>
Right ascension in degrees in the ICRS coordinate system
for the position of each row of the input table. This may simply be a column
name, or it may be an algebraic expression calculated from columns as
explained in SUN/256. If left blank, an attempt is made to guess from UCDs,
column names and unit annotations what expression to use.
- dec=<expr>
Declination in degrees in the ICRS coordinate system for
the position of each row of the input table. This may simply be a column name,
or it may be an algebraic expression calculated from columns as explained in
SUN/256. If left blank, an attempt is made to guess from UCDs, column names
and unit annotations what expression to use.
- radius=<value/arcsec>
Maximum distance from the local table (ra,dec) position
at which counterparts from the remote table will be identified. This is a
fixed value given in arcseconds, and must be in the range [0,180] (this limit
is currently enforced by the CDS Xmatch service).
- cdstable=<value>
Identifier of the table from the CDS crossmatch service
that is to be matched against the local table. This identifier may be the
standard VizieR identifier (e.g. "
II/246/out" for the 2MASS
Point Source Catalogue) or "
simbad" to indicate SIMBAD data.
See for instance the TAPVizieR table searching facility at
http://tapvizier.u-strasbg.fr/adql/ to find VizieR catalogue
identifiers.
- find=all|best|best-remote|each|each-dist
Determines which pair matches are included in the result.
- all: All matches
- best: Matched rows, best remote row for each input row
- best-remote: Matched rows, best input row for each remote row
- each: One row per input row, contains best remote match or
blank
- each-dist: One row per input row, column giving distance only for
best match
Note only the
all mode is symmetric between the two tables.
Note also that there is a bug in best-remote
matching. If the match is done in multiple blocks, it's possible for a
remote table row to appear matched against one local table row per uploaded
block, rather than just once for the whole result. If you're worried about
that, set blocksize >= rowCount. This may be fixed in a future
release.
- blocksize=<int-value>
The CDS Xmatch service operates limits on the maximum
number of rows that can be uploaded and the maximum number of rows that is
returned as a result from a single query. In the case of large input tables,
they are broken down into smaller blocks, and one request is sent to the
external service for each block. This parameter controls the number of rows in
each block. For an input table with fewer rows than this value, the whole
thing is done as a single request.
At time of writing, the maximum upload size is 100Mb (about 3Mrow;
this does not depend on the width of your table), and the maximum return
size is 2Mrow.
Large blocksizes tend to be good (up to a point) for reducing the
total amount of time a large xmatch operation takes, but they can make it
harder to see the job progressing. There is also the danger (for ALL-type
find modes) of exceeding the return size limit, which will result in
truncation of the returned result.
- maxrec=<int-value>
Limit to the number of rows resulting from this
operation. If the value is negative (the default) no limit is imposed. Note
however that there can be truncation of the result if the number of records
returned from a single chunk exceeds the service hard limit (2,000,000 at time
of writing).
- compress=true|false
If true, the service is requested to provide HTTP-level
compression for the response stream (Accept-Encoding header is set to
"
gzip", see RFC 2616). This does not guarantee that
compression will happen but if the service honours this request it may result
in a smaller amount of network traffic at the expense of more processing on
the server and client.
- serviceurl=<url-value>
The URL at which the CDS Xmatch service can be found.
Normally this should not be altered from the default, but if other
implementations of the same service are known, this parameter can be used to
access them.
- usemoc=true|false
If true, first acquire a MOC coverage map from CDS, and
use that to pre-filter rows before uploading them for matching. This should
improve efficiency, but have no effect on the result.
- presort=true|false
If true, the rows are sorted by HEALPix index before they
are uploaded to the CDS X-Match service. If the match is done in multiple
blocks, this may improve efficiency, since when matching against a large
remote catalogue the X-Match service likes to process requests in which
sources are grouped into a small region rather than scattered all over the
sky.
Note this will have a couple of other side effects that may be
undesirable: it will read all the input rows into the task at once, which
may make it harder to assess progress, and it will affect the order of the
rows in the output table.
It is probably only worth setting true for rather large
(multi-million-row?) multi-block matches, where both local and remote
catalogues are spread over a significant fraction of the sky. But feel free
to experiment.
- fixcols=none|dups|all
Determines how input columns are renamed before use in
the output table. The choices are:
- none: columns are not renamed
- dups: columns which would otherwise have duplicate names in the
output will be renamed to indicate which table they came from
- all: all columns will be renamed to indicate which table they came
from
If columns are renamed, the new ones are determined by
suffix*
parameters.
- suffixin=<label>
If the
fixcols parameter is set so that input
columns are renamed for insertion into the output table, this parameter
determines how the renaming is done. It gives a suffix which is appended to
all renamed columns from the input table.
- suffixremote=<label>
If the
fixcols parameter is set so that input
columns are renamed for insertion into the output table, this parameter
determines how the renaming is done. It gives a suffix which is appended to
all renamed columns from the CDS result table.
